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FEATURES 8-Bit Serial Gain Control V/V/LSB Linear Gain Response 36 dB Gain Range 0.20 dB Gain Accuracy Upper Bandwidth: 150 MHz 22 dBm 1 dB Compression Point (75 ) Drives Low Distortion Signals into 75 Load: -57 dBc SFDR at 42 MHz and 12 dBm Out -46 dBc SFDR at 42 MHz and 18 dBm Out Single Supply Operation from 5 V to 12 V Maintains 75 Output Impedance Power-Up and Power-Down Condition Supports SPI Input Control Standard APPLICATIONS Coaxial Cable Driver HFC Cable Telephony Systems HFC High Speed Data Modems Interactive Set-Top Boxes PC Plug-In Modems Interfaces with AD9853 I2C Controlled Digital Modulator High Performance Digitally Controlled Variable Gain Block
Serial Digital Controlled Variable Gain Line Driver AD8320
FUNCTIONAL BLOCK DIAGRAM
VCC GND PWR AMP REFERENCE
AD8320
VOUT
VREF VIN
INV.
ATTENUATOR CORE
REVERSE AMP
BUF. DATA LATCH POWERDOWN/ SWITCH INTER. PD
DATA SHIFT REGISTER
DATEN CLK
SDATA
DESCRIPTION
The AD8320 is a digitally controlled variable gain amplifier optimized for coaxial line driving applications. An 8-bit serial word determines the desired output gain over a 36 dB range (256 gain levels). The AD8320 provides linear gain response. The AD8320 is made up of a digitally controlled variable attenuator of 0 dB to -36 dB, which is preceded by a low noise, fixed gain buffer and followed by a low distortion high power amplifier. The AD8320 has a 220 input impedance and accepts a single-ended input signal with a specified analog input level of up to 0.310 V p-p. The output is specified for driving a 75 load, such as coaxial cable, although the AD8320 is capable of driving other loads. Distortion performance of -57 dBc is achieved with an output level up to 12 dBm (3.1 V p-p) at 42 MHz, while -46 dBc distortion is achieved with an output level up to 18 dBm (6.2 V p-p). A key performance and cost advantage of the AD8320 results from the ability to maintain a constant 75 output impedance during power-up and power-down conditions. This eliminates the need for external 75 back-termination, resulting in twice the effective output voltage when compared to a standard operational amplifier. Additionally, the on-chip 75 termination
results in low glitch output during power-down and power-up transitions, eliminating the need for an external switch. The AD8320 is packaged in a 20-lead SOIC and operates from a single +5 V through +12 V supply and has an operational temperature range of -40C to +85C.
20
30 PO = 18dBm
DISTORTION - dBc
40
50 PO = 8dBm 60
PO = 12dBm
70 PO = 4dBm 80 1 10 FREQUENCY - MHz 100
Figure 1. Worst Harmonic Distortion vs. Frequency for Various Output Levels at VCC = 12 V
REV. 0
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 which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices. One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Tel: 781/329-4700 World Wide Web Site: http://www.analog.com Fax: 781/326-8703 (c) Analog Devices, Inc., 1998
AD8320-SPECIFICATIONS otherwise noted)
Parameter INPUT CHARACTERISTICS Full-Scale Input Voltage Input Resistance Input Capacitance GAIN CONTROL INTERFACE Gain Range Full Scale (Max) Gain Gain Offset (Min) Gain Gain Scaling Factor OUTPUT CHARACTERISTICS Bandwidth (-3 dB) Bandwidth Roll-Off Bandwidth Peaking Output Offset Voltage Output Offset Drift Output Noise Spectral Density Conditions
(@ VCC = 12 V, TA = +25 C, VIN = 0.310 V p-p, RL = 75
Min Typ 0.310 0.155 220 2.0
, RS = 75
unless
Max Units V p-p V p-p pF dB dB (V/V) dB (V/V) V/V/LSB MHz dB dB mV mV/C nV/Hz nV/Hz nV/Hz dBm dBm ns dBc dBc dBc dBc dBc dBc dBm dBm dBm dBm dB dB dB mdB/C mdB/V ns ns ns ns mV dBm dBm +12 85 30 105 37 V mA mA mA mA
Max Gain, POUT = 18 dBm, VCC = 12 V Max Gain, POUT = 12 dBm, VCC = 5 V
36 26 (20) -10.0 (0.316) 0.077 All Gain Codes F = 65 MHz F = 65 MHz All Gain Codes Full Temperature Range Max. Gain, Frequency = 10 MHz Min. Gain, Frequency = 10 MHz PD = 0, Frequency = 10 MHz VCC = 12 V VCC = 5 V Power Up and Power Down Max Gain, VIN = 500 mV p-p F = 42 MHz, POUT = 12 dBm, VCC = 12 V F = 42 MHz, POUT = 12 dBm, VCC = 5 V F = 42 MHz, POUT = 18 dBm, VCC = 12 V F = 65 MHz, POUT = 12 dBm, VCC = 12 V F = 65 MHz, POUT = 12 dBm, VCC = 5 V F = 65 MHz, POUT = 18 dBm, VCC = 12 V F = 42 MHz, POUT = 18 dBm, VCC = 12 V F = 42 MHz, POUT = 12 dBm, VCC = 5 V F = 65 MHz, POUT = 18 dBm, VCC = 12 V F = 65 MHz, POUT = 12 dBm, VCC = 5 V F = 10 MHz F = 10 MHz F = 10 MHz, All Gain Codes Full Temperature Range VCC = +5 V to 12 V Min to Max Gain, VIN = 0.31 V p-p Max Gain, VIN = 0 V to 0.31 V p-p Max Gain, VIN = 0 Max Gain, VIN = 0 Max Gain, VIN = 0 F (PD) = 400 Hz @ 15% Duty Cycle 5 MHz F 65 MHz PD = 0 +5 PD = 1, PD = 0, PD = 1, PD = 0, VCC = +5 V VCC = +5 V VCC = +12 V VCC = +12 V 80 25 97 32 150 0.7 0 40 0.25 73 53 4.5 22.5 16 75 40 -57.0 -43.0 -46.0 -57.0 -42.5 -43.0 34 32 32.5 28.5 0.1 0.2 0.2 0.5 35 30 25 45 65 30 -70 5
1 dB Compression Point Output Impedance Overload Recovery OVERALL PERFORMANCE Worst Harmonic Distortion
65
85
-52.0 -39.0 -42.0 -52.0 -39.0 -40.0
3rd Order Intercept
Full-Scale (Max Gain) Accuracy Gain Offset (Min Gain) Accuracy Gain Accuracy Gain Drift Gain Variation w/Supply Output Settling to 1 mV Gain Change @ TDATEN = 1 Input Change POWER CONTROL Power-Down Settling Time to 1 mV Power-Up Settling Time to 1 mV Power-Down Pedestal Offset Spectral Output Leakage Maximum Reverse Power POWER SUPPLY Specified Operating Range Quiescent Current Power Down Power Up, VCC = +12 V Power Down, VCC =+12 V
-0.75
0.75
-2-
REV. 0
AD8320 LOGIC INPUTS (TTL/CMOS Logic) (DATEN, CLK, SDATA, 5 V V
Parameter Logic "1" Voltage Logic "0" Voltage Logic "1" Current (VINH = 5 V) CLK, SDATA, DATEN Logic "0" Current (VINL = 0 V) CLK, SDATA, DATEN Logic "1" Current (VINH = 5 V) PD Logic "0" Current (VINL = 0 V) PD
CC
12 V; Full Temperature Range)
Typ Max 5.0 0.8 20 -75 190 -70 Units V V nA nA A A
Min 2.1 0 0 -450 0 -320
CC
TIMING REQUIREMENTS (Full Temperature Range, V
Parameter Clock Pulse Width (TWH) Clock Period (TC) Setup Time SDATA vs. Clock (TDS) Setup Time DATEN vs. Clock (TES) Hold Time SDATA vs. Clock (TDH) Hold Time DATEN vs. Clock (TEH) Input Rise and Fall Times, SDATA, DATEN, Clock (TR, TF)
TDS SDATA
VALID DATA WORD G1 MSB. . . .LSB
Supply Range, TR = TF = 4 ns, FCLK = 8 MHz unless otherwise noted.)
Min 12.0 32.0 6.5 17.0 5.0 3.0 10 Typ Max Units ns ns ns ns ns ns ns
VALID DATA WORD G2
TC TWH CLK TES DATEN TEH
8 CLOCK CYCLES
GAIN TRANSFER (G1) TOFF PD TGS
GAIN TRANSFER (G2)
TON ANALOG OUTPUT SIGNAL AMPLITUDE (p-p) PEDESTAL
Figure 2. Serial Interface Timing
VALID DATA BIT
MSB
MSB-1
MSB-2
TDS
TDH
CLK
Figure 3.
REV. 0
-3-
AD8320
ABSOLUTE MAXIMUM RATINGS* PIN CONFIGURATION
SDATA 1 CLK 2 DATEN 3 GND 4 VOCM 5
6 20 VCC 19 VIN 18 VREF 17 VCC
Supply Voltage +VS Pins 7, 8, 9, 17, 20 . . . . . . . . . . . . . . . . . . . -0.8 V to +13 V Input Voltages Pins 19 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 V Pins 1, 2, 3, 6 . . . . . . . . . . . . . . . . . . . . . . . . -0.8 V to +5 V Internal Power Dissipation Small Outline (RP) . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3 W Operating Temperature Range . . . . . . . . . . . -40C to +85C Storage Temperature Range . . . . . . . . . . . . -65C to +150C Lead Temperature, Soldering 60 seconds . . . . . . . . . . +300C
*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 indicated in the operational section of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.
AD8320
16 GND
TOP VIEW 15 GND PD (Not to Scale) 14 BYP 7 VCC VCC 8 VCC
9 13 GND 12 GND 11 GND
VOUT 10
ORDERING GUIDE
Model AD8320ARP AD8320-EB
Temperature Range -40C to +85C
Package Description 20-Lead Thermally Enhanced Power SOIC* Evaluation Board
JA
Package Option RP-20
53C/W
*Shipped in tubes (38 pieces/tube) and dry packed per J-STD-020.
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 the AD8320 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. PIN FUNCTION DESCRIPTIONS
WARNING!
ESD SENSITIVE DEVICE
Pin 1 2
Function SDATA CLK
Description Serial Data Input. This digital input allows for an 8-bit serial (gain) word to be loaded into the internal register with the MSB (most significant bit) first. Clock Input. The clock port controls the serial attenuator data transfer rate to the 8-bit master-slave register. A Logic 0 to 1 transition latches the data bit and a 1 to 0 transfers the data bit to the slave. This requires the input serial data word to be valid at or before this clock transition. Data Enable Low Input. This port controls the 8-bit parallel data latch and shift register. A Logic 0 to 1 transition transfers the latched data to the attenuator core (updates the gain) and simultaneously inhibits serial data transfer into the register. A 1 to 0 transition inhibits the data latch (holds the previous gain state) and simultaneously enables the register for serial data load. Common External Ground Reference. VCC/2 Reference Pin. A dc output reference level that is equal to 1/2 of the supply voltage (VCC). This port should be externally ac decoupled (0.1 F cap). Power-Down Low Logic Input. A Logic 0 powers down (shuts off) the power amplifier disabling the output signal and enabling the reverse amplifier. A Logic 1 enables the output power amplifier and disables the reverse amplifier. Common Positive External Supply Voltage. Output Signal Port. DC biased to approximately VCC/2. Internal Bypass. This pin must be externally ac decoupled (0.1 F cap). Input Reference Voltage (typically 1.9 V at 27C). This port should be externally ac decoupled (0.1 F cap). Analog Voltage Input Signal Port. DC biased to VREF voltage.
3
DATEN
4, 11, 12, 13, 15, 16 5 6
GND VOCM PD
7, 8, 9, 17, 20 VCC 10 14 18 19 VOUT BYP VREF VIN
-4-
REV. 0
Typical Performance Characteristics-AD8320
0.3 VCC = 12V F = 10MHz T = +25 C 0.1
0.6 VCC = 12V T = +25 C
GAIN ERROR - dB
0.45 0.30 F = 10MHz T = +25 C
0.2
GAIN ERROR - dB
0.3
GAIN ERROR - dB
0
10MHz
0.15 VCC = 12V 0
0 T= 0.1 40 C T = +85 C
0.3 42MHz 0.6 65MHz
0.15 VCC = 5V 0.30 0.45
0.2 0.3
0.9 1.2
0
64 128 192 GAIN CONTROL - Decimal
256
0
64 128 192 GAIN CONTROL - Decimal
256
0
64 128 192 GAIN CONTROL - Decimal
256
Figure 4. Gain Error vs. Gain Control at Various Temperatures
Figure 5. Gain Error vs. Gain Control at Various Frequencies
Figure 6. Gain Error vs. Gain Control at Different Supply Voltages
30 255D 20 170D 85D GAIN - dB GAIN - dB 10 VCC = 5V
30
255D 170D 85D
20
VCC = 12V
30
20
MAX GAIN PD = 0V
FEEDTHROUGH - dB
40 50 60 70 80 90 100 100k VCC = 5V, PIN = 14dBm
10
0 01D 10
0 01D 10 00D
00D 20 100k
VCC = 12V, PIN = 1M 10M 100M FREQUENCY - Hz
8dBm 1G
20 100k
1M
10M 100M FREQUENCY - Hz
1G
1M
10M 100M FREQUENCY - Hz
1G
Figure 7. AC Response
Figure 8. AC Response
Figure 9. Input Signal Feedthrough vs. Frequency
80 75 F = 10MHz VCC = 12V
80 75 F = 10MHz
OUTPUT NOISE - nV/ Hz
90
80
OUTPUT NOISE - nV/ Hz
MAX GAIN, VCC = 12V
OUTPUT NOISE - nV/ Hz
70 65 60 55 50 45 40 0
+85 C +25 C
70 65 60 55 50 45 VCC = 5V VCC = 12V
70 MAX GAIN, VCC = 5V 60 MIN GAIN, VCC = 12V
40 C
50 MIN GAIN, VCC = 5V 40 30 100k
128 192 64 GAIN CONTROL - Decimal
256
40
0
128 192 64 GAIN CONTROL - Decimal
256
10M 1M FREQUENCY - Hz
100M
Figure 10. Output Referred Noise vs. Gain Control at Various Temperatures
Figure 11. Output Referred Noise vs. Gain Control at Different Supply Voltages
Figure 12. Output Referred Noise vs. Frequency
REV. 0
-5-
AD8320
30 VCC = 12V 40 F = 65MHz, PO = 18dBm
20
20
30
DISTORTION - dBc
30
DISTORTION - dBc
DISTORTION - dBc
F = 42MHz, PO = 18dBm 50
PO = 12dBm 40 PO = 10dBm 50 60 PO = 8dBm 70 80 PO = 4dBm
40 PO = 10dBm 50 60
PO = 12dBm
60
70 F = 42MHz, PO = 12dBm 80 F = 65MHz, PO = 12dBm 90 0 64 128 192 GAIN CONTROL - Decimal 256
70 80
PO = 8dBm 1
PO = 4dBm 100
1
10 FREQUENCY - MHz
100
10 FREQUENCY - MHz
Figure 13. Worst Harmonic Distortion vs. Gain Control
Figure 14. Worst Harmonic Distortion vs. Frequency for Various Output Levels at VCC = 5 V
Figure 15. Worst Harmonic Distortion vs. Frequency for Various Output Levels at VCC = 6 V
20
20 30
40 VCC = 12V PO = 18dBm F = 42MHz N = 30
30 PO = 18dBm
DISTORTION - dBc
PO = 18dBm
30
PERCENTAGE
100
DISTORTION - dBc
40
40
50 PO = 8dBm 60
PO = 12dBm
50 PO = 8dBm 60
PO = 12dBm
20
10
70 PO = 4dBm 80
80 70 PO = 4dBm
1
10 FREQUENCY - MHz
100
1
10 FREQUENCY - MHz
0 47 46 45 44 HARMONIC DISTORTION - dBc 43
Figure 16. Worst Harmonic Distortion vs. Frequency for Various Output Levels at VCC = 10 V
Figure 17. Worst Harmonic Distortion vs. Frequency for Various Output Levels at VCC = 12 V
Figure 18. Distribution of Worst Harmonic Distortion
40 VCC = 12V PO = 18dBm F = 65MHz N = 30
20 VCC = 12V PO = 12dBm F = 42MHz N = 30
30.0 VCC = 5V PO = 12dBm F = 65MHz N = 30
30 PERCENTAGE
15
22.5
PERCENTAGE
PERCENTAGE
20
10
15.0
10
5
7.5
0 45 44 43 42 HARMONIC DISTORTION dBc 41
0 59 58 57 56 HARMONIC DISTORTION dBc 55
0 44 43 42 41 HARMONIC DISTORTION dBc 40
Figure 19. Distribution of Worst Harmonic Distortion
Figure 20. Distribution of Worst Harmonic Distortion
Figure 21. Distribution of Worst Harmonic Distortion
-6-
REV. 0
AD8320
35 F = 42MHz, VCC = 5V, PO = 12dBm 40
20
45
3RD ORDER INTERCEPT - dBm
dBc
0 POUT - dBm
PO = 18dBm MAX GAIN VCC = 12V
VCC = 12V 40
PO = 12dBm
45 F = 65MHz 50 VCC = 12V PO = 18dBm
DISTORTION
-20
35 PO = 18dBm 30
50
55
F = 42MHz 50 VCC = 12V PO = 18dBm
-40
60 F = 65MHz, VCC = 12V, PO = 12dBm 65 50 25 25 50 0 TEMPERATURE C 75 100
-60
25
-80 41.2 42.0 42.4 41.6 FREQUENCY - MHz 42.8
20
1
10 FREQUENCY - MHz
100
Figure 22. Harmonic Distortion vs. Temperature
Figure 23. Two-Tone Intermodulation Distortion
Figure 24. Third Order Intercept vs. Frequency
30 VCC = 5V MAX GAIN
VCC = 12V VIN = 310mV p-p MIN GAIN F = 10MHz
GAIN - dB
VCC = 12V VIN = 310mV p-p MAX GAIN F = 10MHz
27
CL = 0pF
24 CL = 22pF 21 CL = 100pF
18
CL = 150pF
20mV
12.5nsec
1.2V
12.5nsec
15 100k 1M 10M 100M FREQUENCY - Hz 1G
Figure 25. Transient Response
Figure 26. Transient Response
Figure 27. AC Response for Various Capacitive Loads
30 VCC = 12V MAX GAIN
OUTPUT VOLTAGE - Volts
VCC = 5V MAX GAIN F = 1MHz CL = 0pF
27
CL = 0pF
CL = 22pF CL = 100pF CL = 150pF
GAIN - dB 24 CL = 22pF 21 CL = 100pF
VCC = 12V MAX GAIN F = 1MHz CL = 0pF
CL = 22pF CL = 100pF CL = 150pF
18
CL = 150pF
500mV
TIME - Seconds
5nsec
15 100k 1M 10M 100M FREQUENCY - Hz 1G
1V
5nsec
Figure 28. Transient Response for Various Capacitive Loads
Figure 29. AC Response for Various Capacitive Loads
Figure 30. Transient Response for Various Capacitive Loads
REV. 0
-7-
AD8320
100mV
VCC = 12V MAX GAIN VIN = 0V p-p
5mV
VOUT
VCC = 12V MAX GAIN VIN = 0V p-p
1.25V
VCC = 5V F = 40MHz MAX GAIN
VOUT
VOUT
CLK
VIN
PD
DATEN
5V
75nsec
5V
250nsec
500mV 5V
20nsec
Figure 31. Power-Up/Power-Down Glitch
Figure 32. Clock Feedthrough
Figure 33. Overload Recovery
2.50V
VCC = 12V F = 40MHz MAX GAIN
2.00V
VCC = 12V, F = 40MHz MAX GAIN
2.50V
VCC = 12V, F = 40MHz MIN TO MAX GAIN VIN = .310V p-p
VOUT
VOUT
VOUT
VIN
VIN
DATEN
1V 5V
20nsec
250mV
20nsec
5V
20nsec
Figure 34. Overload Recovery
Figure 35. Output Settling Time Due to Input Change
Figure 36. Output Settling Time Due to Gain Change
80 VCC = 5V RT = 115
90 VCC = 5V, PD = 0 80 SUPPLY CURRENT - mA VCC = 5V, PD = 1
120 100 VCC = 12V, PD = 1
OUTPUT IMPEDANCE -
70
INPUT IMPEDANCE -
80 VCC = 5V, PD = 1 60 VCC = 12V, PD = 0
60
70
VCC = 12V, PD = 1 VCC = 12V, PD = 0
40
50
60
20
VCC = 5V, PD = 0 -25 0 25 50 TEMPERATURE - C 75 100
40 100k
1M
10M 100M FREQUENCY - Hz
1G
50 100k
1M 10M FREQUENCY - Hz
100M
0 -50
Figure 37. Input Impedance vs. Frequency
Figure 38. Output Impedance vs. Frequency
Figure 39. Supply Current vs. Temperature
-8-
REV. 0
AD8320
OPERATIONAL DESCRIPTION
32
The AD8320 is a digitally controlled variable gain power amplifier that is optimized for driving 75 cable. A multifunctional bipolar device on single silicon, it incorporates all the analog features necessary to accommodate reverse path (upstream) high speed (5 MHz to 65 MHz) cable data modem and cable telephony requirements. The AD8320 has an overall gain range of 36 dB (-10 dB to 26 dB) and is capable of greater than 100 MHz of operation at output signal levels exceeding 18 dBm. Overall, when considering the device's wide gain range, low distortion, wide bandwidth and variable load drive, the device can be used in many variable gain block applications. The digitally programmable gain is controlled by the three wire "SPI" compatible inputs. These inputs are called SDATA (serial data input port), DATEN (data enable low input port) and CLK (clock input port). See Pin Function Descriptions and Functional Block diagram. The AD8320 is programmed by an 8-bit "attenuator" word. These eight bits determine the 256 programmable gain settings. See attenuator core description below. The gain is linear in V/V/LSB and can be described by the following equation: AV = 0.316 + 0.077 x Code (RL = 75 ) where code is the decimal equivalent of the 8-bit word. For example, if all 8 bits are at a logic "1," the decimal equivalent is 255 and AV equals 19.95 V/V or 26 dB. The gain scaling factor is 0.077 V/V/LSB, with an offset of 0.316 V/V (-10.0 dB). Figure 40 shows the linear gain versus decimal code and Figure 41 shows the gain in dB versus decimal code. Note the nonlinearity that results when viewed in dB versus code. The dB step size increases as the attenuation increases (i.e., gain decreases) and reaches a maximum step size of approximately 1.9 dB (gain change between 01 and 00 decimal).
22 20 18 16 14
GAIN - V/V
24
16
GAIN - dB
8
0 AV = 20 -8 LOG10 (0.316 + 0.077 CODE)
-16
0
32
64
96 128 160 GAIN - Code - Decimal
192
224
256
Figure 41. Log Gain vs. Gain Control
The attenuator core can be viewed as eight binarily weighted (differential in-differential out) transconductance (gm) stages with the "in phase" current outputs of all eight stages connected in parallel to their respective differential load resistors (not shown). The core differential output signals are also 180 degrees out of phase and equal in amplitude. The input stages are likewise parallel, connected to the inverting input amplifier and buffer outputs as shown. Nine bits plus of accuracy is achieved for all gain settings over the specified frequency, supply voltage and temperature range. The actual total core GM x RL attenuation is determined by which combination of binarily weighted gm stages are selected by the data latch. With 8 bits, 256 levels of attenuation can be programmed. This results in a 36 dB attenuation range (0 dB to -36 dB). See gain equation above.
VCC GND PWR AMP REFERENCE
AD8320
VOUT
AV = 0.316 + 0.077
CODE
VREF VIN
INV.
ATTENUATOR CORE
REVERSE AMP
POWER-UP
12 10 8 6 4 2 0 -2 0 32 64 POWER-DOWN
BUF. DATA LATCH POWER- DOWN SWITCH INTER.
PD
DATA SHIFT REGISTER
DATEN CLK
SDATA
Figure 42. Functional Block Diagram
96 128 160 GAIN - Code - Decimal 192 224 256
Figure 40. Linear Gain vs. Gain Control
The AD8320 is composed of three analog functions in the powerup or forward mode (Figure 42). The input inverter/buffer amplifier provides single-ended to differential output conversion. The output signals are nominally 180 degrees out of phase and equal in amplitude with a differential voltage gain of 2 (6 dB). Maintaining close to 180 degrees and equal amplitude is required for proper gain accuracy of the attenuator core over the specified operating frequency. The input buffer/inverter also provides equal dc voltages to the core inputs via the internal reference. This is required to ensure proper core linearity over the full specified power supply range (5 V to 12 V). REV. 0 -9-
To update the AD8320 gain, the following digital load sequence is required. The attenuation setting is determined by the 8-bit word in the data latch. This 8-bit word is serially loaded (MSB first) into the shift register at each rising edge of the clock. See Figure 43. During this data load time (T), DATEN is low and the data latch is latched holding the previous (T - 1) data word keeping the attenuation level unchanged. After eight clock cycles the new data word is fully loaded and DATEN is switched high. This enables the data latch (becomes transparent) and the loaded register data is passed to the attenuator with the updated gain value. Also at this DATEN transition, the internal clock is disabled, thus inhibiting new serial input data.
AD8320
TDS SDATA
VALID DATA WORD G1 MSB. . . .LSB VALID DATA WORD G2
TC
TWH
CLK TES DATEN TEH
8 CLOCK CYCLES
GAIN TRANSFER (G1) TOFF PD TGS
GAIN TRANSFER (G2)
TON ANALOG OUTPUT SIGNAL AMPLITUDE (p-p) PEDESTAL
Figure 43. Serial Interface Timing
The power amplifier has two basic modes of operation; forward or power-up mode and reverse or power-down mode. In the power-up mode (PD = 1), the power amplifier stage is enabled and the differential output core signal is amplified by 20 dB. With a core attenuation range of 0 dB to -36 dB and 6 dB of input gain, the overall AD8320 gain range is 26 dB to -10 dB. In this mode, the single-ended output signal maintains a dc level of VCC/2. This dc output level provides for optimum large signal linearity and allows for dc coupling the output if necessary. The output stage is unique in that it maintains a dynamic output impedance of 75 . This allows for a direct 75 cable connection and results in 6 dB of added load power versus using a series 75 back-termination resistor as required with traditional low output impedance amplifiers. The power amplifier will also drive lower or higher output loads, although the device's gain (not gain range) will change accordingly (see Applications section). In the power-down mode (PD = 0), the power amplifier is turned off and a "reverse" amplifier (the inner triangle in Figure 42) is enabled. During this 1 to 0 transition, the output power amplifier's input stage is also disabled, resulting in no forward output signal (S21 is 0), although the attenuator core and input amplifier/ buffer signals are not affected (S11 0). The function of the reverse amplifier is to maintain 75 and VCC/2 at the output port (VOUT) during power-down. This is required to minimize line reflections (S22 0) and ensures proper filter operation for any forward mode device sharing the same bus (i.e., in a multiplexed configuration). (See Applications section.) In the time domain, as PD switches states, a transitional glitch and pedestal offset results. (See Figures 31 and 43.) The powered down supply current drops to 32 mA versus 97 mA (VCC = 12 V) in power-up mode. Generally, using the power-down low input (PD) for switching allows for multiple devices to be multiplexed via splitters (N-1 off, 1 on) and reduces overall total power consumption as required for cable data applications. For cable telephony, the power-down current generally needs to be much lower during
what is referred to as sleep and standby modes, and VCC supply switching via PFETS or equivalent, as described in the applications section, would be required.
APPLICATIONS
The AD8320 is primarily intended to be used as the return path (also called upstream path) line driver in cable modem and cable telephony applications. Data to be transmitted is modulated in either QPSK or QAM format. This is done either in DSP or by a dedicated QPSK/QAM modulator such as the AD9853. The amplifier receives its input signal either from the dedicated QPSK/QAM modulator or from a DAC. In both cases, the signal must be low-pass filtered before being applied to the line driving amplifier.
TO MODEM RECEIVE CIRCUITRY SUBSCRIBER CENTRAL OFFICE
DIPLEXER 7TH ORDER ELLIPTIC LOW PASS FILTER 75
AD9853
AD8320
Figure 44. Block Diagram of Cable Modem's Upstream Driver Section
The amplifier drives the line through a diplexer. The insertion loss of a diplexer is typically -3 dB. As a result, the line driver must deliver a power level roughly 3 dB greater than required by the applicable cable modem standard so that diplexer losses are canceled out. Because the distance to the central office varies from subscriber to subscriber, signals from different subscribers will be attenuated by differing amounts. As a result, the line driver is required to vary its gain so that all signals arriving at the central office have the same amplitude.
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Basic Connection
Figure 45 shows the basic schematic for operating the AD8320. Because the amplifier operates from a single supply, the input signal must be ac-coupled using a 0.1 F capacitor. The input pin has a bias level of about 1.9 V. This bias level is available on the VREF pin (Pin 18) and can be used to externally bias signals if dc-coupling is desired. Under all conditions, a 0.1 F decoupling capacitor must be connected to the VREF pin. If the VREF voltage is to be used externally, it should be buffered first. The VIN pin of the AD8320 (Pin 19) has an input impedance of 220 . Typically, in video applications, 75 termination is favored. As a result, an external shunt resistance (R1) to ground of 115 is required to create an overall input impedance of 75 . If 50 termination is required, a 64.9 shunt resistor should be used. Note, to avoid dc loading of the VIN pin, the ac-coupling capacitor should be placed between the input pin and the shunt resistor as shown in Figure 45. On the output side, the VOUT pin also has a dc bias level. In this case the bias level is midway between the supply voltage and ground. The output signal must therefore be ac-coupled before being applied to the load. The dc bias voltage is available on the VOCM pin (Pin 5) and can be used in dc-coupled applications. This node must be decoupled to ground using a 0.1 F capacitor. If the VOCM voltage is to be used externally, it should be buffered. Since the AD8320 has a dynamic output impedance of 75 , no external back termination resistor is required. If the output signal is being evaluated on 50 test equipment such as a spectrum analyzer, a 75 to 50 adapter (commonly called a pad) should be used to maintain a properly matched circuit.
Varying the Gain
The timing diagram for AD8320's serial interface is shown in Figure 43. The write cycle to the device is initiated by the falling edge of DATEN. This is followed by eight clock pulses that clock in the control word. Because the clock signal is level triggered, data is effectively clocked on the falling edge of CLK. After the control word has been clocked in, the DATEN line goes back high, allowing the gain to be updated (this takes about 30 ns). The relationship between gain and control word is given by the equation: Gain (V/V) = 0.077 x Code + 0.316 where code is the decimal equivalent of the gain control word (0 to 255). The gain in dB is given by the equation: Gain (dB) = 20 log10 (0.077 x Code + 0.316) The digital interface also contains an asynchronous power-down mode. The normally high PD line can be pulled low at any time. This turns off the output signal after 45 ns, and reduces the quiescent current to between 25 mA and 32 mA (depending upon the power supply voltage). In this mode, the programmed gain is maintained.
Clock Line Feedthrough
Clock feedthrough results in a 5 mV p-p signal appearing superimposed on the output signal (see Figure 32). If this impinges upon the dynamic range of the application, the clock signal should be noncontinuous, i.e., should only be turned on for eight cycles during programming.
Power Supply and Decoupling
The gain of the AD8320 can be varied over a range of 36 dB, from -10 dB to +26 dB, by varying the 8-bit gain setting word.
The AD8320 should be powered with a good quality (i.e., low noise) single supply of between +5 V and +12 V. In order to achieve an output power level of +18 dBm (6.2 V p-p) into
VCC +5V TO +12V C7 10 F C6 0.1 F C5 0.1 F VCC C4 0.1 F VCC C2 0.1 F VCC C11 0.1 F VCC VCC C3 0.1 F
GND
BYP VOUT C10 0.1 F TO DIPLEXER RIN = 75 C8 0.1 F
C12 0.1 F C1 0.1 F INPUT R1* 115 VREF
REFERENCE
AD8320
VOCM ATTENUATOR CORE
VIN DATA LATCH POWERDOWN / SWITCH INTER.
PD
*FOR A 75 INPUT IMPEDANCE DATA SHIFT REGISTER DATEN CLK
SDATA GND GND GND GND GND
CLK SDATA DATEN PD
Figure 45. Basic Connection
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75 , a supply voltage of at least +10 V is required. To achieve a signal level of +12 dBm (about 3.1 V p-p) into 75 , a minimum supply level of +5 V is required. However, for the lowest possible distortion, the power supply voltage should be raised as high as possible. In varying the power supply from +5 V to +12 V, the quiescent current increases from 80 mA to 97 mA. Careful attention must be paid to decoupling the power supply pins. A 10 F capacitor, located fairly close to the device, is required to provide good decoupling for lower frequency signals. In addition, five 0.1 F decoupling capacitors should be located close to each of the five power supply pins (7, 8, 9, 17 and 20). A 0.1 F capacitor must also be connected to the pin labeled BYP (Pin 14), to provide decoupling to an internal node of the device. All six ground pins should be connected to a low impedance ground plane.
Alternative Power-Down Mode
A HEXFET power MOSFET (International Rectifier part number IRLML5103) is used to turn on and off the current to the supply pins of the AD8320. Under normal operating conditions, the gate (labeled POWER-DOWN) should be grounded. Pulling the gate to within 2 V of the supply will open the switch and reduce the current to the amplifier to zero. In cable modem and cable telephony applications the modem must always present an output impedance of 75 to the line. This forces the line driver to always present a 75 impedance to the diplexer. In this application, a single pole double throw RF switch (AS103, Alpha Semiconductor) is used to switch in an external 75 impedance when the AD8320 is turned off. This resistor then mimics the dynamic output impedance of the AD8320. TTL or CMOS logic can be used to drive the two voltages driving the RF switch (V1 and V2). Before the AD8320 is turned back on again, the gain needs to be set to a known level. This can be done by holding the PD pin of the AD8320 low after POWER-DOWN has gone high. While PD is held low, the 8-bit serial data stream can be clocked into the AD8320. During this time the quiescent current will increase to 32 mA. However, this time period can be as small as about 1 s. In this mode the output settles about 45 ns after the rising edge of PD. Alternatively, if DATEN is held low as the AD8320 is powered on, the device will power up in minimum gain. In this mode, the output settles after about 200 s. Note that for both cases, the capacitor on VOCM has been reduced from 0.1 F to 0.01 F to facilitate a faster turn-on time. All other capacitors in the circuit should be connected as shown in Figure 45.
As previously mentioned, the AD8320 can be put into a low power sleep mode by pulling the PD pin low. If lower power consumption is required during power-down mode, an alternative scheme can be used as shown in Figure 46.
VCC+12V POWERDOWN
S
G
IRLML5103
D
0.1 F
J1
VOUT
PD DATEN CLK SDATA
14
VDD PD VCC 75 VOUT VOCM AS103 0.1 F (SEETEXT)
J2
+5V 0.1 F
AD8320*
J3
GND 0.1 F
0.1 F 75 0.01 F V2 V1
V2 V1 * ADDITIONAL PINS AND DECOUPLING CAPACITORS OMITTED FOR CLARITY 10-12V POWERDOWN 0V 3-5V V1 V2 PD DATEN 0V 0V 3-5V 0V
CLK SDATA OUTPUT
97mA QUIESCENT CURRENT 0mA 32mA
97mA
Figure 46. Alternative Power-Down Mode with Timing
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TO MODEM RECEIVE CHANNEL 58dBmV DIPLEXER +5V +12V 75
VIN 11dBmV 5V
VIN
AD603*
VPOS VOUT 220 20pF 0.1 F 100
AD8320*
75 220 61dBmV
FBDK 41dBmV VNEG COMM GNEG GPOS VDD CLR -5V 1.5 1k +0.5V
REFIN
PD
AD7801*
AGND DGND CS WR LDAC D0-D7 8 D0-D7 CLK DATA CLK ENABLE PD
*ADDITIONAL PINS AND DECOUPLING CAPACITORS OMITTED FOR CLARITY
74HCT164*
AB
Figure 47. Enhanced Dynamic Range Circuit
Enhanced Dynamic Range Application
The AD8320 can be combined with the AD603 to give additional dynamic range as shown in Figure 47. The AD603 is a voltage controlled variable gain amplifier. The gain of the AD603 is determined by the difference in voltage between the GPOS and GNEG pins. This differential voltage has a range of 0.5 V. In this example, the voltage on GNEG is tied to +0.5 V. As the voltage on GPOS is varied from 0 V to 1 V, the gain of the AD603 changes from -10 dB to +30 dB with a slope of 25 mV/dB (i.e., linear in dB). The gain control voltage is supplied by the AD7801 DAC. The output voltage of the DAC (0 V to +2.5 V) is divided down to fit the 0 V to 1 V range of the AD603 using a resistor attenuator network. In order that the same gain control word can be used for both the AD603 and the AD8320, the serial data stream is converted to the parallel format of the AD7801 DAC using a serial-toparallel shift register. The rising edge of the enable pulse simultaneously updates both amplifiers. As the control word is varied from 00Dec to 255Dec, the gain of the signal chain varies from -26 dB to +50 dB (there is 6 dB of attenuation between AD603 and AD8320). In practice, this circuit is not usable at the lower end of the gain range due to the small input signal (11 dBmV or about 10 mV p-p). Figure 48 shows the spectrum of the output signal at a frequency of 42 MHz and an output level of 61 dBmV (3.1 V p-p, max gain). The gain vs. code transfer function of the two amplifiers along with the overall gain is shown in Figure 49. The overall gain transfer function combines a linear in dB transfer function with a linear in Volts/Volt transfer function. It is clear from Figure 49 that the overall gain transfer function can be considered to be approximately linear in dB over the top 50 dB of its range.
Figure 48. Output Spectrum of Enhanced Dynamic Range Circuit (Output Level = 61 dBmV, Frequency = 42 MHz)
60 50 AD8320/AD603 40 30
GAIN
AD8320
20 10 AD603 0 -10 -20 -30 0 21 41 61 81 101 121 141 161 181 201 221 241 GAIN CONTROL WORD - Decimal
Figure 49. Gain Transfer Function of Enhanced Dynamic Range Circuit
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Varying Gain by Varying Load Impedance
As already mentioned, the AD8320 has a dynamic output impedance of 75 . The specified gain range assumes that the output is terminated with a 75 load impedance. Varying the load impedance allows the gain to be varied, up to a maximum of twice the specified gain (for RL = ). The variation in gain with load resistance is shown in Figure 50 for the case of a gain control word of 255Dec (i.e., max gain).
32
and tested to demonstrate the specified high speed performance of the device. Figure 51 shows the schematic of the evaluation board. The silkscreen for the component side layer is shown in Figure 52. The layout of the board is shown in Figure 53 and Figure 54. The evaluation board package includes a fully populated board with BNC-type connectors along with Windows(R)-based software for controlling the board from a PC's printer port via a standard printer cable. A prototyping area is provided to allow for additional circuitry on the board. The single supply and ground to the board are brought over to this area and are available on two strips. There are also two extra strips available on the prototyping area which can be used for additional power supplies. The board should be powered with a good quality (i.e., low noise) single supply of between +5 V and +12 V. Extensive decoupling is provided on the board. A 10 F capacitor, located fairly close to the device, provides good decoupling for lower frequency signals. In addition, and more importantly, five 0.1 F decoupling capacitors are located close to each of the five power supply pins (7, 8, 9, 17 and 20).
30
28
GAIN - dB
26
24
22
20 0
100 RLOAD -
1000
10000
Controlling the Evaluation Board from a PC
Figure 50. Gain vs. RLOAD (Gain Control Word = 255Dec)
The gain can be described by the following equation:
2 RLOAD AV = 20 log 10 0.316 + 0.077 x Code RLOAD + 75
(
)
where Code is the decimal equivalent of the 8-bit word.
Evaluation Board
A two layer evaluation board for the AD8320 is available (part number AD8320-EB). This board has been carefully laid out
VCC TP2 VREF C12 0.1 F TP1 C1 0.1 F R1 115 INPUT VREF C7 10 F C6 0.1 F C5 0.1 F VCC C4 0.1 F VCC C2 0.1 F VCC
The evaluation board ships with Windows-based control software. A standard printer cable can be used to connect the evaluation board to a PC's printer port (also called parallel port). The cable length should be kept to less than about 5 feet. The wiring of a standard printer cable, with respect to the signal lines that are used in this application, is shown in Figure 55. Although the software controls the evaluation board via the PC's parallel port, the AD8320 digital interface is serial. Three of the parallel port's eight bits (and one digital ground line) are used to implement this serial interface. A fourth bit is used to control the PD pin.
C3 0.1 F TP4 VCC VCC GND BYP VOUT C10 0.1 F TP3 OUTPUT
C11 0.1 F
REFERENCE
AD8320
ATTENUATOR CORE VIN DATA LATCH POWERDOWN / SWITCH INTER VOCM C8 0.1 F
VOCM
PD
DATA SHIFT REGISTER CLK R4 0 C9 OPTIONAL 6 5 2 16, 19-30, 33 3 SDATA DATEN GND GND GND GND GND
36-PIN CENTRONICS CONNECTOR
Figure 51. Evaluation Board Schematic
All trademarks are the property of their respective holders.
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The control software requires Windows 3.1 or later to operate. To install the software, insert the disk labeled "Disk # 1 of 2" in the PC and run the file called SETUP.EXE. Additional installation instructions will be given on-screen. Before beginning installation, it is important to close any other Windows applications that are running. When you launch the installed control software from Windows, you will be asked to select the printer port you are using. Most modern PCs have only one printer port, usually called LPT1. However, some laptop computers use the PRN port. Figure 56 shows the main screen of the control software. Using the slider, you can set any gain in the AD8320's 36 dB range. The gain is displayed on-screen in dB and V/V. The 8-bit gain setting byte is also displayed, in binary, hexadecimal and decimal. Each time the slider is moved, the software automatically sends and latches the required 8-bit data stream to the AD8320. You can power down or reset the device simply by clicking the appropriate buttons. The software also offers one volatile storage location that can be used to store a particular gain. This functions in the same way as the memory on a pocket calculator.
Overshoot on PC Printer Ports' Data Lines
The data lines on some printer ports have excessive overshoot. Overshoot on the pin used as the serial clock (Pin 6 on the DSub-25 connector) can cause communication problems. This overshoot can be eliminated by applying mild filtering to the CLK line on the evaluation board. This can be done by putting a small series resistor on the CLK line, combined with a capacitor to ground. Pads are provided (C9, R4) on the component side of the evaluation board to allow easy insertion of these devices. Determining the size of these values will take some experimentation. Depending upon the overshoot from the printer port, this capacitor may need to be as large as 0.01 F, while the resistor is typically in the 50 to 100 range.
Figure 52. Evaluation Board Silkscreen (Component Side)
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Figure 53. Evaluation Board Layout (Component Side)
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Figure 54. Evaluation Board Layout (Solder Side)
36 PIN CENTRONICS 1 19
D-SUB 25 PIN (MALE) 14 1 DATEN PD
SDATA CLK
GND 25 13
SIGNAL DATEN PD DATA CLK DGND 36 18 EVALUATION BOARD
D-SUB-25 2 3 5 6 25
36-PIN CENTRONICS 3 1 5 2 16, 19-30, 33
PC
Figure 55. Interconnection Between AD8320EB and PC Printer Port
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Figure 56. Screen Display of Windows-Based Control Software
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OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
20-Lead Thermally Enhanced Power Small Outline Package (RP-20)
0.5118 (13.00) 0.4961 (12.60)
20 11
0.2992 (7.60) 0.2914 (7.40)
1
HEAT SINK
10
0.1890 (4.80) 0.4193 (10.65) 0.1791 (4.55) 0.3937 (10.00)
PIN 1 0.3340 (8.61) 0.3287 (8.35) 0.1043 (2.65) 0.0926 (2.35) 8 0 0.0118 (0.30) 0.0500 (1.27) 0.0040 (0.10) BSC STANDOFF 0.0201 (0.51) SEATING 0.0500 (1.27) 0.0130 (0.33) PLANE 0.0057 (0.40) 0.0295 (0.75) x 45 0.0098 (0.25)
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C3167-8-1/98
PRINTED IN U.S.A.

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