AD AD8132-EVAL Low-cost, high-speed differential amplifier Datasheet

a
Low-Cost, High-Speed
Differential Amplifier
AD8132
FEATURES
High Speed
350 MHz –3 dB Bandwidth
1200 V/s Slew rate
Resistor-Settable Gain
Internal Common-Mode Feedback to Improve Gain
and Phase Balance –68 dB @ 10 MHz
Separate Input to Set the Common-Mode Output
Voltage
Low Distortion –99 dBc SFDR @ 5 MHz 800 Load
Low Power 10.7 mA @ 5 V
Power Supply Range +2.7 V to 5.5 V
FUNCTIONAL BLOCK DIAGRAM
AD8132
–IN 1
8 +IN
7 NC
VOCM 2
V+ 3
+OUT 4
+
6 V–
5 –OUT
NC = NO CONNECT
APPLICATIONS
Low Power Differential ADC Driver
Differential Gain and Differential Filtering
Video Line Driver
Differential In/Out Level-Shifting
Single-Ended Input to Differential Output Driver
Active Transformer
The AD8132 is a low-cost differential or single-ended input to
differential output amplifier with resistor-settable gain. The
AD8132 is a major advancement over op amps for driving differential input ADCs or for driving signals over long lines. The
AD8132 has a unique internal feedback feature that provides
output gain and phase matching balanced to –68 dB at 10 MHz,
suppressing harmonics, and reducing radiated EMI.
Manufactured on ADI’s next generation of XFCB bipolar process, the AD8132 has a –3 dB bandwidth of 350 MHz and
delivers a differential signal with –99 dBc SFDR at 5 MHz,
despite its low cost. The AD8132 eliminates the need for a
transformer with high-performance ADCs, preserving the low
frequency and dc information. The common-mode level of the
differential output is adjustable by applying a voltage on the VOCM
pin, easily level-shifting the input signals for driving single supply
ADCs. Fast overload recovery preserves sampling accuracy.
Differential signal processing reduces the effects of ground noise
which plagues ground referenced systems. The AD8132 can be
used for differential signal processing (gain and filtering) throughout a signal chain, easily simplifying the conversion between
differential and single-ended components.
The AD8132 is available in both SOIC and µSOIC packages for
operation over –40°C to +85°C temperatures.
6
VS = 5V
G=1
VO,dm = 2V p-p
RL,dm = 499
3
0
GAIN – dB
GENERAL DESCRIPTION
–3
–6
The AD8132 can also be used as a differential driver for the
transmission of high-speed signals over low-cost twisted pair or
coaxial cables. The feedback network can be adjusted to boost
the high-frequency components of the signal. The AD8132 can
be used for either analog or digital video signals or for other highspeed data transmission. The AD8132 is capable of driving either
cat3 or cat5 twisted pair or coaxial with minimal line attenuation. The AD8132 has considerable cost and performance
improvements over discrete line driver solutions.
–9
–12
1
10
100
FREQUENCY – MHz
1k
Figure 1. Large Signal Frequency Response
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
© Analog Devices, Inc., 2000
(@ 25C, V = 5 V, V = 0 V, G = 1, R = 499 , R = R = 348 unless
AD8132–SPECIFICATIONS
otherwise noted. For G = 2, R = 200 , R = 1000 , R = 499 . Refer to TPC 1 and TPC 10 for test setup and label descriptions. All
S
L,dm
F
OCM
L,dm
F
G
G
specifications refer to single-ended input and differential outputs unless otherwise noted.)
Parameter
Conditions
Min
Typ
Max
Unit
VOUT = 2 V p-p
VOUT = 2 V p-p, G = 2
VOUT = 0.2 V p-p
VOUT = 0.2 V p-p, G = 2
VOUT = 0.2 V p-p
VOUT = 0.2 V p-p, G = 2
VOUT = 2 V p-p
0.1%, VOUT = 2 V p-p
VIN = 5 V to 0 V Step, G = 2
300
350
190
360
160
90
50
1200
15
5
MHz
MHz
MHz
MHz
MHz
MHz
V/µs
ns
ns
VOUT = 2 V p-p, 1 MHz, RL,dm = 800 Ω
VOUT = 2 V p-p, 5 MHz, RL,dm = 800 Ω
VOUT = 2 V p-p, 20 MHz, RL,dm = 800 Ω
VOUT = 2 V p-p, 1 MHz, RL,dm = 800 Ω
VOUT = 2 V p-p, 5 MHz, RL,dm = 800 Ω
VOUT = 2 V p-p, 20 MHz, RL,dm = 800 Ω
20 MHz, RL,dm = 800 Ω
20 MHz, RL,dm = 800 Ω
f = 0.1 MHz to 100 MHz
f = 0.1 MHz to 100 MHz
NTSC, G = 2, RL,dm = 150 Ω
NTSC, G = 2, RL,dm = 150 Ω
–96
–83
–73
–102
–98
–67
–76
40
8
1.8
0.01
0.10
dBc
dBc
dBc
dBc
dBc
dBc
dBc
dBm
nV/√Hz
pA/√Hz
%
Degrees
VOS,dm = VOUT,dm/2; VDIN+ = VDIN– = VOCM = 0 V
TMIN to TMAX Variation
± 1.0
10
3
12
3.5
1
–7 to +6
–70
DIN to OUT Specifications
DYNAMIC PERFORMANCE
–3 dB Large Signal Bandwidth
–3 dB Small Signal Bandwidth
Bandwidth for 0.1 dB Flatness
Slew Rate
Settling Time
Overdrive Recovery Time
NOISE/HARMONIC PERFORMANCE
Second Harmonic
Third Harmonic
IMD
IP3
Input Voltage Noise (RTI)
Input Current Noise
Differential Gain Error
Differential Phase Error
INPUT CHARACTERISTICS
Offset Voltage (RTI)
Input Bias Current
Input Resistance
Input Capacitance
Input Common-Mode Voltage
CMRR
OUTPUT CHARACTERISTICS
Output Voltage Swing
Output Current
Output Balance Error
1000
Differential
Common-Mode
∆VOUT,dm/∆VIN,cm; ∆VIN,cm = ± 1 V;
Resistors Matched to 0.01%
Maximum ∆VOUT; Single-Ended Output
± 3.5
7
–60
mV
µV/°C
µA
MΩ
MΩ
pF
V
dB
∆VOUT,cm/∆VOUT,dm; ∆VOUT,dm = 1 V
–3.6 to +3.6
70
–70
V
mA
dB
∆VOCM = 600 mV p-p
∆VOCM = –1 V to +1 V
210
400
MHz
V/µs
VOS,cm = VOUT,cm; VDIN+ = VDIN– = VOCM = 0 V
± 3.6
150
± 1.5
0.5
–68
±7
V
kΩ
mV
µA
dB
1
1.015
V/V
± 5.5
13
–60
V
mA
µA/°C
dB
+85
°C
VOCM to OUT Specifications
DYNAMIC PERFORMANCE
–3 dB Bandwidth
Slew Rate
DC PERFORMANCE
Input Voltage Range
Input Resistance
Input Offset Voltage
Input Bias Current
VOCM CMRR
Gain
POWER SUPPLY
Operating Range
Quiescent Current
Power Supply Rejection Ratio
[∆VOUT,dm/∆VOCM]; ∆VOCM = ± 1 V;
Resistors Matched to 0.01%
∆VOUT,cm/∆VOCM; ∆VOCM = ± 1 V
VDIN+ = VDIN– = VOCM = 0 V
TMIN to TMAX Variation
∆VOUT,dm/∆VS; ∆VS = ± 1 V
OPERATING TEMPERATURE RANGE
0.985
± 1.35
11
–40
12
16
–70
Specifications subject to change without notice.
–2–
REV. 0
AD8132
AD8132–SPECIFICATIONS
(@ 25C, VS = 5 V, VOCM = 2.5 V, G = 1, RL,dm = 499 , RF = RG = 348 unless
otherwise noted. For G = 2, RL,dm = 200 , RF = 1000 , RG = 499 . Refer to TPC 1 and TPC 10 for test setup and label descriptions. All
specifications refer to single-ended input and differential outputs unless otherwise noted.)
Parameter
Conditions
Min
Typ
Max
Unit
VOUT = 2 V p-p
VOUT = 2 V p-p, G = 2
VOUT = 0.2 V p-p
VOUT = 0.2 V p-p, G = 2
VOUT = 0.2 V p-p
VOUT = 0.2 V p-p, G = 2
VOUT = 2 V p-p
0.1%, VOUT = 2 V p-p
VIN = 2.5 V to 0 V Step, G = 2
250
300
180
360
155
65
50
1000
20
5
MHz
MHz
MHz
MHz
MHz
MHz
V/µs
ns
ns
VOUT = 2 V p-p, 1 MHz, RL,dm = 800 Ω
VOUT = 2 V p-p, 5 MHz, RL,dm = 800 Ω
VOUT = 2 V p-p, 20 MHz, RL,dm = 800 Ω
VOUT = 2 V p-p, 1 MHz, RL,dm = 800 Ω
VOUT = 2 V p-p, 5 MHz, RL,dm = 800 Ω
VOUT = 2 V p-p, 20 MHz, RL,dm = 800 Ω
20 MHz, RL,dm = 800 Ω
20 MHz, RL,dm = 800 Ω
f = 0.1 MHz to 100 MHz
f = 0.1 MHz to 100 MHz
NTSC, G = 2, RL,dm = 150 Ω
NTSC, G = 2, RL,dm = 150 Ω
–97
–100
–74
–100
–99
–67
–76
40
8
1.8
0.025
0.15
dBc
dBc
dBc
dBc
dBc
dBc
dBc
dBm
nV/√Hz
pA/√Hz
%
Degree
VOS,dm = VOUT,dm/2; VDIN+ = VDIN– = VOCM = 2.5 V
TMIN to TMAX Variation
± 1.0
6
3
10
3
1
–1 to +4
–70
DIN to OUT Specifications
DYNAMIC PERFORMANCE
–3 dB Large Signal Bandwidth
–3 dB Small Signal Bandwidth
Bandwidth for 0.1 dB Flatness
Slew Rate
Settling Time
Overdrive Recovery Time
NOISE/HARMONIC PERFORMANCE
Second Harmonic
Third Harmonic
IMD
IP3
Input Voltage Noise (RTI)
Input Current Noise
Differential Gain Error
Differential Phase Error
INPUT CHARACTERISTICS
Offset Voltage (RTI)
Input Bias Current
Input Resistance
Input Capacitance
Input Common-Mode Voltage
CMRR
OUTPUT CHARACTERISTICS
Output Voltage Swing
Output Current
Output Balance Error
800
Differential
Common-Mode
∆VOUT,dm/∆VIN,cm; ∆VIN,cm = ± 1 V;
Resistors Matched to 0.01%
Maximum ∆VOUT; Single-Ended Output
± 3.5
7
–60
mV
µV/°C
µA
MΩ
MΩ
pF
V
dB
∆VOUT,cm/∆VOUT,dm; ∆VOUT,dm = 1 V
1 to 3.7
50
–68
V
mA
dB
∆VOCM = 600 mV p-p
∆VOCM = 1.5 V to 3.5 V
210
340
MHz
V/µs
VOS,cm = VOUT,cm; VDIN+ = VDIN– = VOCM = 2.5 V
1 to 3.7
130
±5
0.5
–66
± 11
V
kΩ
mV
µA
dB
1
1.015
V/V
11
12
–60
V
mA
µA/°C
dB
+85
°C
VOCM to OUT Specifications
DYNAMIC PERFORMANCE
–3 dB Bandwidth
Slew Rate
DC PERFORMANCE
Input Voltage Range
Input Resistance
Input Offset Voltage
Input Bias Current
VOCM CMRR
Gain
POWER SUPPLY
Operating Range
Quiescent Current
Power Supply Rejection Ratio
[∆VOUT,dm/∆VOCM]; ∆VOCM = 2.5 ± 1 V;
Resistors Matched to 0.01%
∆VOUT,cm/∆VOCM; ∆VOCM = 2.5 ± 1 V
VDIN+ = VDIN– = VOCM = 2.5 V
TMIN to TMAX Variation
∆VOUT,dm/∆VS; ∆VS = ± 1 V
OPERATING TEMPERATURE RANGE
2.7
9.4
–40
Specifications subject to change without notice.
REV. 0
0.985
–3–
10.7
10
–70
AD8132–SPECIFICATIONS
(@ 25C, VS = 3 V, VOCM = 1.5 V, G = 1, RL,dm = 499 , RF = RG = 348 unless
otherwise noted. For G = 2, RL,dm = 200 , RF = 1000 , RG = 499 . Refer to TPC 1 and TPC 10 for test setup and label descriptions. All
specifications refer to single-ended input and differential outputs unless otherwise noted.)
Parameter
Conditions
Min
Typ
Max
Unit
DIN to OUT Specifications
DYNAMIC PERFORMANCE
–3 dB Large Signal Bandwidth
–3 dB Small Signal Bandwidth
Bandwidth for 0.1 dB Flatness
NOISE/HARMONIC PERFORMANCE
Second Harmonic
Third Harmonic
INPUT CHARACTERISTICS
Offset Voltage (RTI)
Input Bias Current
CMRR
VOUT = 1 V p-p
VOUT = 1 V p-p, G = 2
VOUT = 0.2 V p-p
VOUT = 0.2 V p-p, G = 2
VOUT = 0.2 V p-p
VOUT = 0.2 V p-p, G = 2
350
165
350
150
45
50
MHz
MHz
MHz
MHz
MHz
MHz
VOUT = 1 V p-p, 1 MHz, RL,dm = 800 Ω
VOUT = 1 V p-p, 5 MHz, RL,dm = 800 Ω
VOUT = 1 V p-p, 20 MHz, RL,dm = 800 Ω
VOUT = 1 V p-p, 1 MHz, RL,dm = 800 Ω
VOUT = 1 V p-p, 5 MHz, RL,dm = 800 Ω
VOUT = 1 V p-p, 20 MHz, RL,dm = 800 Ω
–100
–94
–77
–90
–85
–66
dBc
dBc
dBc
dBc
dBc
dBc
VOS,dm = VOUT,dm/2; VDIN+ = VDIN– = VOCM = 1.5 V
± 10
3
–60
mV
µA
dB
±7
1
mV
V/V
∆VOUT,dm/∆VIN,cm; ∆VIN,cm = ± 0.5 V;
Resistors Matched to 0.01%
VOCM to OUT Specifications
DC PERFORMANCE
Input Offset Voltage
Gain
VOS,cm = VOUT,cm; VDIN+ = VDIN– = VOCM = 1.5 V
∆VOUT,cm/∆VOCM; ∆VOCM = ± 0.5 V
POWER SUPPLY
Operating Range
Quiescent Current
Power Supply Rejection Ratio
VDIN+ = VDIN– = VOCM = 0 V
∆VOUT,dm/∆VS; ∆VS = ± 0.5 V
2.7
OPERATING TEMPERATURE RANGE
11
V
mA
dB
+85
°C
7.25
–70
–40
Specifications subject to change without notice.
–4–
REV. 0
AD8132
ABSOLUTE MAXIMUM RATINGS 1, 2
Supply Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ± 5.5 V
VOCM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ± VS
Internal Power Dissipation . . . . . . . . . . . . . . . . . . . . 250 mW
Operating Temperature Range . . . . . . . . . . . –40°C to +85°C
Storage Temperature Range . . . . . . . . . . . . –65°C to +150°C
Lead Temperature (Soldering 10 sec) . . . . . . . . . . . . . 300°C
NOTES
1
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 listed in the operational section of this
specification is not implied. Exposure to Absolute Maximum Ratings for any
extended periods may affect device reliability.
2
Thermal resistance measured on SEMI standard 4-layer board.
8-Lead SOIC: θJA = 121°C/W
8-Lead µSOIC: θJA = 142°C/W
PIN FUNCTION DESCRIPTIONS
Pin No. Name
1
2
Function
–IN
VOCM
Negative Input.
Voltage applied to this pin sets the commonmode output voltage with a ratio of 1:1. For
example, 1 V dc on VOCM will set the dc bias
level on +OUT and –OUT to 1 V.
V+
Positive Supply Voltage.
+OUT Positive Output. Note: the voltage at –DIN is
inverted at +OUT.
–OUT Negative Output. Note: the voltage at +DIN
is inverted at –OUT.
V–
Negative Supply Voltage.
NC
No Connect.
+IN
Positive Input.
3
4
5
6
7
8
MAXIMUM POWER DISSIPATION – Watts
2.0
1.5
8-LEAD SOIC
PACKAGE
PIN CONFIGURATION
TJ = 150C
AD8132
–IN 1
8 +IN
7 NC
VOCM 2
1.0
V+ 3
8-LEAD
microSOIC
+OUT 4
0.5
+
6 V–
5 –OUT
NC = NO CONNECT
0
–50 –40 –30 –20 –10 0 10 20 30 40 50 60 70
AMBIENT TEMPERATURE – C
80 90
Figure 2. Plot of Maximum Power Dissipation vs.
Temperature
ORDERING GUIDE
Model
Temperature Range
Package Description
Package Option
AD8132AR
AD8132AR-REEL1
AD8132AR-REEL72
AD8132ARM
AD8132ARM-REEL3
AD8132ARM-REEL72
AD8132-EVAL
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
8-Lead SOIC
13" Tape and Reel
7" Tape and Reel
8-Lead µSOIC
13" Tape and Reel
7" Tape and Reel
Evaluation Board
SO-8
SM-8
NOTES
1
13" Reels of 2500 each.
2
7" Reels of 1000 each.
3
13" Reels of 3000 each.
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 AD8132 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. 0
–5–
WARNING!
ESD SENSITIVE DEVICE
AD8132–Typical Performance Characteristics
0.5
2
CF
348
24.9
0.3
348
VS = 5V
–1
–2
–3
348
–5
0.0
–0.1
–0.3
–0.4
–0.5
1
VS = 5V
–0.2
10
100
1k
1
TPC 2. Small Signal Frequency
Response
0.2
3
VS = 3V
0.1
–0.2
VS AS SHOWN
G=1
VO,dm = 0.2V p-p
RL,dm = 499
–0.4
1
10
0
–1
VS = 3V
–2
VS AS SHOWN
G=1
VO,dm = 2V p-p FOR VS = 5V, 5V
VO,dm = 1V p-p FOR VS = 3V
RL,dm = 499
–3
–4
100
–5
1k
1
1
0
0
GAIN – dB
GAIN – dB
2
–40C
–1
–2
VS = 5V
G=1
VO,dm = 2V p-p
RL,dm = 499
TEMPERATURE AS SHOWN
1
10
100
10
100
VS AS SHOWN
G=1
VO,dm = 2V p-p FOR VS = 5V, 5V
VO,dm = 1V p-p FOR VS = 3V
RL,dm = 499
–4
1k
1
TPC 7. Large Signal Response vs.
Temperature
TPC 6. Large Signal Frequency
Response; CF = 0.5 pF
RF = 348
RF = 249
–2
VS = 5V
G=1
VO,dm = 2V p-p
RL,dm = 499
RF AS SHOWN
–4
1k
1k
100
–3
FREQUENCY – MHz
100
RF = 499
–1
–5
10
FREQUENCY – MHz
3
2
–4
VS = 3V
–2
–5
1
VS = 5V
–1
–3
TPC 5. Large Signal Frequency
Response; CF = 0 pF
+85C
+25C
–3
VS = 5V
FREQUENCY – MHz
TPC 4. 0.1 dB Flatness vs. Frequency;
CF = 0.5 pF
3
VS = 3V
0
FREQUENCY – MHz
–5
1
IMPEDANCE – –0.3
–0.5
VS = 3V
VS = 5V
GAIN – dB
GAIN – dB
GAIN – dB
VS = 5V
–0.1
1k
2
1
0.0
100
TPC 3. 0.1 dB Flatness vs. Frequency;
CF = 0 pF
VS = 5V
2
VS = 5V
10
FREQUENCY – MHz
FREQUENCY – MHz
TPC 1. Basic Test Circuit, G = 1
VS = 5V
0.1
VS AS SHOWN
G=1
VO,dm = 0.2V p-p
RL,dm = 499
–4
CF
VS = 3V
0.2
GAIN – dB
499
GAIN – dB
0.1F
VS AS SHOWN
G=1
VO,dm = 0.2V p-p
RL,dm = 499
0.4
VS = 5V
0
348
49.9
VS = 3V
1
1
10
1
VS = 5V
VS = 5V
10
100
FREQUENCY – MHz
TPC 8. Large Signal Frequency
Response vs. RF
–6–
1k
0.1
1
10
FREQUENCY – MHz
100
TPC 9. Closed-Loop Single-Ended
ZOUT vs. Frequency; G = 1
REV. 0
AD8132
7
6.1
6
6.0
1000
24.9
200
5.9
GAIN – dB
0.1F
GAIN – dB
49.9
VS = 5V,
+5V
5
499
4
VS = 3V
3
499
1000
5.7
VS AS SHOWN
G=2
VO,dm = 0.2V p-p
RL,dm = 200
2
1
5.8
1
VS = 3V, 5V, 5V
G=2
VO,dm = 0.2V p-p
RL,dm = 200
5.6
10
100
5.5
1k
1
10
TPC 10. Basic Test Circuit, G = 2
7
RF = 1.5k
VS = 5V, 5V
6
6
RF
VS = 3V
4
GAIN – dB
GAIN – dB
RF = 1.0k
5
5
VS AS SHOWN
G=2
VO,dm = 2V p-p FOR
VS = 5V, 5V
VO,dm = 1V p-p FOR
VS = 3V
RL,dm = 200
3
2
1
10
499
RF = 499
VS = 5V
G=2
VO,dm = 0.2V p-p
RL,dm = 200
RF AS SHOWN
3
2
100
1k
49.9
4
1
1
24.9
TPC 13. Large Signal Frequency
Response
200
499
RF
10
100
1k
TPC 14. Small Signal Frequency
Response vs. RF
TPC 15. Test Circuit for Various
Gains
25
–25
G = 10, RF = 4.99k
RF
RG
15 G = 5, RF = 2.49k
49.9
10
G = 2, RF = 1k
RL
0.1F
RL
5
G = 1, RF = 499
0
24.9
–10
1
10
RG
RF
VS = 5V
VO, dm = 2V p-p
RL, dm = 200
RG = 499
–5
G = 1: RF = RG = 348, RL = 249 (RL,dm = 498)
G = 2: RF = 1000, RG = 499, RL = 100
(RL,dm = 200)
100
–40
–45
G=1
–50
–55
–60
G=2
–65
–70
1
10
100
FREQUENCY – MHz
FREQUENCY – MHz
REV. 0
–35
–75
1k
TPC 16. Large Signal Response for
Various Gains
VS = 5V
GAIN AS SHOWN
VOUT,dm = 2V p-p
VOUT,cm/VOUT,dm
–30
RTI BALANCE ERROR – dB
20
GAIN – dB
0.1F
FREQUENCY – MHz
FREQUENCY – MHz
–15
1k
TPC 12. 0.1 dB Flatness vs.
Frequency
TPC 11. Small Signal Frequency
Response
7
1
100
FREQUENCY – MHz
FREQUENCY – MHz
TPC 17. Test Circuit for Output
Balance
–7–
TPC 18. RTI Output Balance
Error vs. Frequency
1k
AD8132
348
2:1 TRANSFORMER
348
300
HPF
ZIN = 50
LPF
49.9
0.1F
348
24.9
300
348
TPC 19. Harmonic Distortion Test Circuit,
G = 1, RL,dm = 800 Ω
–40
HD3 (VS = 3V)
–50
HD2 (VS = 3V)
–70
–80
HD2 (VS = 5V)
–90
–50
HD2 (VS = 5V)
–60
HD3 (VS = 5V)
–70
HD2 (VS = 5V)
–80
–90
–100
–110
10
20
30
40
50
60
70
0
10
20
30
40
50
60
TPC 20. Harmonic Distortion vs.
Frequency, G = 1
TPC 21. Harmonic Distortion vs.
Frequency, G = 1
–40
–40
HD3 (F = 20MHz)
–60
–70
HD2 (F = 20MHz)
–80
HD2 (F = 5MHz)
–90
–100
HD3 (F = 5MHz)
DISTORTION – dBc
–50
HD2 (F = 20MHz)
–70
–80
–90
HD3 (F = 5MHz)
–110
0.25
0.50
0.75
1.00
1.25
1.50
1.75
DIFFERENTIAL OUTPUT VOLTAGE – V p-p
70
FREQUENCY – MHz
FREQUENCY – MHz
VS = 5V
RL,dm = 800
HD3 (F = 20MHz)
HD2 (F = 5MHz)
TPC 22. Harmonic Distortion vs.
Differential Output Voltage, G = 1
–50
–50
VS = 5V
RL,dm = 800
–60
HD2 (F = 20MHz)
HD3 (F = 20MHz)
–60
–70
–80
HD2 (F = 5MHz)
–90
DISTORTION – dBc
0
–60
VS = 3V
RL,dm = 800
–100
–100
HD3 (VS = 5V)
DISTORTION – dBc
HD3 (VS = 5V)
DISTORTION – dBc
–60
RL,dm = 800
–40 VOUT,dm = 2V p-p
DISTORTION – dBc
DISTORTION – dBc
–50
–110
–40
–30
RL,dm = 800
VOUT,dm = 1V p-p
–70
VS = 3V
VO,dm = 1V p-p
HD3 (F = 20MHz)
HD2 (F = 20MHz)
–80
–90
HD3 (F = 5MHz)
–100
–100
HD2 (F = 5MHz)
HD3 (F = 5MHz)
–110
0
1
2
3
4
DIFFERENTIAL OUTPUT VOLTAGE – V p-p
TPC 23. Harmonic Distortion vs.
Differential Output Voltage, G = 1
–110
0
1
2
3
4
5
6
DIFFERENTIAL OUTPUT VOLTAGE – V p-p
TPC 24. Harmonic Distortion vs.
Differential Output Voltage, G = 1
–8–
–110
200 300 400 500 600 700
RLOAD – 800 900 1000
TPC 25. Harmonic Distortion vs.
RLOAD, G = 1
REV. 0
AD8132
–50
VS = 5V
VOUT,dm = 2V p-p
DISTORTION – dBc
–60
–60
HD3 (F = 20MHz)
DISTORTION – dBc
–50
–70
–80
HD2 (F = 20MHz)
HD2 (F = 5MHz)
–90
–100
HD3 (F = 5MHz)
VS = 5V
VOUT,dm = 2V p-p
HD3 (F = 20MHz)
–70
HD2 (F = 20MHz)
–80
HD2 (F = 5MHz)
–90
–100
–110
200 300 400 500 600 700
RLOAD – HD3 (F = 5MHz)
–110
200 300 400 500 600 700 800 900 1000
RLOAD – 800 900 1000
TPC 27. Harmonic Distortion vs.
RLOAD, G = 1
TPC 26. Harmonic Distortion vs.
RLOAD, G = 1
1000
2:1 TRANSFORMER
499
300
HPF
ZIN = 50
LPF
49.9
24.9
0.1F
499
300
1000
TPC 28. Harmonic Distortion Test Circuit, G = 2, RL,dm = 800 Ω
HD3 (VS = 3V)
–60
DISTORTION – dBc
DISTORTION – dBc
–50
–70
HD2 (VS = 5V)
–80
–90
HD2 (VS = 3V)
–30
RL,dm = 800
VOUT,dm = 4V p-p
–40
HD2 (VS = 5V)
–50
HD3 (VS = 5V)
–60
–70
10
20
30
40
50
FREQUENCY – MHz
HD2 (F = 20MHz)
–80
–90
HD2 (F = 5MHz)
–110
HD3 (F = 5MHz)
60
70
TPC 29. Harmonic Distortion vs.
Frequency, G = 2
REV. 0
–70
–100
HD2 (VS = 5V)
–100
0
HD3 (F = 20MHz)
–60
–90
HD3 (VS = 5V)
VS = 5V
RL,dm = 800
HD3 (VS = 5V)
–50
–80
–100
–110
–40
–20
RL,dm = 800
VOUT,dm = 1V p-p
DISTORTION – dBc
–40
–120
0
10
20 30
40
50
60
FREQUENCY – MHz
70
80
TPC 30. Harmonic Distortion vs.
Frequency, G = 2
–9–
0
2
1
3
4
DIFFERENTIAL OUTPUT VOLTAGE – V p-p
TPC 31. Harmonic Distortion vs.
Differential Output Voltage, G = 2
AD8132
VS = 5V
VOUT,dm = 2V p-p
HD3 (F = 20MHz)
–70
–80
–90
VS = 5V
VOUT,dm = 2V p-p
HD3 (F = 20MHz)
–60
HD2 (F = 20MHz)
–70
HD2 (F = 20MHz)
–80
HD2 (F = 5MHz)
–90
DISTORTION – dBc
HD2 (F = 20MHz)
–60
HD3 (F = 20MHz)
–60
DISTORTION – dBc
VS = 5V
RL,dm = 800
–50
DISTORTION – dBc
–50
–50
–40
–70
–80
HD2 (F = 5MHz)
–90
HD3 (F = 5MHz)
–100
–110
0
4
1
2
3
5
6
DIFFERENTIAL OUTPUT VOLTAGE – V p-p
–110
200 300 400 500 600 700 800 900 1000
RLOAD – TPC 33. Harmonic Distortion vs.
RLOAD, G = 2
45
10
fC = 20MHz
VS = 5V
RL,dm = 800
INTERCEPT – dBm (Re:50)
POUT – dBm (Re:50)
–10
HD3 (F = 5MHz)
HD3 (F = 5MHz)
TPC 32. Harmonic Distortion vs.
Differential Output Voltage, G = 2
0
–100
–100
HD2 (F = 5MHz)
–20
–30
–40
–50
–60
–70
–110
200 300 400 500 600 700 800 900 1000
RLOAD – TPC 34. Harmonic Distortion vs.
RLOAD, G = 2
VS = 5V, 5V
RL,dm = 800
VS = 5V, 5V, 3V
40
35
30
25
20
–80
40mV
–90
19.5
15
20
FREQUENCY – MHz
20.5
TPC 35. Intermodulation
Distortion, G = 1
10
20
30
40
50
FREQUENCY – MHz
60
CF = 0pF
TPC 37. Small Signal Transient
Response, G = 1
VS = 5V
VOUT,dm = 2V p-p
VS = 5V
VOUT,dm = 2V p-p
CF = 0pF
CF = 0.5pF
CF = 0.5pF
300mV
5ns
70
TPC 36. Third Order Intercept vs.
Frequency, G = 1
VS = 3V
VOUT,dm = 1.5V p-p
CF = 0pF
0
5ns
TPC 38. Large Signal Transient
Response, G = 1
400mV
CF = 0.5pF
5ns
TPC 39. Large Signal Transient
Response, G = 1
–10–
400mV
5ns
TPC 40. Large Signal Transient
Response, G = 1
REV. 0
AD8132
VS = 5, 5, 3V
VS = 3V
VOUT,dm
VOUT–
VOUT+
V+DIN
1V
40mV
5ns
TPC 41. Large Signal Transient
Response, G = 1
5ns
TPC 42. Small Signal Transient
Response, G = 2
5ns
300mV
TPC 43. Large Signal Transient
Response, G = 2
VS = 5V
VS = 5, 5V
VS = 5V
G=1
VO,dm = 2V p-p
RL,dm = 499
VOUT,dm
0.1%/DIV
VOUT–
VOUT+
V+DIN
400mV
5ns
1V
5ns
2mV
0
TPC 44. Large Signal Transient
Response, G = 2
TPC 45. Large Signal Transient
Response, G = 2
0
–10
CL = 5pF
348
CL = 20pF
CL
PSRR – dB
49.9
0.1F
453
24.9
40
VOUT,dm
VS
–PSRR
+PSRR (VS = 5V, 5V)
–30 –PSRR (V = 5V)
S
–50
–70
5ns
400mV
–80
–90
0.1
TPC 47. Test Circuit for Cap Load
Drive
+PSRR
–40
–60
348
REV. 0
5ns
30 35
–20
24.9
348
24.9
10 15 20 25
5ns/DIV
TPC 46. 0.1% Settling Time
CL = 0pF
348
5
TPC 48. Large Signal Transient
Response for Various Capacitor
Loads
–11–
1
10
100
FREQUENCY – MHz
1000
TPC 49. PSRR vs. Frequency
AD8132
–20
6
VS = 5V
VIN,cm = 2V p-p
–30
348
348
VOUT, cm
VOCM = 600mV p-p
VOCM = 2V p-p
–3
dB
VOUT,dm
49.9
CMRR – dB
249
VS = 5V
0
VOUT,cm
VIN,cm
–40
348
VOUT,cm
VOCM
3
–50
–6
–60
249
348
–9
VOUT,dm
VIN,cm
–70
–12
NOTE: RESISTORS MATCHED TO 0.01%.
–80
–10
VOUT,dm
VOCM
VOCM CMRR – dB
–20
1000
TPC 52. VOCM Gain Response
VOCM = 2V p-p
–40
–50
–60
100
8nV/ Hz
10
5ns
TPC 53. VOCM Transient Response
1
10
100
FREQUENCY – MHz
1
10
1000
TPC 54. VOCM CMRR vs. Frequency
100
1k
10k 100k 1M
FREQUENCY – Hz
10M 100M
TPC 55. Input Voltage Noise vs.
Frequency
15
1000
VOUT,dm (0.5V/DIV)
SUPPLY CURRENT – mA
INPUT CURRENT NOISE – pA/ Hz
10
100
FREQUENCY – MHz
1000
–30
–80
VIN,sm (1V/DIV)
100
10
1.8pA/ Hz
1
10
1
VOCM = 600mV p-p
–70
400mV
–15
1000
TPC 51. CMRR vs. Frequency
VS = 5V
VOCM = –1V TO +1V
VOUT,cm
10
100
FREQUENCY – MHz
INPUT VOLTAGE NOISE – nV/ Hz
TPC 50. CMRR Test Circuit
1
100
1k
10k 100k 1M
FREQUENCY – Hz
VS = 5V
VIN = 2.5V STEP
G=2
RF = 1k
RL,dm = 200
V/DIV AS SHOWN
VS = 5V
11
VS = 5V
9
7
5ns
5
–50
10M 100M
TPC 56. Input Current Noise vs.
Frequency
13
TPC 57. Overdrive Recovery
–12–
–30
–10
10
30
50
TEMPERATURE – C
70
90
TPC 58. Quiescent Current vs.
Temperature
REV. 0
AD8132
DIFFERENTIAL OUTPUT OFFSET – mV
0
VS = 5V
–0.5
–1.0
VS = 5V
–1.5
–2.0
–2.5
–40
–20
0
20
40
60
TEMPERATURE – C
80
100
TPC 59. Differential Offset Voltage vs. Temperature
Table I indicates the gain from any type of input to either type
of output.
OPERATIONAL DESCRIPTION
Definition of Terms
CF
Table I. Differential and Common-Mode Gains
RF
+DIN
RG
+IN
AD8132
VOCM
–DIN
–OUT
RG
–IN
RL, dm
VOUT, dm
+OUT
RF
CF
Figure 3. Circuit Definitions
Differential voltage refers to the difference between two node
voltages. For example, the output differential voltage (or
equivalently output differential-mode voltage) is defined as:
VOUT,dm = (V+OUT – V–OUT)
V+OUT and V–OUT refer to the voltages at the +OUT and –OUT
terminals with respect to a common reference.
Common-mode voltage refers to the average of two node voltages. The output common-mode voltage is defined as:
VOUT,cm = (V+OUT + V–OUT)/2
Basic Circuit Operation
One of the more useful and easy to understand ways to use the
AD8132 is to provide two equal-ratio feedback networks. To
match the effect of parasitics, these networks should actually be
comprised of two equal-value feedback resistors, RF and two
equal-value gain resistors, RG. This circuit is diagrammed in
Figure 3.
Like a conventional op amp, the AD8132 has two differential
inputs that can be driven with both a differential-mode input
voltage, VIN,dm, and a common-mode input voltage, VIN,cm.
There is another input, VOCM, which is not present on conventional op amps, but provides another input to consider on the
AD8132. It is totally separate from the above inputs.
There are two complementary outputs whose response can be
defined by a differential-mode output, VOUT,dm and a commonmode output, VOUT,cm.
REV. 0
Input
VOUT,dm
VOUT,cm
VIN,dm
VIN,cm
VOCM
RF/RG
0
0
0 (By Design)
0 (By Design)
1 (By Design)
The differential output (VOUT,dm) is equal to the differential
input voltage (VIN,dm) times RF/RG. In this case, it does not
matter if both differential inputs are driven, or only one output
is driven and the other is tied to a reference voltage, like ground.
As can be seen from the two zero entries in the first column,
neither of the common-mode inputs has any effect on this gain.
The gain from VIN,dm to VOUT,cm is 0 and to first order does not
depend on the ratio matching of the feedback networks. The
common-mode feedback loop within the AD8132 provides a
corrective action to keep this gain term minimized. The term
“balance error” describes the degree to which this gain term
differs from zero.
The gain from VIN,cm to VOUT,dm does directly depend on the
matching of the feedback networks. The analogous term for this
transfer function, which is used in conventional op amps, is
“common-mode rejection ratio” or CMRR. Thus, if it is desirable
to have a high CMRR, the feedback ratios must be well matched.
The gain from VIN,cm to VOUT,cm is also ideally 0, and is firstorder independent of the feedback ratio matching. As in the
case of VIN,dm to VOUT,cm, the common-mode feedback loop
keeps this term minimized.
The gain from VOCM to VOUT,dm is ideally 0 only when the feedback ratios are matched. The amount of differential output
signal that will be created by varying VOCM is related to the
degree of mismatch in the feedback networks.
VOCM controls the output common-mode voltage VOUT,cm with a
unity-gain transfer function. With equal-ratio feedback networks
(as assumed above), its effect on each output will be the same,
which is another way to say that the gain from VOCM to VOUT,dm
is zero. If not driven, the output common-mode will be at midsupplies. It is recommended that a 0.1 µF bypass capacitor be
connected to VOCM.
–13–
AD8132
When unequal feedback ratios are used, the two gains associated
with VOUT,dm become nonzero. This significantly complicates
the mathematical analysis along with any intuitive understanding of how the part operates. Some of these configurations will
be in another section.
THEORY OF OPERATION
The AD8132 differs from conventional op amps by the external
presence of an additional input and output. The additional
input, VOCM, controls the output common-mode voltage. The
additional output is the analog complement of the single output
of a conventional op amp. For its operation, the AD8132 makes
use of two feedback loops as compared to the single loop of
conventional op amps. While this provides significant freedom
to create various novel circuits, basic op amp theory can still be
used to analyze the operation.
One of the feedback loops controls the output common-mode
voltage, VOUT,cm. Its input is VOCM (Pin 2) and the output is the
common-mode, or average voltage, of the two differential outputs
(+OUT and –OUT). The gain of this circuit is internally set to
unity. When the AD8132 is operating in its linear region, this
establishes one of the operational constraints: VOUT,cm = VOCM.
The second feedback loop controls the differential operation.
Similar to an op amp, the gain and gain-shaping of the transfer
function is controllable by adding passive feedback networks. However, only one feedback network is required to “close the loop” and
fully constrain the operation. But depending on the function
desired, two feedback networks can be used. This is possible as
a result of having two outputs that are each inverted with respect
to the differential inputs.
General Usage of the AD8132
Several assumptions are made here for a first-order analysis, which
are the typical assumptions used for the analysis of op amps:
• The input impedances are arbitrarily large and their loading
effect can be ignored.
• The input bias currents are sufficiently small so they can be
neglected.
• The open-loop gain is arbitrarily large, which drives the
amplifier to a state where the input differential voltage is
effectively zero.
This expression is not very intuitive, but some further examples
can provide better understanding of its implications. One observation that can be made right away is that a tolerance error in β1
does not have the same effect on gain as the same tolerance
error in β2.
Resistorless Differential Amplifier (High Input Impedance
Inverting Amplifier)
The simplest closed-loop circuit that can be made does not require
any resistors and is shown in Figure 7. In this circuit, β1 is equal
to zero, and β2 is equal to one. The gain is equal to two.
A more intuitive means to figure the gain is by simple inspection. +OUT is connected to –IN, whose voltage is equal to the
voltage at +IN under equilibrium conditions. Thus, +VOUT is
equal to VIN, and there is unity gain in this path. Since –OUT
has to swing in the opposite direction from +OUT due to the
common-mode constraint, its effect will double the output
signal and produce a gain of two.
One useful function that this circuit provides is a high inputimpedance inverter. If +OUT is ignored, there is a unity-gain,
high-input-impedance amplifier formed from +IN to –OUT.
Most traditional op amp inverters have relatively low input
impedances, unless they are buffered with another amplifier.
VOCM has been assumed to be at midsupply. Since there is
still the constraint from the above discussion that +VOUT must
equal VIN, changing the VOCM voltage will not change +VOUT
(= VIN). Therefore, all of the effect of changing VOCM must
show up at –OUT.
The above simple configuration with β2 = 1 and its gain-of-two
is the highest gain circuit that can be made under this condition.
Since β1 was equal to zero, only higher β1 values are possible.
All of these circuits with higher values of β1 will have gains lower
than two. However, circuits with β1 equal to one are not practical,
because they have no effective input, and result in a gain of 0.
While it is possible to operate the AD8132 with a purely differential input, many of its applications call for a circuit that has a
single-ended input with a differential output.
For a single-ended-to-differential circuit, the RG of the undriven
input will be tied to a reference voltage. For now this is ground,
and other conditions will be discussed later. Also, the voltage at
VOCM, and hence VOUT,cm will be assumed to be ground for now.
Figure 4 shows a generalized schematic of such a circuit using
an AD8132 with two feedback paths.
β1 = RG1/(RG1+ RF1)
G = 2 × (1–β1)/(β1 + β2)
Other 2 = 1 Circuits
• Offset voltages are assumed to be zero.
β2 = RG2/(RG2 + RF2)
A single-ended-to-differential gain equation can be derived
which is true for all values of β1 and β2:
For example, if VOCM is raised by 1 V, then –VOUT must go up
by 2 V. This makes VOUT,cm also go up by 1 V, since it is defined
as the average of the two differential output voltages. This means
that the gain from VOCM to the differential output is two.
• The output impedances are arbitrarily low.
For each feedback network, a feedback factor can be defined,
which is the fraction of the output signal that is fed back to the
opposite-sign input. These terms are:
The feedback factor β1 is for the side that is driven, while the
feedback factor β2 is for the side that is tied to a reference voltage, (ground for now). Note also that each feedback factor can
vary anywhere between 0 and 1.
To increase β1 from zero, it is necessary to add two resistors in
a feedback network. A generalized circuit that has β1 with a
value higher than zero is shown in Figure 6. A couple of different convenient gains that can be created are a gain of 1, when
β1 is equal to 1/3, and a gain of 0.5 when β1 equals 0.6.
In all of these circuits with β2 equal to 1, VOCM serves as the
reference voltage from which to measure the input voltage and
the individual output voltages. In general, when VOCM is varied
in these circuits, a differential output signal will be generated in
addition to VOUT,cm changing the same amount as the voltage
change of VOCM.
–14–
REV. 0
AD8132
Varying 2
While the circuit above sets β2 to 1, another class of simple
circuits can be made that set β2 equal to zero. This means that
there is no feedback from +OUT to –IN. This class of circuits is
very similar to a conventional inverting op amp. However, the
AD8132 circuits have an additional output and common-mode
input which can be analyzed separately (see Figure 8).
With –IN connected to ground, +IN becomes a “virtual ground”
in the same sense that the term is used in conventional op amps.
Both inputs must maintain the same voltage for equilibrium
operation, so if one is set to ground, the other will be driven to
ground. The input impedance can also be seen to be equal to
RG, just as in a conventional op amp.
In this case, however, the positive input and negative output are
used for the feedback network. Since a conventional op amp
does not have a negative output, only its inverting input can be
used for the feedback network. The AD8132 is symmetrical, so the
feedback network on either side can be used to produce the same
results.
Since +IN is a summing junction, by analogy to conventional op
amps, the gain from VIN to –OUT will be –RF/RG. This will hold
true regardless of the voltage on VOCM. And since +OUT will
move the same amount in the opposite direction from –OUT,
the overall gain will be –2 (RF/RG).
To compute the total output referred noise for the circuit of
Figure 3, consideration must also be given to the contribution of
the resistors RF and RG. Refer to Table II for estimated output
noise voltage densities at various closed-loop gains.
Table II. Recommended Resistor Values and
Noise Performance for Specific Gains
Gain
RG RF
() ()
Bandwidth Output Noise Output Noise
–3 dB
AD8132 Only AD8132 + RG, RF
1
2
5
10
499
499
499
499
360 MHz
160 MHz
65 MHz
20 MHz
1 = 0
There is yet another class of circuits where there is no feedback
from –OUT to +IN. This is the case where β1 = 0. The resistorless
differential amplifier described above meets this condition, but
it was presented only with the condition that β2 = 1. Recall that
this circuit had a gain equal to two.
If β2 is decreased in this circuit from unity, a smaller part of
+VOUT will be fed back to –IN and the gain will increase. See
Figure 5. This circuit is very similar to a noninverting op amp
configuration, except for the presence of the additional complementary output. Therefore, the overall gain is twice that of a
noninverting op amp or 2 × (1 + RF2/RG2) or 2 × (1/ β2).
Once again, varying VOCM will not affect both outputs in the
same way, so in addition to varying VOUT,cm with unity gain,
there will also be an affect on VOUT,dm by changing VOCM.
Estimating the Output Noise Voltage
Similar to the case of a conventional op amp, the differential
output errors (noise and offset voltages) can be estimated by
multiplying the input referred terms, at +IN and –IN, by the
circuit noise gain. The noise gain is defined as:
17 nV/√Hz
26.1 nV/√Hz
53.3 nV/√Hz
98.6 nV/√Hz
Calculating an Application Circuit’s Input Impedance
RIN,dm = 2 × RG
In the case of a single-ended input signal (for example if –DIN is
grounded and the input signal is applied to +DIN), the input
impedance becomes:
RIN , dm


RG
= 
RF
1 −

2 × RG + RF

(
)







The circuit’s input impedance is effectively higher than it would
be for a conventional op amp connected as an inverter because a
fraction of the differential output voltage appears at the inputs
as a common-mode signal, partially bootstrapping the voltage
across the input resistor RG.
Input Common-Mode Voltage Range in Single Supply
Applications
The AD8132 is optimized for level-shifting “ground” referenced
input signals. For a single-ended input this would imply, for
example, that the voltage at –DIN in Figure 3 would be zero
volts when the amplifier’s negative power supply voltage (at V–)
was also set to zero volts.
Setting the Output Common-Mode Voltage
The AD8132’s VOCM pin is internally biased at a voltage approximately equal to the midsupply point (average value of the voltages
on V+ and V–). Relying on this internal bias will result in an
output common-mode voltage that is within about 100 mV of
the expected value.
In cases where more accurate control of the output common-mode
level is required, it is recommended that an external source,
or resistor divider (with RSOURCE < 10K), be used. The output
common-mode offset specified on pages 2 and 3 assume the
VOCM input is driven by a low impedance voltage source.
R 
GN = 1 +  F 
 RG 
REV. 0
16 nV/√Hz
24.1 nV/√Hz
48.4 nV/√Hz
88.9 nV/√Hz
The effective input impedance of a circuit such as that in Figure 3, at +DIN and –DIN, will depend on whether the amplifier is
being driven by a single-ended or differential signal source. For
balanced differential input signals, the input impedance (RIN,dm)
between the inputs (+DIN and –DIN) is simply:
VOCM still governs VOUT,cm, so +OUT must be the only output
that moves when VOCM is varied. Since VOUT,cm is the average
of the two outputs, +OUT must move twice as fast and in the
same direction as VOCM to create the proper VOUT,cm. Therefore,
the gain from VOCM to +OUT must be two.
In these circuits with β2 equal to zero, the gain can theoretically
be set to any value from close to zero to infinity, just as it can
with a conventional op amp in the inverting mode. However,
practical real-world limitations and parasitics will limit the range
of acceptable gains to more modest values.
499
1.0 k
2.49 k
4.99 k
–15–
AD8132
Driving a Capacitive Load
CIRCUITS
A purely capacitive load can react with the pin and bondwire
inductance of the AD8132 resulting in high frequency ringing in
the pulse response. One way to minimize this effect is to place a
small capacitor across each of the feedback resistors. The added
capacitance should be small to avoid destabilizing the amplifier.
An alternative technique is to place a small resistor in series with
the amplifier’s outputs as shown in TPC 47.
RF1
RG1
+
RG2
RF2
Figure 4. Typical Four-Resistor Feedback Circuit
LAYOUT, GROUNDING AND BYPASSING
As a high-speed part, the AD8132 is sensitive to the PCB
environment in which it has to operate. Realizing its superior
specifications requires attention to various details of good highspeed PCB design.
+
VIN
The first requirement is a good solid ground plane that covers as
much of the board area around the AD8132 as possible. The
only exception to this is that the two input pins (Pins 1 and 8)
should be kept a few mm from the ground plane, and ground
should be removed from inner layers and the opposite side of
the board under the input pins. This will minimize the stray
capacitance on these nodes and help preserve the gain flatness
vs. frequency.
RG2
RF2
Figure 5. Typical Circuit with β1 = 0
The power supply pins should be bypassed as close as possible
to the device to the nearby ground plane. Good high-frequency
ceramic chip capacitors should be used. This bypassing should
be done with a capacitance value of 0.01 µF to 0.1 µF for each
supply. Further away, low frequency bypassing should be provided
with 10 µF tantalum capacitors from each supply to ground.
RF1
RG1
+
Figure 6. Typical Circuit with β2 = 1
The signal routing should be short and direct in order to avoid
parasitic effects. Wherever there are complementary signals, a
symmetrical layout should be provided to the extent possible to
maximize the balance performance. When running differential
signals over a long distance, the traces on PCB should be close
together or any differential wiring should be twisted together to
minimize the area of the loop that is formed. This will reduce
the radiated energy and make the circuit less susceptible to
interference.
+
VIN
Figure 7. Resistorless G = 2 Circuit with β1 = 0
RF1
VIN
RG1
+
Figure 8. Typical Circuit with β2 = 0
–16–
REV. 0
AD8132
3V
3V
3V
10k
+
10F
0.1F
1V p-p
348
10k
0.1F
0.1F
60.4
348
AVDD
DIGITAL
OUTPUTS
AD9203
AD8132
0.1F
DRVDD
AINN
20pF
49.9
20pF
348
AINP
60.4
AVSS
DRVSS
24.9
348
Figure 9. AD8132 Driving AD9203, a 10-Bit 40 MSPS A/D Converter
APPLICATIONS
A/D Driver
10
FUND
–10
–20
OUTPUT – dBc
–30
In Figure 9 a 1 V p-p signal drives the input of an AD8132
configured for unity gain. Both the AD8132 and the AD9203
are powered from a single 3 V supply. A voltage divider biases
VOCM at midsupply, which in turn drives VOUT,cm to be half the
supply voltage. This is within the common-mode range of the
AD9203.
Between the A/D and the driver is a one-pole, differential filter
that helps to filter some of the noise and assists the switchedcapacitor inputs of the A/D. Each of the A/D inputs will be driven
by a 0.5 V p-p signal that goes from 1.25 V dc to 1.75 V dc.
Figure 10 is an FFT plot of the performance of the circuit when
running at a clock rate of 40 MSPS and an input frequency of
2.5 MHz.
fS = 40MHz
fIN = 2.5MHz
0
Many of the newer high-speed A/D converters are single-supply
and have differential inputs. Thus, the driver for these devices
should be able to convert from a single-ended to a differential
signal and provide output common-mode level-shifting in
addition to having low distortion and noise. The AD8132 conveniently performs these functions when driving the AD9203, a
10-bit, 40 MSPS A/D converter.
–40
–50
–60
–70
2ND
5TH
–80
3RD
–90
–110
–120
0
2.5
5.0
7.5
10.0
12.5
499
50
SOURCE
When driving a twisted pair cable, it is desirable to drive only a
pure differential signal onto the line. If the signal is purely differential (i.e., fully balanced), and the transmission line is twisted
and balanced, there will be a minimum radiation of any signal.
+5V
+
10F
1
100
2
TWISTED
PAIR
+
AD830
7
3
VOUT
4
1k
10F
+
10F
0.1F
49.9
0.1F
5
–5V
10F
+
0.1F
–5V
Figure 11. Balanced Line Driver and Receiver Using AD8132 and AD830
REV. 0
20.0
Balanced Cable Driver
49.9
523
17.5
Figure 10. FFT Response for AD8132 Driving AD9203
AD8132
0.1F
15.0
INPUT FREQUENCY – MHz
1k
49.9
9TH 8TH
7TH
–100
+5V
0.1F
6TH
4TH
–17–
AD8132
The complementary electrical fields will mostly be confined to
the space between the two twisted conductors and will not significantly radiate out from the cable. The current in the cable will
create magnetic fields that will radiate to some degree. However,
the amount of radiation is mitigated by the twists, because for
each twist, the two adjacent twists will have an opposite polarity
magnetic field. If the twist pitch is tight enough, these small
magnetic field loops will contain most of the magnetic flux,
and the magnetic far-field strength will be negligible.
Any imbalance in the differential drive signal will appear as a
common-mode signal on the cable. This is the equivalent of a
single wire that is driven with the common-mode signal. In this
case, the wire will act as an antenna and radiate. Thus, in order
to minimize radiation when driving differential twisted pair
cables, the differential drive signal should be very well balanced.
Low-Pass Differential Filter
Similar to an op amp, various types of active filters can be created with the AD8132. These can have single-ended inputs and
differential outputs, which can provide an antialias function
when driving a differential A/D converter.
Figure 14 is a schematic of a low-pass, multiple-feedback filter.
The active section contains two poles, and an additional pole is
added at the output. The filter was designed to have a –3 dB
frequency of 1 MHz. The actual –3 dB frequency was measured
to be 1.12 MHz as shown in Figure 15.
2.15k
VIN
200pF
200pF
33pF
549
Figure 14. 1 MHz, 3-Pole Differential Output Low-Pass
Multiple Feedback Filter
10
0
–10
VOUT/VIN – dB
–20
10pF
–40
–50
–70
–80
–90
10k
499
100k
1M
FREQUENCY – Hz
10M
100M
Figure 15. Frequency Response of 1 MHz Low-Pass Filter
49.9
High Common-Mode-Output-Impedance Amplifier
249
249
100
VOUT
Changing the connection to VOCM (Pin 2) can change the
common-mode from low impedance to high impedance. If
VOCM is actively set to a particular voltage, the AD8132 will try
to force VOUT,cm to the same voltage with a relatively low output
impedance. All the previous analysis assumed that this output
impedance is arbitrarily low enough to drive the load condition
in the circuit.
49.9
24.9
10pF
–30
–60
By lowering the impedance of the RG component of the feedback network at higher frequency, the gain can be increased at
high frequency. Figure 12 shows a gain-of-two line driver that
has its RGs shunted by 10 pF resistors. The effect of this is shown
in the frequency response plot of Figure 13.
499
Figure 12. Frequency Boost Circuit
20
10
However, the are some applications that benefit from a high
common-mode output impedance. This can be accomplished
with the circuit shown in Figure 16.
0
–10
RF
348
–20
–30
RG
348
–40
10
–50
RG
348
–60
–70
–80
VOUT
953
2.15k
Any length of transmission line will attenuate the signals it carries.
This effect is worse at higher frequencies than at low frequencies. One way to compensate for this is to provide an equalizer
circuit that boosts the higher frequencies in the transmitter
circuit, so that at the receive end of the cable, the attenuation
effects are diminished.
VOUT/VIN – dB
100pF
100pF
24.9
Transmit Equalizer
49.9
49.9
549
953
2k
The common-mode feedback loop in the AD8132 helps to
minimize the amount of common-mode voltage at the output,
and can therefore be used to create a well-balanced differential
line driver. Figure 11 shows an application that uses an AD8132
as a balanced line driver and AD830 as a differential receiver
configured for unity gain. This circuit was operated with 10 m
of Category 5 cable.
VIN
33pF
2k
1k
49.9
1k
49.9
10
RF
348
1
10
100
FREQUENCY – MHz
1000
Figure 13. Frequency Response for Transmit Boost Circuit
Figure 16. High Common-Mode Output Impedance Differential Amplifier
–18–
REV. 0
AD8132
VOCM is driven by a resistor divider that “measures” the output
common- mode voltage. Thus, the common-mode output voltage takes on the value that is set by the driven circuit. In this
case it comes from the center point of the termination at the
receive end of a 10 m length of Category 5 twisted pair cable.
VOCM
VDIFF
Figure 18. Transformer with High Output Impedance
Secondary
If the receive end common-mode voltage is set to “ground,” it
will be well-defined at the receive end. Any common-mode
signal that is picked up over the cable length due to noise, will
appear at the transmit end, and must be “absorbed” by the
transmitter. Thus, it is important that the transmitter have
adequate common-mode output range to absorb the full amplitude of the common-mode signal coupled onto the cable and
thus prevent clipping.
Full-Wave Rectifier
The balanced outputs of the AD8132, along with a couple of
Schottky diodes, can create a very high-speed full-wave rectifier.
Such circuits are useful for measuring ac voltages and other
computational tasks.
Another way to look at this is that the circuit performs what is
sometimes called “transformer action.” One main difference is
that the AD8132 passes dc while transformers do not.
A transformer can also be easily configured to have either a high
or low common-mode output impedance. If the transformer’s
center tap is connected to a solid voltage reference, it will set the
common-mode voltage on the secondary side of the transformer.
In this case, if one of the differential outputs is grounded, the
other output will have only half of the differential output signal.
This keeps the common-mode voltage at ground, where it is
required to be due to the center tap connection. This is analogous to the AD8132 operating with a low output impedance
common-mode. See Figure 17.
Figure 19 shows the configuration of such a circuit. Each of the
AD8132 outputs drives the anode of an HP 2835 Schottky diode.
These Schottky diodes were chosen for their high-speed operation. At lower frequencies (approximately lower than 10 MHz),
a silicon signal diode, like a 1N4148 can be used. The cathodes
of the two diodes are connected together and this output node is
connected to ground by a 50 Ω resistor.
+5V
VIN
RG1
348
RF1
348
RT1
49.9
RT2
24.9
RG2
348
5V
HP2835
RF2
348
RL
100
VOUT
–5V
10k
VOCM
VDIFF
CR1
Figure 19. Full-Wave Rectifier
Figure 17. Transformer Whose Low Output Impedance
Secondary Is Set at VOCM
The diodes should be operated such that they are slightly forwardbiased when the differential output voltage is zero. For the
Schottky diodes, this is about 400 mV. The forward biasing can
be conveniently adjusted by CR1, which, in this circuit, raises
and lowers VOUT,CM without creating a differential output voltage.
If the center tap of the secondary of a transformer is allowed to
float (or there is no center tap), the transformer will have a high
common-mode output impedance. This means that the commonmode of the secondary will be determined by what it is connected
to, and not by anything to do with the transformer itself.
One advantage of this circuit is that the feedback loop is never
momentarily opened while the diodes reverse their polarity within
the loop. This is the scheme that is sometimes used for full-wave
rectifiers that use conventional op amps. These conventional
circuits do not work well at frequencies above about 1 MHz.
If one of the differential ends of the transformer is grounded, the
other end will swing with the full output voltage. This means
that the common-mode of the output voltage is one-half of the
differential output voltage. But this shows that the common-mode
is not forced via a low impedance to a given voltage. The commonmode output voltage can easily be changed to any voltage through
its other output terminals.
The AD8132 can exhibit the same performance when one of the
outputs in Figure 16 is grounded. The other output will swing
at the full differential output voltage. The common-mode signal
is “measured” by the voltage divider across the outputs and input
to VOCM. This then drives VOUT,cm to the same level. At higher
frequencies, it is important to minimize the capacitance on the
VOCM node or else phase shifts can compromise the performance.
The voltage divider resistances can also be lowered for better
frequency response.
REV. 0
If there is not enough forward bias (VOUT,cm too low), the lower
sharp cusps of the full-wave rectified output waveform will be
rounded off. Also, as the frequency increases, there tends to be
some rounding of the lower cusps. The forward bias can be
increased to yield sharper cusps at higher frequencies.
There is not a reliable, entirely quantifiable, means to measure
the performance of a full-wave rectifier. Since the ideal waveform has periodic sharp discontinuities, it should have (mostly
even) harmonics that have no upper bound on the frequency.
However, for a practical circuit, as the frequency increases, the
higher harmonics become attenuated and the sharp cusps that
are present at low frequencies become significantly rounded.
–19–
AD8132
The circuit was run at a frequency up to 300 MHz and, while
it was still functional, the major harmonic that remained in
the output was the second. This made it look like a sine
wave at 600 MHz. Figure 20 is an oscilloscope plot of the
output when driven by a 100 MHz, 2.5 V p-p input.
1V
100mV
C3846–8–4/00 (rev. 0) 01035
Sometimes a second harmonic generator is actually useful, as
for creating a clock to oversample a DAC by a factor of two.
If the output of this circuit is run through a low-pass filter, it
can be used as a second harmonic generator.
2ns
Figure 20. Full-Wave Rectifier Response with
100 MHz Input
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
8-Lead SOIC
(SO-8)
0.1968 (5.00)
0.1890 (4.80)
0.1574 (4.00)
0.1497 (3.80)
8
5
1
4
0.2440 (6.20)
0.2284 (5.80)
PIN 1
0.0196 (0.50)
45
0.0099 (0.25)
0.0500 (1.27)
BSC
0.0688 (1.75)
0.0532 (1.35)
0.0098 (0.25)
0.0040 (0.10)
0.0192 (0.49)
0.0138 (0.35)
SEATING
PLANE
8
0.0500 (1.27)
0.0098 (0.25) 0
0.0160 (0.41)
0.0075 (0.19)
8-Lead microSOIC
(SM-8)
8
PRINTED IN U.S.A.
0.122 (3.10)
0.114 (2.90)
5
0.122 (3.10)
0.114 (2.90)
0.199 (5.05)
0.187 (4.75)
1
4
PIN 1
0.0256 (0.65) BSC
0.120 (3.05)
0.112 (2.84)
0.006 (0.15)
0.002 (0.05)
0.018 (0.46)
SEATING 0.008 (0.20)
PLANE
0.120 (3.05)
0.112 (2.84)
0.043 (1.09)
0.037 (0.94)
0.011 (0.28)
0.003 (0.08)
–20–
33
27
0.028 (0.71)
0.016 (0.41)
REV. 0