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Energy Metering IC with On-Chip Fault and Missing Neutral Detection ADE7761
FEATURES
High accuracy active energy measurement IC, supports IEC 687/61036 Less than 0.1% error over a dynamic range of 500 to 1 Supplies active power on the frequency outputs F1 and F2 High frequency output CF is intended for calibration and supplies instantaneous active power Continuous monitoring of the phase and neutral current allows fault detection in 2-wire distribution systems Current channels input level best suited for current transformer sensors Uses the larger of the two currents (phase or neutral) to bill--even during a fault condition Continuous monitoring of the voltage and current inputs allows missing neutral detection Uses one current input (phase or neutral) to bill when missing neutral is detected Two logic outputs (FAULT and REVP) can be used to indicate a potential miswiring, fault, or missing neutral condition Direct drive for electromechanical counters and 2-phase stepper motors (F1 and F2) Proprietary ADCs and DSP provide high accuracy over large variations in environmental conditions and time Reference 2.5 V 8% (drift 30 ppm/C typical) with external overdrive capability Single 5 V supply, low power
GENERAL DESCRIPTION
The ADE7761 is a high accuracy, fault tolerant, electrical energy measurement IC intended for use with 2-wire distribution systems. The part specifications surpass the accuracy requirements as quoted in the IEC61036 standard. The only analog circuitry used on the ADE7761 is in the ADCs and reference circuit. All other signal processing (such as multiplication and filtering) is carried out in the digital domain. This approach provides superior stability and accuracy over extremes in environmental conditions and over time. The ADE7761 incorporates a fault detection scheme similar to the ADE7751 by continuously monitoring both the phase and neutral currents. A fault is indicated when these currents differ by more than 6.25%. (continued on Page 3)
FUNCTIONAL BLOCK DIAGRAM
AGND
8
FAULT
15
VDD
1
POWER SUPPLY MONITOR V1A 2 V1N 4 ADC V1B 3 MISCAL 7 V2P 6 V2N 5 4k 2.5V REFERENCE INTERNAL OSCILLATOR ADC B>A A<>B ZERO CROSSING DETECTION MISSING NEUTRAL GAIN ADJUST ADC A>B HPF
ADE7761
SIGNAL PROCESSING BLOCK LPF
ADC MISSING NEUTRAL DETECTION DIGITAL-TO-FREQUENCY CONVERTER
04407-0-001
9
14
17
10
11
12
16
18
19
20
REFIN/OUT
RCLKIN
DGND
SCF
S1
S0 REVP CF
F2
F1
Figure 1.
Rev. A
Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other rights of third parties that may result from its use. Specifications subject to change without notice. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices. Trademarks and registered trademarks are the property of their respective owners.
.
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ADE7761 TABLE OF CONTENTS
General Description ......................................................................... 3 Specifications..................................................................................... 4 Timing Characteristics..................................................................... 6 Absolute Maximum Ratings............................................................ 7 ESD Caution.................................................................................. 7 Terminology ...................................................................................... 8 Pin Configuration and Function Descriptions............................. 9 Typical Performance Characteristics ........................................... 11 Operation......................................................................................... 13 Power Supply Monitor ............................................................... 13 Analog Inputs.............................................................................. 13 Internal Oscillator ...................................................................... 14 Analog-to-Digital Conversion.................................................. 14 Active Power Calculation .......................................................... 15 Digital-to-Frequency Conversion ............................................ 18 Transfer Function....................................................................... 18 Fault Detection ........................................................................... 19 Missing Neutral Mode ............................................................... 20 Applications..................................................................................... 23 Interfacing to a Microcontroller for Energy Measurement .. 23 Selecting a Frequency for an Energy Meter Application....... 23 Negative Power Information..................................................... 24 Outline Dimensions ....................................................................... 25 Ordering Guide .......................................................................... 25 Disclaimer........................................................................................ 26
REVISION HISTORY
2/04--Changed from Rev. 0 to Rev. A. Changes to Ordering Guide .......................................................... 25 1/04--Revision 0: Initial Version
Rev. A | Page 2 of 28
ADE7761 GENERAL DESCRIPTION
(continued from Page 1) The ADE7761 incorporates a missing neutral detection scheme by continuously monitoring the input voltage. When a missing neutral condition is detected--no voltage input--the ADE7761 continues billing based on the active current signal (see the Missing Neutral Mode section). The missing neutral condition is indicated when the FAULT pin goes high. The ADE7761 supplies average active power information on the low frequency outputs F1 and F2. The CF logic output gives instantaneous active power information. The ADE7761 includes a power supply monitoring circuit on the VDD supply pin. Internal phase matching circuitry ensures that the voltage and current channels are matched. An internal no-load threshold ensures that the ADE7761 does not exhibit any creep when there is no load.
Rev. A | Page 3 of 28
ADE7761 SPECIFICATIONS
VDD = 5 V 5%, AGND = DGND = 0 V, on-chip reference, on-chip oscillator, TMIN to TMAX = -40C to +85C. Table 1.
Parameter ACCURACY1 Measurement Error2 Phase Error between Channels (PF = 0.8 Capacitive) (PF = 0.5 Inductive) AC Power Supply Rejection2 Output Frequency Variation DC Power Supply Rejection2 Output Frequency Variation FAULT DETECTION2, 3 Fault Detection Threshold Inactive Input <> Active Input Input Swap Threshold Inactive Input <> Active Input Accuracy Fault Mode Operation V1A Active, V1B = AGND V1B Active, V1A = AGND Fault Detection Delay Swap Delay MISSING NEUTRAL MODE2, 4 Missing Neutral Detection Threshold V2P - V2N Accuracy Missing Neutral Mode V1A Active, V1B = V2P = AGND V1B Active, V1A = V2P = AGND Missing Neutral Detection Delay ANALOG INPUTS Maximum Signal Levels Input Impedance (DC) Bandwidth (-3 dB) ADC Offset Error2 Gain Error REFERENCE INPUT REFIN/OUT Input Voltage Range Input Impedance Input Capacitance ON-CHIP REFERENCE Reference Error Temperature Coefficient Current Source ON-CHIP OSCILLATOR Oscillator Frequency Oscillator Frequency Tolerance Temperature Coefficient
See footnotes on next page.
Value 0.1 0.05 0.05 0.01 0.01
Unit % of reading, typ Degrees, max Degrees, max %, typ %, typ
Test Conditions/Comments Over a dynamic range of 500 to 1 Phase lead 37 Phase lag 60 V1A = V1B = V2P = 100 mV rms V1A = V1B = V2P = 100 mV rms See the Fault Detection section (V1A or V1B active) (V1A or V1B active) Over a dynamic range of 500 to 1 Over a dynamic range of 500 to 1
6.25 6.25 0.1 0.1 3 3
%, typ % of larger, typ % of reading, typ % of reading, typ Seconds, typ Seconds, typ
See the Missing Neutral Detection section 59.4 0.1 0.1 3 660 660 400 7 10 4 2.7 2.3 4 10 200 30 20 450 12 30 mV peak, min % of reading, typ % of reading, typ Seconds, typ mV peak, max mV peak, max k, min kHz, typ mV, max %, typ V, max V, min k, min pF, max mV, max ppm/C, typ A, min kHz % of reading, typ ppm/C, typ Over a dynamic range of 500 to 1 Over a dynamic range of 500 to 1 V1A - V1N, V1B - V1N, V2P - V2N Differential input Differential input MISCAL - V2N
Uncalibrated error, see the Terminology section for details External 2.5 V reference 2.5 V + 8% 2.5 V - 8%
Rev. A | Page 4 of 28
ADE7761
Parameter LOGIC INPUTS5 SCF, S1, and S0 Input High Voltage, VINH Input Low Voltage, VINL Input Current, IIN Input Capacitance, CIN LOGIC OUTPUTS5 CF, REVP, and FAULT Output High Voltage, VOH Output Low Voltage, VOH F1 and F2 Output High Voltage, VOH Output Low Voltage, VOH POWER SUPPLY VDD VDD Value Unit Test Conditions/Comments
2.4 0.8 3 10
V, min V, max A, max pF, max
VDD = 5 V 5% VDD = 5 V 5% Typical 10 nA, VIN = 0 V to VDD
4 1 4 1 4.75 5.25 4
V, min V, max V, min V, max V, min V, max mA, max
VDD = 5 V 5% VDD = 5 V 5% VDD = 5 V 5%, ISOURCE = 10 mA VDD = 5 V 5%, ISINK = 10 mA For specified performance 5 V - 5% 5 V + 5%
1 2
See plots in the Typical Performance Characteristics section. See the Terminology section for explanation of specifications. 3 See the Fault Detection section for explanation of fault detection functionality. 4 See the Missing Neutral Detection section for explanation of missing neutral detection functionality. 5 Sample tested during initial release and after any redesign or process change that may affect this parameter.
Rev. A | Page 5 of 28
ADE7761 TIMING CHARACTERISTICS
VDD = 5 V 5%, AGND = DGND = 0 V, on-chip reference, on-chip oscillator, TMIN to TMAX = -40C to +85C. Sample tested during initial release and after any redesign or process change that may affect this parameter. See Figure 2. Table 2. Parameter
t1 t2 t3 t41 t5 t6
1
Value
120 See Table 6 1/2 t2 90 See Table 7 CLKIN/4
Unit
ms s s ms s s
Test Conditions/Comments
F1 and F2 Pulse Width (Logic High). Output Pulse Period. See the Transfer Function section. Time between F1 Falling Edge and F2 Falling Edge. CF Pulse Width (Logic High). CF Pulse Period. See the Transfer Function section. Minimum Time between F1 and F2 Pulse.
1
The pulse widths of F1, F2, and CF are not fixed for higher output frequencies. See the Transfer Function section.
t1
F1
t6
F2
t2
t3
04407-0-002
t4
CF
t5
Figure 2. Timing Diagram for Frequency Outputs
Rev. A | Page 6 of 28
ADE7761 ABSOLUTE MAXIMUM RATINGS
TA = 25C, unless otherwise noted. Table 3.
Parameter VDD to AGND Analog Input Voltage to AGND V1AP, V1BP, V1N, V2N, V2P, MISCAL Reference Input Voltage to AGND Digital Input Voltage to DGND Digital Output Voltage to DGND Operating Temperature Range Industrial Storage Temperature Range Junction Temperature 20-Lead SSOP, Power Dissipation JA Thermal Impedance Lead Temperature, Soldering Vapor Phase (60 s) Infrared (15 s) Rating -0.3 V to +7 V -6 V to +6 V -0.3 V to VDD + 0.3 V -0.3 V to VDD + 0.3 V -0.3 V to VDD + 0.3 V -40C to +85C -65C to +150C 150C 450 mW 112C/W 215C 220C
Stresses above those listed under Absolute Maximum Ratings may cause permanent damage to the device. This is a stress rating only; functional operation of the device at these or any other conditions above those listed in the operational sections of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.
ESD CAUTION
ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily accumulate on the human body and test equipment and can discharge without detection. Although this product features proprietary ESD protection circuitry, permanent damage may occur on devices subjected to high energy electrostatic discharges. Therefore, proper ESD precautions are recommended to avoid performance degradation or loss of functionality.
Rev. A | Page 7 of 28
ADE7761 TERMINOLOGY
Measurement Error The error associated with the energy measurement made by the ADE7761 is defined by the following formula:
Percentage Error = Energy registered by ADE7761 - True Energy x 100% True Energy
Phase Error between Channels The high-pass filter (HPF) in the current channel has a phase lead response. To offset this phase response and equalize the phase response between channels, a phase correction network is also placed in the current channel. The phase correction network ensures a phase match between the current channels and voltage channels to within 0.1 over a range of 45 Hz to 65 Hz and 0.2 over a range 40 Hz to 1 kHz. Power Supply Rejection This quantifies the ADE7761 measurement error as a percentage of reading when the power supplies are varied. For the ac PSR measurement, a reading at nominal supplies (5 V) is taken. A second reading is obtained with the same input signal levels when an ac (175 mV rms/100 Hz) signal is introduced onto the supplies. Any error introduced by this ac signal is expressed as a percentage of reading (see the Measurement Error definition above).
For the dc PSR measurement, a reading at nominal supplies (5 V) is taken. A second reading is obtained with the same input signal levels when the power supplies are varied 5%. Any error introduced is again expressed as a percentage of reading.
ADC Offset Error This refers to the dc offset associated with the analog inputs to the ADCs. It means that with the analog inputs connected to AGND, the ADCs still see a dc analog input signal. The magnitude of the offset depends on the input range selection (see the Typical Performance Characteristics section). However, when HPFs are switched on, the offset is removed from the current channels and the power calculation is not affected by this offset. Gain Error The gain error in the ADE7761 ADCs is defined as the difference between the measured output frequency (minus the offset) and the ideal output frequency. The difference is expressed as a percentage of the ideal frequency, which is obtained from the transfer function (see the Transfer Function section).
Rev. A | Page 8 of 28
ADE7761 PIN CONFIGURATION AND FUNCTION DESCRIPTIONS
VDD V1A V1B V1N V2N V2P
1 2 3 4 5 6 20 19 18 17
F1 F2 CF DGND REVP FAULT RCLKIN INT S0 S1
04407-0-003
ADE7761
TOP VIEW (Not to Scale)
16 15 14 13 12 11
MISCAL 7 AGND 8 REFIN/OUT 9 SCF 10
Figure 3. Pin Configuration (SSOP)
Table 4. Pin Function Descriptions Pin No.
1
Mnemonic
VDD
Description
Power Supply. This pin provides the supply voltage for the digital circuitry in the ADE7761. The supply voltage should be maintained at 5 V 5% for specified operation. This pin should be decoupled with a 10 F capacitor in parallel with a ceramic 100 nF capacitor. Analog Inputs for Channel 1 (Current Channel). These inputs are fully differential voltage inputs with maximum differential input signal levels of 660 mV with respect to V1N for specified operation. The maximum signal level at these pins is 1 V with respect to AGND. Both inputs have internal ESD protection circuitry, and an overvoltage of 6 V can also be sustained on these inputs without risk of permanent damage. Negative Input Pin for Differential Voltage Inputs V1A and V1B. The maximum signal level at this pin is 1 V with respect to AGND. The input has internal ESD protection circuitry, and an overvoltage of 6 V can also be sustained on these inputs without risk of permanent damage. The input should be directly connected to the burden resistor and held at a fixed potential,that is, AGND. See the Analog Inputs section. Negative Input Pin for Differential Voltage Inputs V2P and MISCAL. The maximum signal level at this pin is 1 V with respect to AGND. The input has internal ESD protection circuitry, and an overvoltage of 6 V can also be sustained on these inputs without risk of permanent damage. The input should be held at a fixed potential, that is, AGND. See the Analog Inputs section. Analog Inputs for Channel 2 (Voltage Channel). This input is fully differential voltage input with maximum differential input signal levels of 660 mV with respect to V2N for specified operation. The maximum signal level at these pins is 1 V with respect to AGND. This input has internal ESD protection circuitry, and an overvoltage of 6 V can also be sustained on these inputs without risk of permanent damage. Analog Input for Missing Neutral Calibration. This pin can be used to calibrate the CF-F1-F2 frequencies in the missing neutral condition. This input is fully differential voltage input with maximum differential input signal levels of +660 mV with respect to V2N for specified operation. The maximum signal level at this pin is 1 V with respect to AGND. This input has internal ESD protection circuitry, and an overvoltage of 6 V can also be sustained on these inputs without risk of permanent damage. This pin provides the ground reference for the analog circuitry in the ADE7761, that is, ADCs and reference. This pin should be tied to the analog ground plane of the PCB. The analog ground plane is the ground reference for all analog circuitry such as antialiasing filters, and current and voltage transducers. For good noise suppression, the analog ground plane should be connected only to the digital ground plane at the DGND pin. This pin provides access to the on-chip voltage reference. The on-chip reference has a nominal value of 2.5 V 8% and a typical temperature coefficient of 30 ppm/C. An external reference source can also be connected at this pin. In either case, this pin should be decoupled to AGND with a 1 F ceramic capacitor and 100 nF ceramic capacitor. Select Calibration Frequency. This logic input is used to select the frequency on the calibration output CF. Table 6 shows how the calibration frequencies are selected. These logic inputs are used to select one of four possible frequencies for the digital-to-frequency conversion. This offers the designer greater flexibility when designing the energy meter. See the Selecting a Frequency for an Energy Meter Application section.
Rev. A | Page 9 of 28
2, 3
V1A, V1B
4
V1N
5
V2N
6
V2P
7
MISCAL
8
AGND
9
REFIN/OUT
10 11, 12
SCF S1, S0
ADE7761
Pin No.
13 14 15
Mnemonic
INT RCLKIN FAULT
Description
This pin is internally used and should be connected to DGND. To enable the internal oscillator as a clock source on the chip, a precise low temperature drift resistor at nominal value of 6.2 k must be connected from this pin to DGND. This logic output goes active high when a fault or missing neutral condition occurs. A fault is defined as a condition under which the signals on V1A and V1B differ by more than 6.25%. A missing neutral condition is defined when the chip is powered up with no voltage at the input. The logic output is reset to zero when a fault or missing neutral condition is no longer detected. See the Fault Detection section and the Missing Neutral Mode section. This logic output goes logic high when negative power is detected, that is, when the phase angle between the voltage and current signals is greater than 90. This output is not latched and is reset when positive power is once again detected. The output goes high or low at the same time as a pulse is issued on CF. This pin provides the ground reference for the digital circuitry in the ADE7761, that is, multiplier, filters, and digital-to-frequency converter. This pin should be tied to the digital ground plane of the PCB. The digital ground plane is the ground reference for all digital circuitry such as counters (mechanical and digital), MCUs, and indicator LEDs. For good noise suppression, the analog ground plane should be connected only to the digital ground plane at the DGND pin. Calibration Frequency Logic Output. The CF logic output, active high, gives instantaneous active power information. This output is intended to be used for operational and calibration purposes. See the Digitalto-Frequency Conversion section. Low Frequency Logic Outputs. F1 and F2 supply average active power information. The logic outputs can be used to directly drive electromechanical counters and 2-phase stepper motors.
16
REVP
17
DGND
18
CF
19, 20
F2, F1
Rev. A | Page 10 of 28
ADE7761 TYPICAL PERFORMANCE CHARACTERISTICS
1.0 0.8 0.6 0.4 +25C 0.2 0.2 PF = 1 ON-CHIP REFERENCE -40C 1.0 0.8 0.6 0.4 PF = 1 ON-CHIP REFERENCE 5.25V
% ERROR
0 -0.2 -0.4 -0.6 -0.8
04407-0-037
% ERROR
0 -0.2 -0.4 -0.6 -0.8 -1.0 0.1
5.00V
+85C
4.75V
-1.0 0.1
1.0 10.0 CURRENT (% of Full Scale)
100.0
1.0 10.0 CURRENT (% of Full Scale)
100.0
Figure 4. Active Power Error as a Percentage of Reading with Internal Reference
1.5 PF = 0.5 ON-CHIP REFERENCE 2.0 1.5 1.0 -40C; PF = 0.5 0.5 1.0 0.5
Figure 6. Active Power Error as a Percentage of Reading over Power Supply with Internal Reference
ON-CHIP REFERENCE
+85C
% ERROR
% ERROR
+25C; PF = 1 0 +85C; PF = 0.5 -0.5 +25C; PF = 0.5
0 -0.5 -1.0 -1.5
+25C -40C
04407-0-038
-1.0 0.1
1.0 10.0 CURRENT (% of Full Scale)
100.0
-2.0 0.1
1.0 10.0 CURRENT (% of Full Scale)
100.0
Figure 5. Active Power Error as a Percentage of Reading over Power Factor with Internal Reference
Figure 7. Ampere Hour Error as a Percentage of Reading in Missing Neutral Mode with Internal Reference
Rev. A | Page 11 of 28
04434-0-040
04434-0-039
ADE7761
VDD +
10F 40A TO 80mA I RB
100nF
1
1k 33nF
VDD
2
CF 18
2k
PS2501-1
1 4
V1A
ADE7761
2 3
TO FREQ. COUNTER
1k RB 33nF 1k
3
V1B
FAULT 15 RCLKIN 14
2k 6.2k
4
RB = 18
V1N 10k
33nF 1k 33nF
5
V2N
S0 12 S1 11 SCF 10
1M
6
V2P
REFIN/OUT 9 100nF
220V
1k
33nF 560k
6
+
10F
MISCAL INT
13
100k
33nF
AGND DGND
8 17
04407-0-036
Figure 8. Test Circuit for Performances Curves
Rev. A | Page 12 of 28
ADE7761 OPERATION
POWER SUPPLY MONITOR
The ADE7761 contains an on-chip power supply monitor. The power supply (VDD) is continuously monitored by the ADE7761. If the supply is less than 4 V 5%, the ADE7761 goes into an inactive state, that is, no energy is accumulated and the CF, F1, and F2 outputs are disabled. This is useful to ensure correct device operation at power-up and during power-down. The power supply monitor has built-in hysteresis and filtering. This gives a high degree of immunity to false triggering due to noisy supplies. The power supply and decoupling for the part should be such that the ripple at VDD does not exceed 5 V 5% as specified for normal operation.
VDD
5V 4V
Channel V2 (Voltage Channel)
The output of the line voltage transducer is connected to the ADE7761 at this analog input. Channel V2 is a single-ended voltage input. The maximum peak differential signal on Channel 2 is 660 mV with respect to V2N. Figure 11 shows the maximum signal levels that can be connected to Channel 2.
V2 +660mV + VCM DIFFERENTIAL INPUT 660mV MAX PEAK VCM -660mV + VCM COMMON MODE 100mV MAX V2 V2N VCM
04407-0-012
V2P
Figure 11. Maximum Signal Levels, Channel 2
0V
The differential voltage V2P-V2N must be referenced to a common mode (usually AGND). The analog inputs of the ADE7761 can be driven with common-mode voltages of up to 100 mV with respect to AGND. However, the best results are achieved using a common mode equal to AGND.
TIME
04407-0-010
MISCAL Input
The input for the power calibration in missing neutral mode is connected to the ADE7761 at this analog input. MISCAL is a single-ended voltage input. It is recommended to use a dc signal derived from the voltage reference to drive this pin. The maximum peak differential signal on MISCAL is +660 mV with respect to V2N. Figure 12 shows the maximum signal levels that can be connected to the MISCAL pin.
MISCAL MISCAL +660mV + VCM DIFFERENTIAL INPUT 660mV MAX PEAK MISCAL V2N COMMON MODE 100mV MAX VCM
04407-0-013
ADE7761
REVP - FAULT - CF - INACTIVE F1 - F2 OUTPUTS ACTIVE INACTIVE
Figure 9. On-Chip Power Supply Monitoring
ANALOG INPUTS
Channel V1 (Current Channel)
The voltage outputs from the current transducers are connected to the ADE7761 here. Channel V1 has two voltage inputs, V1A and V1B. These inputs are fully differential with respect to V1N. However, at any one time, only one is selected to perform the power calculation (see the Fault Detection section). The maximum peak differential signal on V1A-V1N and V1B-V1N is 660 mV. Figure 10 shows the maximum signal levels on V1A, V1B, and V1N. The differential voltage signal on the inputs must be referenced to a common mode (usually AGND).
V1A, V1B +660mV + VCM COMMON MODE 100mV MAX VCM
04407-0-011
VCM
AGND
Figure 12. Maximum Signal Levels, MISCAL
DIFFERENTIAL INPUT A 660mV MAX PEAK
V1A V1 V1N
VCM V1 V1B
The differential voltage MISCAL-V2N must be referenced to a common mode (usually AGND). The analog inputs of the ADE7761 can be driven with common-mode voltages of up to 100 mV with respect to AGND. However, best results are achieved using a common mode equal to AGND.
Typical Connection Diagrams
Figure 13 shows a typical connection diagram for Channel V1. The analog inputs are being used to monitor both the phase and neutral currents. Because of the large potential difference between the phase and neutral, two current transformers (CTs) must be used to provide the isolation. Note that both CTs are referenced to analog ground (AGND); the common-mode
AGND -660mV + VCM DIFFERENTIAL INPUT B 660mV MAX PEAK
Figure 10. Maximum Signal Levels, Channel 1
Rev. A | Page 13 of 28
ADE7761
voltage, therefore, is 0 V. The CT turns ratio and burden resistor (RB) are selected to give a peak differential voltage of 660 mV. Figure 14 shows two typical connections for Channel V2. The first option uses a potential transformer (PT) to provide complete isolation from the main voltage. In the second option, the ADE7761 is biased around the neutral wire, and a resistor divider is used to provide a voltage signal that is proportional to the line voltage. Adjusting the ratio of RA and RB + VR is a convenient way of carrying out a gain calibration on the meter. Figure 15 shows a typical connection for MISCAL input. The voltage reference input (REFIN/OUT) is used as a dc reference to set the MISCAL voltage. Adjusting the level of MISCAL to calibrate the meter in missing neutral mode can be done by changing the ratio of RC and RD + VR1. When the internal reference is used, the values of RC, RD, and VR1 must be chosen to limit the current sourced by the internal reference sourcing current to below the specified 20 A. Therefore, because VREF internal = 2.5 V, RC + RD + VR1 > 600 k.
CT RB IP IN AGND V1N RF V1A
INTERNAL OSCILLATOR
The nominal internal oscillator frequency is 450 kHz when used with the recommended ROSC resistor value of 6.2 k between RCLKIN and DGND (see Figure 16). The internal oscillator frequency is inversely proportional to the value of this resistor. Although the internal oscillator operates when used with a ROSC resistor value between 5 k and 12 k, it is recommended to choose a value within the range of the nominal value. The output frequencies on CF, F1, and F2 are directly proportional to the internal oscillator frequency; therefore, the resistor ROSC must have a low tolerance and low temperature drift. A low tolerance resistor limits the variation of the internal oscillator frequency. Small variation of the clock frequency and consequently of the output frequencies from meter to meter contributes to a smaller calibration range of the meter. A low temperature drift resistor directly limits the variation of the internal clock frequency over temperature. The stability of the meter to external variation is then better ensured by design.
ADE7761
4k 2.5V REFERENCE
CF
INTERNAL OSCILLATOR
NEUTRAL
RB CT RF
CF
04407-0-014
PHASE
9
14
17
V1B
REFIN/OUT ROSC
RCLKIN
DGND
04407-0-017
Figure 13. Typical Connection for Channel 1 Figure 16. Internal Oscillator Connection
RF V2P CF CF V2N
NEUTRAL
660mV
PHASE
RF
ANALOG-TO-DIGITAL CONVERSION
The analog-to-digital conversion in the ADE7761 is carried out using second-order - ADCs. Figure 17 shows a first-order (for simplicity) - ADC. The converter is made up of two parts: the - modulator and the digital low-pass filter.
AGND
RA* RB* CF V2P RF CT *RB + VR = RF V2N
04407-0-015
NEUTRAL
PHASE
VR*
MCLK ANALOG LOW-PASS FILTER R C ....10100101....
04407-0-019
INTEGRATOR
LATCHED COMPARATOR
DIGITAL LOW-PASS FILTER 1 24
VREF
Figure 14. Typical Connection for Channel 2
REFIN/OUT RC RD VR1
04407-0-016
1-BIT DAC
CF MISCAL RF CF V2N
Figure 17. First-Order - ADC
Figure 15. Typical Connection for MISCAL
A - modulator converts the input signal into a continuous serial stream of 1s and 0s at a rate determined by the sampling clock. In the ADE7761, the sampling clock is equal to CLKIN. The 1-bit DAC in the feedback loop is driven by the serial data stream. The DAC output is subtracted from the input signal. If the loop gain is high enough, the average value of the DAC
Rev. A | Page 14 of 28
ADE7761
output (and, therefore, the bit stream) approaches that of the input signal level. For any given input value in a single sampling interval, the data from the 1-bit ADC is virtually meaningless. Only when a large number of samples are averaged is a meaningful result obtained. This averaging is carried out in the second part of the ADC, the digital low-pass filter. By averaging a large number of bits from the modulator, the low-pass filter can produce 24-bit data words that are proportional to the input signal level. The - converter uses two techniques to achieve high resolution from what is essentially a 1-bit conversion technique. The first is oversampling, which means that the signal is sampled at a rate (frequency) that is many times higher than the bandwidth of interest. For example, the sampling rate in the ADE7761 is CLKIN (450 kHz) and the band of interest is 40 Hz to 1 kHz. Oversampling has the effect of spreading the quantization noise (noise due to sampling) over a wider bandwidth. With the noise spread more thinly over a wider bandwidth, the quantization noise in the band of interest is lowered (see Figure 18). However, oversampling alone is not an efficient enough method to improve the signal-to-noise ratio (SNR) in the band of interest. For example, an oversampling ratio of 4 is required just to increase the SNR by only 6 dB (1 bit). To keep the oversampling ratio at a reasonable level, it is possible to shape the quantization noise so that the majority of the noise lies at the higher frequencies. This is what happens in the - modulator; the noise is shaped by the integrator, which has a high-pass type response for the quantization noise. The result is that most of the noise is at the higher frequencies where it can be removed by the digital low-pass filter. This noise shaping is also shown in Figure 18.
ANTIALIAS FILTER (RC) DIGITAL FILTER SIGNAL SAMPLING FREQUENCY SHAPED NOISE
Antialias Filter
Figure 18 also shows an analog low-pass filter (RC) on input to the modulator. This filter is present to prevent aliasing. Aliasing is an artifact of all sampled systems, which means that frequency components in the input signal to the ADC that are higher than half the sampling rate of the ADC appear in the sampled signal frequency below half the sampling rate. Figure 19 illustrates the effect. In Figure 19, frequency components (arrows shown in black) above half the sampling frequency (also known as the Nyquist frequency), that is, 225 kHz, are imaged or folded back down below 225 kHz (arrows shown in gray). This happens with all ADCs no matter what the architecture. In the example shown, only frequencies near the sampling frequency (450 kHz) move into the band of interest for metering (40 Hz to 1 kHz). This fact allows the use of a very simple low-pass filter to attenuate these frequencies (near 250 kHz) and thereby prevent distortion in the band of interest. A simple RC filter (single pole) with a corner frequency of 10 kHz produces an attenuation of approximately 33 dB at 450 kHz (see Figure 19). This is sufficient to eliminate the effects of aliasing.
ANTIALIASING EFFECTS SAMPLING FREQUENCY
IMAGE FREQUENCIES
0
1kHz
225kHz FREQUENCY (Hz)
450kHz
Figure 19. ADC and Signal Processing in Current Channel or Voltage Channel
ACTIVE POWER CALCULATION
The ADCs digitize the voltage signals from the current and voltage transducers. A high-pass filter in the current channel removes any dc component from the current signal. This eliminates any inaccuracies in the active power calculation due to offsets in the voltage or current signals (see the HPF and Offset Effects section). The active power calculation is derived from the instantaneous power signal. The instantaneous power signal is generated by a direct multiplication of the current and voltage signals. To extract the active power component (dc component), the instantaneous power signal is low-pass filtered. Figure 20 illustrates the instantaneous active power signal and shows how the active power information can be extracted by low-pass filtering the instantaneous power signal. This scheme correctly calculates active power for nonsinusoidal current and voltage waveforms at all power factors. All signal processing is carried out in the digital domain for superior stability over temperature and time.
NOISE
0
1kHz
225kHz FREQUENCY (Hz)
450kHz
SIGNAL
HIGH RESOLUTION OUTPUT FROM DIGITAL LFP
NOISE
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0
1kHz
225kHz FREQUENCY (Hz)
450kHz
Figure 18. Noise Reduction Due to Oversampling and Noise Shaping in the Analog Modulator
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ADE7761
The low frequency output of the ADE7761 is generated by accumulating this active power information. This low frequency inherently means a long accumulation time between output pulses. The output frequency is, therefore, proportional to the average active power. This average active power information can in turn be accumulated (for example, by a counter) to generate active energy information. Because of its high output frequency and therefore shorter integration time, the CF output is proportional to the instantaneous active power. This is useful for system calibration purposes that would take place under steady load conditions.
DIGITAL-TOFREQUENCY F1 F2 HPF MULTIPLIER CH2 ADC LPF DIGITAL-TOFREQUENCY CF
INSTANTANEOUS POWER SIGNAL
INSTANTANEOUS ACTIVE POWER SIGNAL
VxI 2
0V CURRENT VOLTAGE
INSTANTANEOUS POWER SIGNAL
INSTANTANEOUS ACTIVE POWER SIGNAL
VxI 2
x cos(60) 0V
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CH1
ADC
VOLTAGE 60
CURRENT
INSTANTANEOUS POWER SIGNAL -p(t) VxI
INSTANTANEOUS ACTIVE POWER SIGNAL
Figure 21. Active Power Calculation over PF
TIME
Figure 20. Signal Processing Block Diagram
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p(t) = i(t).v(t) WHERE: VxI v(t) = V x cos(t) 2 i(t) = I x cos(t) p(t) = V x I {1 + cos (2t)} 2
Nonsinusoidal Voltage and Current
The active power calculation method also holds true for nonsinusoidal current and voltage waveforms. All voltage and current waveforms in practical applications have some harmonic content. Using the Fourier transform, instantaneous voltage and current waveforms can be expressed in terms of their harmonic content:
Power Factor Considerations
The method used to extract the active power information from the instantaneous power signal (by low-pass filtering) is still valid even when the voltage and current signals are not in phase. Figure 21 displays the unity power factor condition and a displacement power factor (DPF = 0.5), that is, current signal lagging the voltage by 60. If one assumes that the voltage and current waveforms are sinusoidal, the active power component of the instantaneous power signal (dc term) is given by (V x I/2) x cos(60) This is the correct active power calculation.
v(t ) = VO + 2 x Vh x sin(ht + h )
h0
(1)
where: v(t) is the instantaneous voltage. VO is the average value. Vh is the rms value of voltage harmonic h. h is the phase angle of the voltage harmonic. i(t ) = I O + 2 x I h x sin(ht + h )
h0
(2)
where: i(t) is the instantaneous current. IO is the dc component. Ih is the rms value of current harmonic h. h is the phase angle of the current harmonic.
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ADE7761
Using Equations 1 and 2, the active power P can be expressed in terms of its fundamental active power (P1) and harmonic active power (PH):
V1 x I1 DC COMPONENT (INCLUDING ERROR TERM) IS EXTRACTED BY THE LPF FOR ACTIVE POWER CALCULATION 2
P = P1 + PH
where:
P1 = V1 x I 1 cos( 1 ) 1 = 1 - 1
V1 x I 0
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(3)
0v
V0 x I 1 2v FREQUENCY (RAD/S)
and
PH = Vh x I h x cos( h )
h=2
Figure 22. Effect of Channel Offsets on the Active Power Calculation
0.30 0.25 0.20
(4)
h = h - h As can be seen from Equation 4, a harmonic active power component is generated for every harmonic, provided that harmonic is present in both the voltage and current waveforms. The power factor calculation has previously been shown to be accurate in the case of a pure sinusoid; the harmonic active power must, therefore, also correctly account for power factor, because it is made up of a series of pure sinusoids. Note that the input bandwidth of the analog inputs is 7 kHz with the internal oscillator frequency of 450 kHz.
PHASE (Degrees)
0.15 0.10 0.05 0
-0.05 -0.10
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0
100
200
300
HPF and Offset Effects
Equation 5 shows the effect of offsets on the active power calculation. Figure 22 shows the effect of offsets on the active power calculation in the frequency domain.
400 500 600 700 FREQUENCY (Hz)
800
900
1000
Figure 23. Phase Error between Channels (0 Hz to 1 kHz)
0.30 0.25 0.20
V (t ) x I (t ) = V1 x I 1 2
PHASE (Degrees)
(V0 + V1 x cos(t )) x (I 0 + I 1 x cos(t )) = V0 x I 1 + + V0 x I 1 x cos(t ) + V1 x I 0 x cos(t )
(5)
0.15 0.10 0.05 0
As can be seen from Equation 5 and Figure 22, an offset on Channel 1 and Channel 2 contributes a dc component after multiplication. Because this dc component is extracted by the LPF and used to generate the active power information, the offsets contribute a constant error to the active power calculation. This problem is easily avoided in the ADE7761 with the HPF in Channel 1. By removing the offset from at least one channel, no error component can be generated at dc by the multiplication. Error terms at cos(t) are removed by the LPF and the digital-to-frequency conversion (see the Digital-toFrequency Conversion section). The HPF in Channel 1 has an associated phase response that is compensated for on-chip. Figure 23 and Figure 24 show the phase error between channels with the compensation network activated. The ADE7761 is phase compensated up to 1 kHz as shown, which ensures correct active harmonic power calculation even at low power factors.
-0.05 -0.10 40 45 50 55 60 FREQUENCY (Hz) 65 70
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Figure 24. Phase Error between Channels (40 Hz to 70 Hz)
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ADE7761
DIGITAL-TO-FREQUENCY CONVERSION
As previously described, the digital output of the low-pass filter after multiplication contains the active power information. However, because this LPF is not an ideal "brick wall" filter implementation, the output signal also contains attenuated components at the line frequency and its harmonics, that is, cos(ht), where h = 1, 2, 3, ..., and so on. The magnitude response of the filter is given by
H( f ) = 1 1 = ( f / 4.5 Hz) 2 (6) The output frequency on CF can be up to 2048 times higher than the frequency on F1 and F2. This higher output frequency is generated by accumulating the instantaneous active power signal over a much shorter time while converting it to a frequency. This shorter accumulation period means less averaging of the cos(2t) component. As a consequence, some of this instantaneous power signal passes through the digital-tofrequency conversion. This is not a problem in the application. Where CF is used for calibration purposes, the frequency should be averaged by the frequency counter, which removes any ripple. If CF is being used to measure energy, such as in a microprocessor-based application, the CF output should also be averaged to calculate power. Because the outputs F1 and F2 operate at a much lower frequency, a lot more averaging of the instantaneous active power signal is carried out. The result is a greatly attenuated sinusoidal content and a virtually ripple-free frequency output.
For a line frequency of 50 Hz, this gives an attenuation of the 2 (100 Hz) component of approximately -26.9 dB. The dominating harmonic is at twice the line frequency, cos(2t), due to the instantaneous power signal. Figure 25 shows the instantaneous active power signal output of the LPF, which still contains a significant amount of instantaneous power information, cos(2t). This signal is then passed to the digital-to-frequency converter, where it is integrated (accumulated) over time to produce an output frequency. This accumulation of the signal suppresses or averages out any nondc components in the instantaneous active power signal. The average value of a sinusoidal signal is zero. Therefore, the frequency generated by the ADE7761 is proportional to the average active power. Figure 25 also shows the digital-to-frequency conversion for steady load conditions: constant voltage and current. As can be seen in Figure 25, the frequency output CF varies over time, even under steady load conditions. This frequency variation is primarily due to the cos(2t) component in the instantaneous active power signal.
F1
FREQUENCY
TRANSFER FUNCTION
Frequency Outputs F1 and F2
The ADE7761 calculates the product of two voltage signals (on Channel 1 and Channel 2) and then low-pass filters this product to extract active power information. This active power information is then converted to a frequency. The frequency information is output on F1 and F2 in the form of active high pulses. The pulse rate at these outputs is relatively low, for example, 0.34 Hz maximum for ac signals with S0 = S1 = 0 (see Table 7). This means that the frequency at these outputs is generated from active power information accumulated over a relatively long period of time. The result is an output frequency that is proportional to the average active power. The averaging of the active power signal is implicit to the digital-to-frequency conversion. The output frequency or pulse rate is related to the input voltage signals by the following equation:
F1 - F2 Frequency = 5.70 x V 1rms x V 2 rms x F1- 4 V REF 2
(7)
V MULTIPLIER I LPF TO EXTRACT ACTIVE POWER (DC TERM) LPF
DIGITAL-TOFREQUENCY F1 F2 DIGITAL-TOFREQUENCY CF
where:
TIME
FOUT
FREQUENCY
F1 - F2 Frequency is the output frequency on F1 and F2 (Hz). V1rms is the differential rms voltage signal on Channel 1 (V). V2rms is the differential rms voltage signal on Channel 2 (V). VREF is the reference voltage (2.5 V 8%) (V). F1-4 is one of four possible frequencies selected by using the logic inputs S0 and S1 (see Table 5).
TIME 0 2 FREQUENCY (Rad/s)
INSTANTANEOUS ACTIVE POWER SIGNAL (FREQUENCY DOMAIN)
Figure 25. Active Power to Frequency Conversion
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ADE7761
Table 5. F1-4 Frequency Selection
S1 0 0 1 1
1 2
S0 0 1 0 1
F1-4 (Hz)1 1.72 3.44 6.86 13.7
OSC/CLKIN2 OSC/218 OSC/217 OSC/216 OSC/215
Note that if the on-chip reference is used, actual output frequencies may vary from device to device due to a reference tolerance of 8%. 2 x 2 x 2.5 2 CF Frequency = F1 - F2 x 64 = 22.0 Hz F1 - F2 Frequency = 5.70 x 0.66 x 0.66 x 1.72 Hz
= 0.34 Hz
Values are generated using the nominal frequency of 450 kHz. F1-4 are a binary fraction of the master clock and, therefore, vary with the internal oscillator frequency (OSC).
Frequency Output CF
The pulse output calibration frequency (CF) is intended for use during calibration. The output pulse rate on CF can be up to 2048 times the pulse rate on F1 and F2. The lower the F1-4 frequency selected, the higher the CF scaling. Table 6 shows how the two frequencies are related, depending on the states of the logic inputs S0, S1, and SCF. Because of its relatively high pulse rate, the frequency at this logic output is proportional to the instantaneous active power. As with F1 and F2, the frequency is derived from the output of the low-pass filter after multiplication. However, because the output frequency is high, this active power information is accumulated over a much shorter time. Therefore, less averaging is carried out in the digital-to-frequency conversion. With much less averaging of the active power signal, the CF output is much more responsive to power fluctuations (see Figure 20).
Table 6. Relationship between CF and F1, F2 Frequency Outputs
SCF 1 0 1 0 1 0 1 0 S1 0 0 0 0 1 1 1 1 S0 0 0 1 1 0 0 1 1 F1-4 (Hz) 1.72 1.72 3.44 3.44 6.86 6.86 13.7 13.7 CF Frequency Output 128 x F1, F2 64 x F1, F2 64 x F1, F2 32 x F1, F2 32 x F1, F2 16 x F1, F2 16 x F1, F2 2048 x F1, F2
As can be seen from these two example calculations, the maximum output frequency for ac inputs is always half of that for dc input signals. Table 7 shows a complete listing of all maximum output frequencies for ac signals.
Table 7. Maximum Output Frequency on CF, F1, and F2 for AC Inputs
SCF 1 0 1 0 1 0 1 0 S1 0 0 0 0 1 1 1 1 S0 0 0 1 1 0 0 1 1 F1, F2 Maximum Frequency (Hz) 0.34 0.34 0.68 0.68 1.36 1.36 2.72 2.72 CF Maximum Frequency (Hz) 43.52 21.76 43.52 21.76 43.52 21.76 43.52 5570 CF to F1 Ratio 128 64 64 32 32 16 16 2048
FAULT DETECTION
The ADE7761 incorporates a novel fault detection scheme that warns of fault conditions and allows the ADE7761 to continue accurate billing during a fault event. The ADE7761 does this by continuously monitoring both the phase and neutral (return) currents. A fault is indicated when these currents differ by more than 6.25%. However, even during a fault, the output pulse rate on F1 and F2 is generated using the larger of the two currents. Because the ADE7761 looks for a difference between the voltage signals on V1A and V1B, it is important that both current transducers be closely matched. On power-up, the output pulse rate of the ADE7761 is proportional to the product of the voltage signals on V1A and Channel 2. If there is a difference of greater than 6.25% between V1A and V1B on power-up, the fault indicator (FAULT) becomes active after about 1 s. In addition, if V1B is greater than V1A, the ADE7761 selects V1B as the input. The fault detection is automatically disabled when the voltage signal on Channel 1 is less than 0.3% of the full-scale input range. This eliminates false detection of a fault due to noise at light loads.
Example
In this example, if ac voltages of 660 mV peak are applied to V1 and V2, then the expected output frequency on CF, F1, and F2 is calculated as follows: F1-4 = 1.7 Hz, SCF = S1 = S0 = 0 V1rms = rms of 660 mV peak ac = 0.66/2 V V2rms = rms of 660 mV peak ac = 0.66/2 V VREF = 2.5 V (nominal reference value)
Rev. A | Page 19 of 28
ADE7761
Fault with Active Input Greater than Inactive Input
If V1A is the active current input (that is, being used for billing), and the voltage signal on V1B (inactive input) falls below 93.75% of V1A, the fault indicator becomes active. Both analog inputs are filtered and averaged to prevent false triggering of this logic output. As a consequence of the filtering, there is a time delay of approximately 3 s on the logic output FAULT after the fault event. The FAULT logic output is independent of any activity on outputs F1 or F2. Figure 26 shows one condition under which FAULT becomes active. Because V1A is the active input and it is still greater than V1B, billing is maintained on V1A, that is, no swap to the V1B input occurs. V1A remains the active input.
V1A V1B V1A 0V V1N AGND V1B < 93.75% OF V1A FAULT V1B V1B B V1A A FAULT FILTER AND COMPARE TO MULTIPLIER
Calibration Concerns
Typically, when a meter is being calibrated, the voltage and current circuits are separated as shown in Figure 28. This means that current passes through only the phase or neutral circuit. Figure 28 shows current being passed through the phase circuit. This is the preferred option, because the ADE7761 starts billing on the input V1A on power-up. The phase circuit CT is connected to V1A in the diagram. Since there is no current in the neutral circuit, the FAULT indicator comes on under these conditions. However, this does not affect the accuracy of the calibration and can be used as a means to test the functionality of the fault detection.
IB CT RB AGND V1N TEST CURRENT IB RF V1A
V1A
CF
NEUTRAL
RB CT RA*
0V
CF
PHASE
RF CF V2P RF V2N CT *RB + VR = RF
V1B
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<0
>0 6.25% OF ACTIVE INPUT
RB* VR* V 240V RMS
ACTIVE POINT - INACTIVE INPUT
Figure 26. Fault Conditions for Active Input Greater than Inactive Input
Fault with Inactive Input Greater than Active Input
Figure 27 illustrates another fault condition. If the difference between V1B, the inactive input, and V1A, the active input (that is, being used for billing), becomes greater than 6.25% of V1B, the FAULT indicator goes active, and there is also a swap over to the V1B input. The analog input V1B becomes the active input. Again, there is a time constant of about 3 s associated with this swap. V1A does not swap back to being the active channel until V1A is greater than V1B and the difference between V1A and V1B--in this order--becomes greater than 6.25% of V1A. However, the FAULT indicator becomes inactive as soon as V1A is within 6.25% of V1B. This threshold eliminates potential chatter between V1A and V1B.
V1A V1B V1A 0V V1N AGND V1A < 93.75% OF V1B V1B V1B B V1A A FILTER FAULT AND COMPARE TO MULTIPLIER
Figure 28. Fault Conditions for Inactive Input Greater than Active Input
If the neutral circuit is chosen for the current circuit in the arrangement shown in Figure 28, this may have implications for the calibration accuracy. The ADE7761 powers up with the V1A input active as normal. However, because there is no current in the phase circuit, the signal on V1A is zero. This causes a fault to be flagged and the active input to be swapped to V1B (neutral). The meter can be calibrated in this mode, but the phase and neutral CTs might differ slightly. Because under no-fault conditions all billing is carried out using the phase CT, the meter should be calibrated using the phase circuit. Of course, both phase and neutral circuits can be calibrated.
MISSING NEUTRAL MODE
The ADE7761 integrates a novel fault detection that warns and allows the ADE7761 to continue to bill in case a meter is connected to only one wire (see Figure 29). For correct operation of the ADE7761 in this mode, the VDD pin of the ADE7761 must be maintained within the specified range (5 V 5%). The missing neutral detection algorithm is designed to work over a line frequency of 45 Hz to 55 Hz.
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FAULT + SWAP <0 >0
ACTIVE POINT - INACTIVE INPUT
6.25% OF INACTIVE INPUT
Figure 27. Fault Conditions for Inactive Input Greater than Active Input
Rev. A | Page 20 of 28
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ADE7761
IB 240V RMS POWER GENERATOR CT RB V1A RF V1A
Important Note for Billing of Active Energy
The ADE7761 provides pulse outputs--CF, F1, and F2-- intended to be used for the billing of active energy. Pulses are generated at these outputs in two different situations.
Case 1: When the analog input V2P - V2N complies with the conditions described in Figure 32, CF, F1, and F2 frequencies are proportional to active power and can be used to bill active energy. Case 2: When the analog input V2P - V2N does not comply with the conditions described in Figure 32, the ADE7761 does not measure active energy, but a quantity proportional to kAh. This quantity is used to generate pulses on the same CF, F1, and F2. This situation is indicated when the FAULT pin is high.
CF
V1N 0V RB CT LOAD RA* RB* VR* RF CT *RB + VR = RF CF V2P V2N
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CF
RF
V1B
Figure 29. Missing Neutral System Diagram
Analog Devices Inc. cautions users of the ADE7761:
*
The ADE7761 detects a missing neutral condition by continuously monitoring the voltage channel input (V2P-V2N). The FAULT pin is held high when a missing neutral condition is detected. In this mode, the ADE7761 continues to bill the energy based on the signal level on the current channel (see Figure 30). The billing rate or frequency outputs can be adjusted by changing the dc level on the MISCAL pin.
V1A V1N V1B ADC B>A B <> A ADC A>B HPF
ZERO CROSSING DETECTION
Billing active energy in Case 1 is consistent with the understanding of the quantity represented by pulses on CF, F1, and F2 outputs (watt-hour). Billing active energy while the ADE7761 is in Case 2 must be decided knowing that the entity measured by the ADE7761 in this case is ampere-hour and not watt-hour. Users should be aware of this limitation and decide if the ADE7761 is appropriate for their application.
*
Missing Neutral Detection
LPF
MISCAL
ADC
MISSING NEUTRAL GAIN ADJUSMENT
DIGITALTOFREQUENCY CONVERTERS
04407-0-032
The ADE7761 continuously monitors the voltage input and detects a missing neutral condition when the voltage input peak value is smaller than 9% of the analog full scale or when no zero crossings are detected on this input (see Figure 31).
V2P V2 V2N ADC FILTER AND THRESHOLD MISSING NEUTRAL
CF F1 F2
Figure 30. Energy Calculation in Missing Neutral Mode
AGND
|V2|PEAK < 9% OF FULL SCALE V2P-V2N FS 9% OF FS 0V
OR V2P-V2N FS
NO ZERO-CROSSING ON V2 V2P-V2N FS
0V
0V
04407-0-033
Figure 31. Missing Neutral Detection
Rev. A | Page 21 of 28
ADE7761
The ADE7761 leaves the missing neutral mode for normal operation when both conditions are no longer valid--voltage peak value greater than 9% of full scale and zero crossing on the voltage channel detected (see Figure 32).
V2P V2 V2N ADC FILTER AND THRESHOLD MISSING NEUTRAL
where: F1, F2 Frequency is the output frequency on F1 and F2 (Hz). V1rms is the differential rms voltage signal on Channel 1 (V). MISCALrms is the differential rms voltage signal on the MISCAL pin (V). VREF is the reference voltage (2.5 V 8%) (V). F1-4 is one of four possible frequencies selected by using the logic inputs S0 and S1 (see Table 5).
AGND
|V2|PEAK > 9% OF FULL SCALE AND ZERO-CROSSING ON V2 V2P-V2N FS +9% OF FS
04407-0-034
Example
In normal mode, ac voltages of 330 mV peak are applied to V1 and V2, and then the expected output frequency on F1 and F2 is calculated as follows: F1-4 = 1.7 Hz, SCF = S1 = S0 = 0 V1 = rms of 330 mV peak ac = 0.33/2 V V2 = rms of 330 mV peak ac = 0.33/2 V VREF = 2.5 V (nominal reference value)
= 0.084 Hz 2 x 2 x 2.5 2 CF Frequency = F1 - F2 frequency x 64 = 5.4 Hz
-9% OF FS
Figure 32. Return to Normal Mode after Missing Neutral Detection
Missing Neutral Gain Calibration
When the ADE7761 is in missing neutral mode, the energy is billed based on the active current input signal level. The calibration of the frequency outputs in this mode can be done with the MISCAL analog input pin. In this mode, applying a dc voltage of 330 mV on MISCAL is equivalent to applying, in normal mode, a pure sine wave on the voltage input with a peak value of 330 mV. The MISCAL input can vary from 0 V to 660 mV (see the Analog Inputs section). When set to 0 V, the frequency outputs are close to zero. When set to 660 mV dc, the frequency outputs are twice that when MISCAL is at 330 mV dc. In other words, Equation 7 can be used in missing neutral mode by replacing V2rms by MISCALrms 2: F1 , F2 Frequency = 5.70 x MISCALrms / 2 x F1- 4 V REF 2 (8)
F1 , F2 Frequency =
5.70 x 0.33 x 0.33 x 1.7 Hz
In missing neutral mode, ac voltage of 330 mV peak is applied to V1, no signal is connected on V2, and 330 mV dc input is applied to MISCAL. With the ADE7761 in the same configuration as the previous example, the expected output frequencies on CF, F1, and F2 are
F1 , F2 Frequency =
5.70 x 0.33 x 0.33 / 2 x 1.7 Hz 2 x 2.5 2
= 0.084 Hz
CF Frequency = F1 , F2 frequency x 64 = 5.4 Hz
Rev. A | Page 22 of 28
ADE7761 APPLICATIONS
INTERFACING TO A MICROCONTROLLER FOR ENERGY MEASUREMENT
The easiest way to interface the ADE7761 to a microcontroller is to use the CF high frequency output with the output frequency scaling set to 2048 x F1, F2. This is done by setting SCF = 0 and S0 = S1 = 1 (see Table 7). With full-scale ac signals on the analog inputs, the output frequency on CF is approximately 5.5 kHz. Figure 33 illustrates one scheme that could be used to digitize the output frequency and carry out the necessary averaging mentioned in the previous section.
CF FREQUENCY RIPPLE AVERAGE FREQUENCY
For the purpose of calibration, this integration time could be 10 s to 20 s in order to accumulate enough pulses to ensure correct averaging of the frequency. In normal operation, the integration time could be reduced to 1 s or 2 s depending, for example, on the required update rate of a display. With shorter integration times on the MCU, the amount of energy in each update may still have a small amount of ripple, even under steady load conditions. However, over a minute or more, the measured energy has no ripple.
SELECTING A FREQUENCY FOR AN ENERGY METER APPLICATION
As shown in Table 5, the user can select one of four frequencies. This frequency selection determines the maximum frequency on F1 and F2. These outputs are intended to be used to drive the energy register (electromechanical or other). Because only four different output frequencies can be selected, the available frequency selection has been optimized for a meter constant of 100 impulses/kWh with a maximum current of between 10 A and 120 A. Table 8 shows the output frequency for several maximum currents (IMAX) with a line voltage of 240 V. In all cases, the meter constant is 100 impulses/kWh.
Table 8. F1 and F2 Frequency at 100 Impulses/kWh
IMAX (A) 12.5 25 40 60 80 120 F1 and F2 (Hz) 0.083 0.166 0.266 0.4 0.533 0.8
10%
TIME
MCU
ADE7761
CF REVP*
COUNTER
UP/DOWN
FAULT**
LOGIC
*REVP MUST BE USED IF THE METER IS BIDIRECTIONAL OR DIRECTION OF ENERGY FLOW IS NEEDED. **FAULT MUST BE USED TO RECORD ENERGY IN FAULT CONDITION.
Figure 33. Interfacing the ADE7761 to an MCU
As shown, the frequency output CF is connected to an MCU counter or port, which counts the number of pulses in a given integration time, determined by an MCU internal timer. The average power, proportional to the average frequency, is given by
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Average Frequency = Average Active Power =
Counter Timer
The energy consumed during an integration period is given by
Energy = Average Power x Time =
Counter x Time = Counter Time
The F1-4 frequencies allow complete coverage of this range of output frequencies on F1 and F2. When designing an energy meter, the nominal design voltage on Channel 2 (voltage) should be set to half-scale to allow for calibration of the meter constant. The current channel should also be no more than halfscale when the meter sees maximum load, which accommodates overcurrent signals and signals with high crest factors. Table 9 shows the output frequency on F1 and F2 when both analog inputs are half-scale. The frequencies listed in Table 9 align well with those listed in Table 8 for maximum load.
Rev. A | Page 23 of 28
ADE7761
Table 9. F1 and F2 Frequency with Half-Scale AC Inputs
Frequency on F1 and F2, Ch 1 and Ch 2, Half-Scale AC Inputs (Hz) 0.085 0.17 0.34 0.68
No-Load Threshold
The ADE7761 includes a no-load threshold and startup current feature that eliminates creep effects in the meter. The ADE7761 is designed to issue a minimum output frequency. Any load generating a frequency lower than this minimum frequency does not cause a pulse to be issued on F1, F2, or CF. The minimum output frequency is given as 0.0045% of the full-scale output frequency. (See Table 7 for maximum output frequencies for ac signals). For example, an energy meter with a meter constant of 100 impulses/kWh on F1, F2 using SCF = 1, S1 = 0, and S0 = 1, the maximum output frequency at F1 or F2 is 0.68 Hz and 43.52 Hz on CF. The minimum output frequency at F1 or F2 is 0.0045% of 0.68 Hz or 3.06 x 10-5 Hz. This is 1.96 x 10-3 Hz at CF (64 x F1 Hz). In this example, the no-load threshold is equivalent to 1.1 W of load or a startup current of 4.6 mA at 240 V. Compare this value to the IEC61036 specification, which states that the meter must start up with a load equal to or less than 0.4% IB. For a 5 A (IB) meter, 0.4% of IB is equivalent to 20 mA. Note that the no-load threshold is not enabled when using the high CF frequency mode: SCF = 0, S1 = S0 = 1.
S0 0 0 1 1
S1 0 1 0 1
F1-4 1.72 3.44 6.86 13.5
When selecting a suitable F1-4 frequency for a meter design, the frequency output at IMAX (maximum load) with a meter constant of 100 impulses/kWh should be compared with Column 4 of Table 9. The frequency that is closest in Table 9 determines the best choice of frequency (F1-4). For example, if a meter with a maximum current of 40 A is being designed, the output frequency on F1 and F2 with a meter constant of 100 impulses/kWh is 0.266 Hz at 40 A and 240 V (from Table 8). Looking at Table 9, the closest frequency to 0.266 Hz in Column 4 is 0.17 Hz. Therefore, F2 (3.4 Hz; see Table 5) is selected for this design.
Frequency Outputs
Figure 2 shows a timing diagram for the various frequency outputs. The high frequency CF output is intended to be used for communications and calibration purposes. CF produces a 90 ms wide, active high pulse (t4) at a frequency that is proportional to active power. The CF output frequencies are given in Table 7. As in the case of F1 and F2, if the period of CF (t5) falls below 180 ms, the CF pulse width is set to half the period. For example, if the CF frequency is 20 Hz, the CF pulse width is 25 ms.
NEGATIVE POWER INFORMATION
The ADE7761 detects when the current and voltage channels have a phase shift greater than 90. This mechanism can detect wrong connection of the meter or generation of negative power. The REVP pin output goes active high when negative power is detected, and active low when positive power is detected. The REVP pin output changes state as a pulse is issued on CF.
Rev. A | Page 24 of 28
ADE7761 OUTLINE DIMENSIONS
7.50 7.20 6.90
20
11
1
10
5.60 5.30 5.00 8.20 7.80 7.40
2.00 MAX
1.85 1.75 1.65
0.25 0.09 8 4 0 0.95 0.75 0.55
0.05 MIN COPLANARITY 0.10
0.65 BSC
0.38 0.22
SEATING PLANE
COMPLIANT TO JEDEC STANDARDS MO-150AE
Figure 34. 20-Lead Shrink Small Outline Package [SSOP] (RS-20) Dimensions shown in millimeters
ORDERING GUIDE
Model
ADE7761ARS ADE7761ARSRL ADE7761ARS-REF
Temperature Range
-40C to +85C -40C to +85C
Package Description
Shrink Small Outline Package Shrink Small Outline Package Reference Board
Package Option
RS-20 RS-20
Rev. A | Page 25 of 28
ADE7761 DISCLAIMER
The ADE7761 provides pulse outputs--CF, F1, and F2-- intended to be used for the billing of active energy. Pulses are generated at these outputs in two different situations.
Case 1: When the analog input V2P - V2N complies with the conditions described in Figure 32, CF, F1, and F2 frequencies are proportional to active power and can be used to bill active energy. Case 2: When the analog input V2P - V2N does not comply with the conditions described in Figure 32, the ADE7761 does not measure active energy, but a quantity proportional to kAh. This quantity is used to generate pulses on the same CF, F1, and F2. This situation is indicated when the FAULT pin is high.
Analog Devices Inc. cautions users of the ADE7761:
*
Billing active energy in Case 1 is consistent with the understanding of the quantity represented by pulses on CF, F1, and F2 outputs (watt-hour). Billing active energy while the ADE7761 is in Case 2 must be decided knowing that the entity measured by the ADE7761 in this case is ampere-hour and not watt-hour. Users should be aware of this limitation and decide if the ADE7761 is appropriate for their application.
*
Rev. A | Page 26 of 28
ADE7761 NOTES
Rev. A | Page 27 of 28
ADE7761 NOTES
(c) 2004 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. D04407-0-2/04(A)
Rev. A | Page 28 of 28
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