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LT1920 Single Resistor Gain Programmable, Precision Instrumentation Amplifier FEATURES
s s s s s s s s s s s s s s
DESCRIPTIO
Single Gain Set Resistor: G = 1 to 10,000 Gain Error: G = 10, 0.3% Max Gain Nonlinearity: G = 10, 30ppm Max Input Offset Voltage: G = 10, 225V Max Input Offset Voltage Drift: 1V/C Max Input Bias Current: 2nA Max PSRR at G = 1: 80dB Min CMRR at G = 1: 75dB Min Supply Current: 1.3mA Max Wide Supply Range: 2.3V to 18V 1kHz Voltage Noise: 7.5nV/Hz 0.1Hz to 10Hz Noise: 0.28VP-P Available in 8-Pin PDIP and SO Packages Meets IEC 1000-4-2 Level 4 ESD Tests with Two External 5k Resistors
APPLICATIO S
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Bridge Amplifiers Strain Gauge Amplifiers Thermocouple Amplifiers Differential to Single-Ended Converters Medical Instrumentation
The LT (R)1920 is a low power, precision instrumentation amplifier that requires only one external resistor to set gains of 1 to 10,000. The low voltage noise of 7.5nV/Hz (at 1kHz) is not compromised by low power dissipation (0.9mA typical for 2.3V to 15V supplies). The high accuracy of 30ppm maximum nonlinearity and 0.3% max gain error (G = 10) is not degraded even for load resistors as low as 2k (previous monolithic instrumentation amps used 10k for their nonlinearity specifications). The LT1920 is laser trimmed for very low input offset voltage (125V max), drift (1V/C), high CMRR (75dB, G = 1) and PSRR (80dB, G = 1). Low input bias currents of 2nA max are achieved with the use of superbeta processing. The output can handle capacitive loads up to 1000pF in any gain configuration while the inputs are ESD protected up to 13kV (human body). The LT1920 with two external 5k resistors passes the IEC 1000-4-2 level 4 specification. The LT1920, offered in 8-pin PDIP and SO packages, is a pin for pin and spec for spec improved replacement for the AD620. The LT1920 is the most cost effective solution for precision instrumentation amplifier applications. For even better guaranteed performance, see the LT1167.
, LTC and LT are registered trademarks of Linear Technology Corporation.
TYPICAL APPLICATIO
VS R5 392k 1 LT1634CCZ-1.25 2 2
Single Supply Barometer Gain Nonlinearity
- -
5k R6 1k 5k R1 825 2 6 R4 50k R3 50k 6 R8 100k RSET 5 5k 5k R2 12 8 3 LT1920 G = 60 5 4 TO 4-DIGIT DVM 6 1 2 1
+ -
1/2 LT1490 4
1
4
7
+
1/2 LT1490 7 R7 50k
5
-
VS = 8V TO 30V
+
+
3
NONLINEARITY (100ppm/DIV) OUTPUT VOLTAGE (2V/DIV) G = 1000 RL = 1k VOUT = 10V
3
8
LUCAS NOVA SENOR NPC-1220-015-A-3L
VS
VOLTS 2.800 3.000 3.200
INCHES Hg 28.00 30.00 32.00
1920 TA01
U
1167 TA02
U
U
1
LT1920 ABSOLUTE MAXIMUM RATINGS
(Note 1)
PACKAGE/ORDER INFORMATION
TOP VIEW RG 1 -IN 2 +IN 3 -VS 4 N8 PACKAGE 8-LEAD PDIP S8 PACKAGE 8-LEAD PLASTIC SO TJMAX = 150C, JA = 130C/ W (N8) TJMAX = 150C, JA = 190C/ W (S8) - + 8 7 6 5 RG +VS OUTPUT REF
Supply Voltage ...................................................... 20V Differential Input Voltage (Within the Supply Voltage) ..................................................... 40V Input Voltage (Equal to Supply Voltage) ................ 20V Input Current (Note 3) ........................................ 20mA Output Short-Circuit Duration .......................... Indefinite Operating Temperature Range ................ - 40C to 85C Specified Temperature Range LT1920C (Note 4) .................................... 0C to 70C LT1920I .............................................. - 40C to 85C Storage Temperature Range ................. - 65C to 150C Lead Temperature (Soldering, 10 sec).................. 300C
ORDER PART NUMBER LT1920CN8 LT1920CS8 LT1920IN8 LT1920IS8 S8 PART MARKING 1920 1920I
Consult factory for Military grade parts.
ELECTRICAL CHARACTERISTICS
SYMBOL G PARAMETER Gain Range Gain Error
VS = 15V, VCM = 0V, TA = 25C, R L = 2k, unless otherwise noted.
MIN 1 0.008 0.010 0.025 0.040
q
CONDITIONS (Note 6) G = 1 + (49.4k/RG ) G=1 G = 10 (Note 2) G = 100 (Note 2) G = 1000 (Note 2) G < 1000 (Note 2) VO = 10V, G = 1 VO = 10V, G = 10 and 100 VO = 10V, G = 100 and 1000 VOST = VOSI + VOSO/G G = 1000, VS = 5V to 15V G = 1000, VS = 5V to 15V (Note 3) G = 1, VS = 5V to 15V G = 1, VS = 5V to 15V (Note 3)
TYP
MAX 10k 0.1 0.3 0.3 0.35 50 30
UNITS % % % % ppm/C ppm ppm ppm V V V/C V V V/C nA nA VP-P VP-P VP-P nV/Hz nV/Hz pAP-P fA/Hz G pF
G/T
Gain vs Temperature Gain Nonlinearity (Note 5)
20 10 10 20 30
VOST VOSI VOSI/T VOSO VOSO /T IOS IB en
Total Input Referred Offset Voltage Input Offset Voltage Input Offset Drift (RTI) Output Offset Voltage Output Offset Drift Input Offset Current Input Bias Current Input Noise Voltage, RTI
q q
125 185 1 1000 1500 15 1 2
400
q q
5 0.3 0.5
0.1Hz to 10Hz, G = 1 0.1Hz to 10Hz, G = 10 0.1Hz to 10Hz, G = 100 and 1000 fO = 1kHz fO = 1kHz fO = 0.1Hz to 10Hz fO = 10Hz VIN = 10V fO = 100kHz
2.00 0.50 0.28 7.5 67 10 124 200 1.6
Total RTI Noise = eni 2 + (eno /G)2 eni eno in RIN CIN(DIFF) Input Noise Voltage Density, RTI Output Noise Voltage Density, RTI Input Noise Current Input Noise Current Density Input Resistance Differential Input Capacitance
2
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W
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WW
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LT1920 ELECTRICAL CHARACTERISTICS
SYMBOL CIN(CM) VCM PARAMETER Common Mode Input Capacitance Input Voltage Range fO = 100kHz G = 1, Other Input Grounded VS = 2.3V to 5V VS = 5V to 18V VS = 2.3V to 5V VS = 5V to 18V 1k Source Imbalance, VCM = 0V to 10V G=1 G = 10 G = 100 G = 1000 VS = 2.3 to 18V G=1 G = 10 G = 100 G = 1000 VS = 2.3V to 18V RL = 10k VS = 2.3V to 5V VS = 5V to 18V VS = 2.3V to 5V VS = 5V to 18V G=1 G = 10 G = 100 G = 1000 G = 1, VOUT = 10V 10V Step G = 1 to 100 G = 1000 VREF = 0V - VS + 1.6 1 0.0001 Note 5: This parameter is measured in a high speed automatic tester that does not measure the thermal effects with longer time constants. The magnitude of these thermal effects are dependent on the package used, heat sinking and air flow conditions. Note 6: Typical parameters are defined as the 60% of the yield parameter distribution. -VS + 1.1 -VS + 1.2 -VS + 1.4 -VS + 1.6 20 27 1000 800 120 12 1.2 14 130 20 50 +VS - 1.6 -VS + 1.9 -VS + 1.9 -VS + 2.1 -VS + 2.1
VS = 15V, V CM = 0V, TA = 25C, R L = 2k, unless otherwise noted.
MIN TYP 1.6 +VS - 1.2 +VS - 1.4 +VS - 1.3 +VS - 1.4 MAX UNITS pF V V V V
CONDITIONS (Note 6)
q q
CMRR
Common Mode Rejection Ratio
75 95 110 110 80 100 120 120
95 115 125 140 120 135 140 150 0.9 1.3 +VS - 1.2 +VS - 1.3 +VS - 1.3 +VS - 1.5
dB dB dB dB dB dB dB dB mA V V V V mA kHz kHz kHz kHz V/s s s k A V
PSRR
Power Supply Rejection Ratio
IS VOUT
Supply Current Output Voltage Swing
q q
IOUT BW
Output Current Bandwidth
SR
Slew Rate Settling Time to 0.01%
RREFIN IREFIN VREF AVREF
Reference Input Resistance Reference Input Current Reference Voltage Range Reference Gain to Output
The q denotes specifications that apply over the full specified temperature range. Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired. Note 2: Does not include the effect of the external gain resistor RG. Note 3: This parameter is not 100% tested. Note 4: The LT1920C is designed, characterized and expected to meet the industrial temperature limits, but is not tested at - 40C and 85C. I-grade parts are guaranteed.
3
LT1920 TYPICAL PERFOR A CE CHARACTERISTICS
Gain Nonlinearity, G = 1
NONLINEARITY (10ppm/DIV) NONLINEARITY (1ppm/DIV)
NONLINEARITY (10ppm/DIV)
G=1 OUTPUT VOLTAGE (2V/DIV) RL = 2k VOUT = 10V
Gain Nonlinearity, G = 1000
0.20 NONLINEARITY (100ppm/DIV) 0.15 0.10 GAIN ERROR (%) 0.05
CHANGE IN OFFSET VOLTAGE (V)
G = 1000 OUTPUT VOLTAGE (2V/DIV) RL = 2k VOUT = 10V
Input Bias Current vs Common Mode Input Voltage
500 400 INPUT BIAS CURRENT (pA) 300 200 100 0 -100 - 200 - 300 - 400 - 500 -15 -12 - 9 - 6 - 3 0 3 6 9 12 15 COMMON MODE INPUT VOLTAGE (V)
1920 G13
160 COMMON MODE REJECTION RATIO (dB) 140 120 100 80 60 40 20 0 0.1 1 10 1k 100 FREQUENCY (Hz) G = 1000 G = 100 G = 10 G=1
NEGATIVE POWER SUPPLY REJECTION RATIO (dB)
85C 0C - 40C
70C
25C
4
UW
1167 G01 1167 G04
Gain Nonlinearity, G = 10
Gain Nonlinearity, G = 100
OUTPUT VOLTAGE (2V/DIV) G = 10 RL = 2k V OUT = 10V
1167 G02
G = 100 OUTPUT VOLTAGE (2V/DIV) RL = 2k VOUT = 10V
1167 G03
Gain Error vs Temperature
14 12 10 8
Warm-Up Drift
VS = 15V TA = 25C G=1
S8
G=1 0 - 0.05 - 0.10 - 0.15 VS = 15V G = 10* VOUT = 10V RL = 2k G = 100* *DOES NOT INCLUDE G = 1000* TEMPERATURE EFFECTS OF R G - 25 0 25 50 TEMPERATURE (C) 75 100
1920 G06
N8 6 4 2 0 0 1 2 3 4 TIME AFTER POWER ON (MINUTES) 5
- 0.20 - 50
1920 G09
Common Mode Rejection Ratio vs Frequency
VS = 15V TA = 25C 1k SOURCE IMBALANCE 160 140 120 100 80 60 40 20
Negative Power Supply Rejection Ratio vs Frequency
G = 100 G = 10 G=1 G = 1000 V + = 15V TA = 25C
10k
100k
1920 G14
0 0.1
1
10
1k 100 FREQUENCY (Hz)
10k
100k
1920 G15
LT1920 TYPICAL PERFOR A CE CHARACTERISTICS
Positive Power Supply Rejection Ratio vs Frequency
POSITIVE POWER SUPPLY REJECTION RATIO (dB) 160 140 120 100 80 60 40 20 0 0.1 1 10 1k 100 FREQUENCY (Hz) 10k 100k
1920 G16
V - = - 15V TA = 25C G = 10 G = 100 G=1 GAIN (dB) G = 1000
SUPPLY CURRENT (mA)
Voltage Noise Density vs Frequency
1000 VOLTAGE NOISE DENSITY (nVHz) VS = 15V TA = 25C 1/fCORNER = 10Hz GAIN = 1 1/fCORNER = 9Hz GAIN = 10 10 1/fCORNER = 7Hz GAIN = 100, 1000 NOISE VOLTAGE (2V/DIV)
100
BW LIMIT GAIN = 1000 0 1 10 100 1k FREQUENCY (Hz) 10k 100k
1920 G19
NOISE VOLTAGE (0.2V/DIV)
Current Noise Density vs Frequency
1000 CURRENT NOISE DENSITY (fA/Hz) VS = 15V TA = 25C CURRENT NOISE (5pA/DIV)
OUTPUT CURRENT (mA) (SINK) (SOURCE)
100 RS
10 1 10 100 FREQUENCY (Hz) 1000
1920 G22
UW
Gain vs Frequency
60 50 40 30 20 10 0 -10 G=1 VS = 15V TA = 25C 0.1 1 10 FREQUENCY (kHz) 100 1000
1920 G17
Supply Current vs Supply Voltage
1.50
G = 1000 G = 100
1.25 85C 1.00 25C - 40C 0.75
G = 10
- 20 0.01
0.50
0
10 15 5 SUPPLY VOLTAGE ( V)
20
1920 G18
0.1Hz to 10Hz Noise Voltage, G=1
VS = 15V TA = 25C
0.1Hz to 10Hz Noise Voltage, RTI G = 1000
VS = 15V TA = 25C
0
1
2
3
456 TIME (SEC)
7
8
9
10
0
1
2
3
456 TIME (SEC)
7
8
9
10
1920 G20
1920 G21
0.1Hz to 10Hz Current Noise
VS = 15V TA = 25C 50 40 30 20 10 0 - 10 - 20 - 30 - 40 - 50 0 1 2 3 456 TIME (SEC) 7 8 9 10
Short-Circuit Current vs Time
VS = 15V TA = - 40C TA = 25C TA = 85C
TA = 85C TA = - 40C TA = 25C
2 1 0 3 TIME FROM OUTPUT SHORT TO GROUND (MINUTES)
1920 G24
1920 G23
5
LT1920 TYPICAL PERFOR A CE CHARACTERISTICS
Large-Signal Transient Response Small-Signal Transient Response
100 90 80 20mV/DIV OVERSHOOT (%) 70 60 50 40 30 20 10 0 10 AV = 10 AV 100 100 1000 CAPACITIVE LOAD (pF) 10000
1920 G25
5V/DIV
G=1 VS = 15V RL = 2k CL = 60pF
10s/DIV
Large-Signal Transient Response
OUTPUT IMPEDANCE ()
20mV/DIV
5V/DIV
G = 10 VS = 15V RL = 2k CL = 60pF
10s/DIV
Large-Signal Transient Response
PEAK-TO-PEAK OUTPUT SWING (V)
20mV/DIV
5V/DIV
G = 100 VS = 15V RL = 2k CL = 60pF
10s/DIV
6
UW
1167 G28 1167 G31 1167 G34
Overshoot vs Capacitive Load
VS = 15V VOUT = 50mV RL =
AV = 1
G=1 V S = 15V R L = 2k C L = 60pF
10s/DIV
1167 G29
Small-Signal Transient Response
1000
Output Impedance vs Frequency
VS = 15V TA = 25C G = 1 TO 1000
100
10
1
G = 10 VS = 15V RL = 2k CL = 60pF
10s/DIV
1167 G32
0.1 1 10 100 FREQUENCY (kHz) 1000
1920 G26
Small-Signal Transient Response
Undistorted Output Swing vs Frequency
35 30 G = 10, 100, 1000 G=1 25 20 15 10 5 0 1 10 100 FREQUENCY (kHz) 1000
1920 G27
VS = 15V TA = 25C
G = 100 VS = 15V RL = 2k CL = 60pF
10s/DIV
1167 G35
LT1920 TYPICAL PERFOR A CE CHARACTERISTICS
Large-Signal Transient Response Small-Signal Transient Response
1000
SETTLING TIME (s)
20mV/DIV
5V/DIV
G = 1000 V S = 15V RL = 2k C L = 60pF
50s/DIV
Settling Time vs Step Size
10 8 6 OUTPUT STEP (V) 4 2 0 -2 -4 -6 -8 -10 2 3 4 TO 0.1% 5 6 7 8 9 10 11 12 SETTLING TIME (s)
1920 G33
0V 0V
VOUT
SLEW RATE (V/s)
VS = 15 G=1 TA = 25C CL = 30pF RL = 1k
OUTPUT VOLTAGE SWING (V) (REFERRED TO SUPPLY VOLTAGE)
UW
1167 G37
Settling Time vs Gain
VS = 15V TA = 25C VOUT = 10V 1mV = 0.01%
100
10
G = 1000 VS = 15V RL = 2k CL = 60pF
50s/DIV
1167 G38
1 1 10 GAIN (dB)
1920 G30
100
1000
Slew Rate vs Temperature
1.8 VS = 15V VOUT = 10V G=1
TO 0.1% 1.6 TO 0.01% 1.4
+ SLEW 1.2 - SLEW 1.0
VOUT TO 0.01%
0.8 - 50 -25
50 0 25 75 TEMPERATURE (C)
100
125
1920 G36
Output Voltage Swing vs Load Current
+ VS + VS - 0.5 + VS - 1.0 + VS - 1.5 + VS - 2.0 - VS + 2.0 - VS + 1.5 - VS + 1.0 - VS + 0.5 - VS 0.01 0.1 1 10 OUTPUT CURRENT (mA) 100
1920 G39
VS = 15V
85C 25C - 40C SOURCE
SINK
7
LT1920 BLOCK DIAGRAM W
V+ VB
+
A1 R3 400 -IN 2 Q1
R5 10k
R6 10k 6 OUTPUT
-
C1 R1 24.7k
V-
RG 1 RG 8 V+ VB
+
A2
R7 10k
R4 400 +IN 3 Q2
-
C2 V- R2 24.7k 7 4 PREAMP STAGE DIFFERENCE AMPLIFIER STAGE V+ V-
V-
Figure 1. Block Diagram
THEORY OF OPERATIO
The LT1920 is a modified version of the three op amp instrumentation amplifier. Laser trimming and monolithic construction allow tight matching and tracking of circuit parameters over the specified temperature range. Refer to the block diagram (Figure 1) to understand the following circuit description. The collector currents in Q1 and Q2 are trimmed to minimize offset voltage drift, thus assuring a high level of performance. R1 and R2 are trimmed to an absolute value of 24.7k to assure that the gain can be set accurately (0.3% at G = 100) with only one external resistor RG. The value of RG in parallel with R1 (R2) determines the transconductance of the preamp stage. As RG is reduced for larger programmed gains, the transconductance of the input preamp stage increases to that of the input transistors Q1 and Q2. This increases the open-loop gain when the programmed gain is increased, reducing the input referred gain related errors and noise. The input voltage noise at gains greater than 50 is determined only by Q1 and Q2. At lower gains the noise of the difference amplifier and preamp gain setting resistors increase the noise. The gain bandwidth product is determined by C1, C2 and the preamp transconductance which increases
with programmed gain. Therefore, the bandwidth does not drop proportional to gain. The input transistors Q1 and Q2 offer excellent matching, which is inherent in NPN bipolar transistors, as well as picoampere input bias current due to superbeta processing. The collector currents in Q1 and Q2 are held constant due to the feedback through the Q1-A1-R1 loop and Q2-A2-R2 loop which in turn impresses the differential input voltage across the external gain set resistor RG. Since the current that flows through RG also flows through R1 and R2, the ratios provide a gained-up differential voltage,G = (R1 + R2)/RG, to the unity-gain difference amplifier A3. The common mode voltage is removed by A3, resulting in a single-ended output voltage referenced to the voltage on the REF pin. The resulting gain equation is: VOUT - VREF = G(VIN+ - VIN-) where: G = (49.4k / RG) + 1 solving for the gain set resistor gives: RG = 49.4k /(G - 1)
8
+
V- R8 10k 5 REF
1920 F01
-
A3
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LT1920 THEORY OF OPERATIO
Input and Output Offset Voltage The offset voltage of the LT1920 has two components: the output offset and the input offset. The total offset voltage referred to the input (RTI) is found by dividing the output offset by the programmed gain (G) and adding it to the input offset. At high gains the input offset voltage dominates, whereas at low gains the output offset voltage dominates. The total offset voltage is: Total input offset voltage (RTI) = input offset + (output offset/G) Total output offset voltage (RTO) = (input offset * G) + output offset Reference Terminal The reference terminal is one end of one of the four 10k resistors around the difference amplifier. The output voltage of the LT1920 (Pin 6) is referenced to the voltage on the reference terminal (Pin 5). Resistance in series with the REF pin must be minimized for best common mode rejection. For example, a 2 resistance from the REF pin to ground will not only increase the gain error by 0.02% but will lower the CMRR to 80dB. Single Supply Operation For single supply operation, the REF pin can be at the same potential as the negative supply (Pin 4) provided the output of the instrumentation amplifier remains inside the specified operating range and that one of the inputs is at least 2.5V above ground. The barometer application on the front page of this data sheet is an example that satisfies these conditions. The resistance RSET from the bridge transducer to ground sets the operating current for the bridge and also has the effect of raising the input common mode voltage. The output of the LT1920 is always inside the specified range since the barometric pressure rarely goes low enough to cause the output to rail (30.00 inches of Hg corresponds to 3.000V). For applications that require the output to swing at or below the REF potential, the voltage on the REF pin can be level shifted. An op amp is used to buffer the voltage on the REF pin since a parasitic series resistance will degrade the CMRR. The application in the back of this data sheet, Four Digit Pressure Sensor, is an example.
+IN
-IN
1
RG 8
1 10mV ADJUSTMENT RANGE
1/2 LT1112
Figure 2. Optional Trimming of Output Offset Voltage
Input Bias Current Return Path The low input bias current of the LT1920 (2nA) and the high input impedance (200G) allow the use of high impedance sources without introducing additional offset voltage errors, even when the full common mode range is required. However, a path must be provided for the input bias currents of both inputs when a purely differential signal is being amplified. Without this path the inputs will float to either rail and exceed the input common mode range of the LT1920, resulting in a saturated input stage. Figure 3 shows three examples of an input bias current path. The first example is of a purely differential signal source with a 10k input current path to ground. Since the impedance of the signal source is low, only one resistor is needed. Two matching resistors are needed for higher impedance signal sources as shown in the second example. Balancing the input impedance improves both common mode rejection and DC offset. The need for input resistors is eliminated if a center tap is present as shown in the third example.
-
+
+
3
-
U
Output Offset Trimming The LT1920 is laser trimmed for low offset voltage so that no external offset trimming is required for most applications. In the event that the offset needs to be adjusted, the circuit in Figure 2 is an example of an optional offset adjust circuit. The op amp buffer provides a low impedance to the REF pin where resistance must be kept to minimum for best CMRR and lowest gain error.
2 LT1920 REF 5 2 10mV 100 3 10k 100 -10mV 6 OUTPUT V+ V-
1920 F02
9
LT1920 THEORY OF OPERATIO
-
THERMOCOUPLE RG LT1920 MICROPHONE, HYDROPHONE, ETC RG 10k
LT1920
RG
200k
200k CENTER-TAP PROVIDES BIAS CURRENT RETURN
1920 F03
Figure 3. Providing an Input Common Mode Current Path
APPLICATIONS INFORMATION
The LT1920 is a low power precision instrumentation amplifier that requires only one external resistor to accurately set the gain anywhere from 1 to 1000. The output can handle capacitive loads up to 1000pF in any gain configuration and the inputs are protected against ESD strikes up to 13kV (human body). Input Protection The LT1920 can safely handle up to 20mA of input current in an overload condition. Adding an external 5k input resistor in series with each input allows DC input fault voltages up to 100V and improves the ESD immunity to 8kV (contact) and 15kV (air discharge), which is the IEC 1000-4-2 level 4 specification. If lower value input resistors are needed, a clamp diode from the positive supply to each input will maintain the IEC 1000-4-2 specification to level 4 for both air and contact discharge. A 2N4393 drain/source to gate is a good low leakage diode for use with 1k resistors, see Figure 4. The input resistors should be carbon and not metal film or carbon film. RFI Reduction In many industrial and data acquisition applications, instrumentation amplifiers are used to accurately amplify small signals in the presence of large common mode voltages or high levels of noise. Typically, the sources of these very small signals (on the order of microvolts or millivolts) are sensors that can be a significant distance from the signal conditioning circuit. Although these senVCC J1 2N4393 RIN VCC J2 2N4393 OPTIONAL FOR HIGHEST ESD PROTECTION VCC
RG RIN
Figure 4. Input Protection
sors may be connected to signal conditioning circuitry, using shielded or unshielded twisted-pair cabling, the cabling may act as antennae, conveying very high frequency interference directly into the input stage of the LT1920. The amplitude and frequency of the interference can have an adverse effect on an instrumentation amplifier's input stage by causing an unwanted DC shift in the amplifier's input offset voltage. This well known effect is called RFI rectification and is produced when out-of-band interference is coupled (inductively, capacitively or via radiation) and rectified by the instrumentation amplifier's input transistors. These transistors act as high frequency signal detectors, in the same way diodes were used as RF envelope detectors in early radio designs. Regardless of the type of interference or the method by which it is coupled into the circuit, an out-of-band error signal appears in series with the instrumentation amplifier's inputs.
10
-
LT1920
+
+
-
- +
U
U
LT1920 OUT REF VEE
1920 F04
W
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+
LT1920 APPLICATIONS INFORMATION
To significantly reduce the effect of these out-of-band signals on the input offset voltage of instrumentation amplifiers, simple lowpass filters can be used at the inputs. This filter should be located very close to the input pins of the circuit. An effective filter configuration is illustrated in Figure 5, where three capacitors have been added to the inputs of the LT1920. Capacitors CXCM1 and CXCM2 form lowpass filters with the external series resistors RS1, 2 to any out-of-band signal appearing on each of the input traces. Capacitor CXD forms a filter to reduce any unwanted signal that would appear across the input traces. An added benefit to using CXD is that the circuit's AC common mode rejection is not degraded due to common mode capacitive imbalance. The differential mode and common mode time constants associated with the capacitors are: tDM(LPF) = (2)(RS)(CXD) tCM(LPF) = (RS1, 2)(CXCM1, 2) Setting the time constants requires a knowledge of the frequency, or frequencies of the interference. Once this frequency is known, the common mode time constants can be set followed by the differential mode time constant. Set the common mode time constants such that they do not degrade the LT1920's inherent AC CMR. Then the differential mode time constant can be set for the bandwidth required for the application. Setting the differential
IN - RS2 1.6k CXCM2 0.001F EXTERNAL RFI FILTER f(-3dB) 500Hz
CXD 0.1F
RG
Figure 5. Adding a Simple RC Filter at the Inputs to an Instrumentation Amplifier is Effective in Reducing Rectification of High Frequency Out-of-Band Signals
PACKAGE DESCRIPTION
Dimensions in inches (millimeters) unless otherwise noted. N8 Package 8-Lead PDIP (Narrow 0.300)
(LTC DWG # 05-08-1510)
0.130 0.005 (3.302 0.127) 0.400* (10.160) MAX 8 7 6 5
0.300 - 0.325 (7.620 - 8.255)
0.045 - 0.065 (1.143 - 1.651)
0.009 - 0.015 (0.229 - 0.381)
0.065 (1.651) TYP 0.125 (3.175) 0.020 MIN (0.508) MIN 0.018 0.003 (0.457 0.076)
0.255 0.015* (6.477 0.381)
(
+0.035 0.325 -0.015 8.255 +0.889 -0.381
)
0.100 0.010 (2.540 0.254)
*THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS. MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.010 INCH (0.254mm)
Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
+
-
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U
U
mode time constant close to the sensor's BW also minimizes any noise pickup along the leads. To avoid any possibility of inadvertently affecting the signal to be processed, set the common mode time constant an order of magnitude (or more) larger than the differential mode time constant. To avoid any possibility of common mode to differential mode signal conversion, match the common mode time constants to 1% or better. If the sensor is an RTD or a resistive strain gauge, then the series resistors RS1, 2 can be omitted, if the sensor is in proximity to the instrumentation amplifier.
IN +
RS1 1.6k
CXCM1 0.001F
V+
LT1920
VOUT
V-
1920 F05
1
2
3
4
N8 1197
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LT1920 TYPICAL APPLICATION
Nerve Impulse Amplifier
PATIENT/CIRCUIT PROTECTION/ISOLATION +IN C1 0.01F R2 1M R1 12k R3 30k R4 30k 2 3V 3 8 7 C2 0.47F RG 6k 1 2 1/2 LT1112 LT1920 G = 10 5 6 R6 1M 5 0.3Hz HIGHPASS 3V
PATIENT GROUND
1
3 AV = 101 POLE AT 1kHz
-IN
PACKAGE DESCRIPTION
0.010 - 0.020 x 45 (0.254 - 0.508) 0.008 - 0.010 (0.203 - 0.254) 0- 8 TYP
0.016 - 0.050 0.406 - 1.270
*DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE **DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE
RELATED PARTS
PART NUMBER LTC1100 LT1101 LT1102 LT1167 LTC(R)1418 LT1460 LTC1562 LTC1605 DESCRIPTION Precision Chopper-Stabilized Instrumentation Amplifier Precision, Micropower, Single Supply Instrumentation Amplifier High Speed, JFET Instrumentation Amplifier Single Resistor Gain Programmable Precision Instrumentation Amplifier 14-Bit, Low Power, 200ksps ADC with Serial and Parallel I/O Precision Series Reference Active RC Filter 16-Bit, 100ksps, Sampling ADC COMMENTS Best DC Accuracy Fixed Gain of 10 or 100, IS < 105A Fixed Gain of 10 or 100, 30V/s Slew Rate Upgraded Version of the LT1920 Single Supply 5V or 5V Operation, 1.5LSB INL and 1LSB DNL Max Micropower; 2.5V, 5V, 10V Versions; High Precision Lowpass, Bandpass, Highpass Responses; Low Noise, Low Distortion, Four 2nd Order Filter Sections Single 5V Supply, Bipolar Input Range: 10V, Power Dissipation: 55mW Typ
1920f LT/TP 0299 4K * PRINTED IN USA
12
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408)432-1900 q FAX: (408) 434-0507 q www.linear-tech.com
U
U
+
+ -
8 1/2 LT1112 4 -3V 7 OUTPUT 1V/mV
6
-
4 -3V R8 100
R7 10k
- +
C3 15nF
1920 TA03
Dimensions in inches (millimeters) unless otherwise noted.
S8 Package 8-Lead Plastic Small Outline (Narrow 0.150)
(LTC DWG # 05-08-1610)
0.189 - 0.197* (4.801 - 5.004) 0.053 - 0.069 (1.346 - 1.752) 0.004 - 0.010 (0.101 - 0.254) 8 7 6 5
0.014 - 0.019 (0.355 - 0.483)
0.050 (1.270) TYP
0.228 - 0.244 (5.791 - 6.197)
0.150 - 0.157** (3.810 - 3.988)
SO8 0996
1
2
3
4
(c) LINEAR TECHNOLOGY CORPORATION 1998

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