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5 –OUT
6 NC
8 INP
7 –VS
FUNCTIONAL BLOCK DIAGRAM
10kΩ
10kΩ
10kΩ
AD8476
NOTES
1. NC = NO CONNECT.
DO NOT CONNECT TO THIS PIN.
10195-001
+OUT 4
VOCM 3
10kΩ
+VS 2
Very low power
330 μA supply current
Fully differential or single-ended inputs/outputs
Differential output designed to drive precision ADCs
Drives switched capacitor and Σ-Δ ADCs
Rail-to-rail outputs
VOCM pin adjusts output common mode
Robust overvoltage up to 18 V beyond supplies
High performance
Suitable for driving 16-bit converter up to 250 kSPS
39 nV/√Hz output noise
1 ppm/°C gain drift maximum
200 μV maximum output offset
10 V/μs slew rate
5 MHz bandwidth
Single supply: 3 V to 18 V
Dual supplies: ±1.5 V to ±9 V
INN 1
FEATURES
Figure 1.
APPLICATIONS
ADC driver
Differential instrumentation amplifier building block
Single-ended-to-differential converter
Battery-powered instruments
GENERAL DESCRIPTION
The AD8476 is a very low power, fully differential precision
amplifier with integrated gain resistors for unity gain. It is an ideal
choice for driving low power, high performance ADCs as a
single-ended-to-differential or differential-to-differential
amplifier. It provides a precision gain of 1, common-mode level
shifting, low temperature drift, and rail-to-rail outputs for
maximum dynamic range.
The AD8476 also provides overvoltage protection from large
industrial input voltages up to ±23 V while operating on a dual 5 V
supply. Power dissipation on a single 5 V supply is only 1.5 mW.
The AD8476 works well with SAR, Σ-Δ, and pipeline converters.
The high current output stage of the part allows it to drive the
switched capacitor front-end circuits of many ADCs with
minimal error.
Unlike many differential drivers on the market, the AD8476 is
a high precision amplifier. With 200 µV maximum output
offset, 39 nV/√Hz noise, and −102 dB THD + N at 10 kHz, the
AD8476 pairs well with low power, high accuracy converters.
Considering its low power consumption and high precision, the
slew-enhanced AD8476 has excellent speed, settling to 16-bit
precision for 250 kSPS acquisition times.
The AD8476 is available in a space-saving 8-lead MSOP
package. It is fully specified over the −40°C to +125°C
temperature range.
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TABLE OF CONTENTS
Features .............................................................................................. 1
Theory of Operation ...................................................................... 16
Applications ....................................................................................... 1
Overview ..................................................................................... 16
Functional Block Diagram .............................................................. 1
Circuit Information .................................................................... 16
General Description ........................................................................... 1
DC Precision ............................................................................... 16
Revision History ............................................................................... 2
Input Voltage Range ................................................................... 17
Specifications..................................................................................... 3
Driving the AD8476................................................................... 17
Absolute Maximum Ratings ............................................................ 5
Power Supplies ............................................................................ 17
Thermal Resistance ...................................................................... 5
Applications Information .............................................................. 18
Maximum Power Dissipation ..................................................... 5
Typical Configuration ................................................................ 18
ESD Caution .................................................................................. 5
Single-Ended-to-Differential Conversion............................... 18
Pin Configuration and Function Descriptions ............................. 6
Setting the Output Common-Mode Voltage .......................... 18
Typical Performance Characteristics ............................................. 7
Outline Dimensions ....................................................................... 19
Terminology .................................................................................... 15
Ordering Guide .......................................................................... 19
REVISION HISTORY
11/11—Rev. 0 to Rev. A
Changes to Table 1 ............................................................................ 3
Changes to Typical Performance Characteristics ......................... 7
Added Figure 39; Renumbered Sequentially .............................. 13
Added Table 5.................................................................................. 18
Removed Low Power ADC Driving Section ............................... 19
Removed Figure 52 ......................................................................... 19
10/11—Revision 0: Initial Version
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SPECIFICATIONS
VS = +5 to ±5 V, VOCM = midsupply, VOUT = V+OUT − V−OUT, RL = 2 kΩ differential, referred to output (RTO), TA = 25°C, unless
otherwise noted.
Table 1.
Parameter
DYNAMIC PERFORMANCE
−3 dB Small Signal Bandwidth
−3 dB Large Signal Bandwidth
Slew Rate
Settling Time to 0.01%
Settling Time to 0.001%
NOISE/DISTORTION 1
THD + N
HD2
HD3
IMD3
Output Voltage Noise
Spectral Noise Density
GAIN
Gain Error
Gain Drift
Gain Nonlinearity
OFFSET AND CMRR
Differential Offset2
vs. Temperature
Average TC
vs. Power Supply (PSRR)
Common-Mode Offset2
Common-Mode Rejection
Ratio
INPUT CHARACTERISTICS
Input Voltage Range3
Impedance4
Single-Ended Input
Differential Input
Common-Mode Input
OUTPUT CHARACTERISTICS
Output Swing
Output Balance Error
Output Impedance
Capacitive Load
Short-Circuit Current Limit
VOCM CHARACTERISTICS
VOCM Input Voltage Range
VOCM Input Impedance
VOCM Gain Error
Test Conditions/Comments
Min
B Grade
Typ
Max
Min
A Grade
Typ
Max
Unit
VOUT = 200 mV p-p
VOUT = 2 V p-p
VOUT = 2 V step
VOUT = 2 V step
VOUT = 2 V step
5
1
10
1.0
1.6
5
1
10
1.0
1.6
MHz
MHz
V/µs
µs
µs
f = 10 kHz, VOUT = 2 V p-p,
22 kHz filter
f = 10 kHz, VOUT = 2 V p-p
f = 10 kHz, VOUT = 2 V p-p
f1 = 95 kHz, f2 = 105 kHz,
VOUT = 2 V p-p
f = 0.1 Hz to 10 Hz
f = 10 kHz
−102
−102
dB
−120
−122
−82
−120
−122
−82
dB
dB
dBc
6
39
1
6
39
1
µV p-p
nV/√Hz
V/V
%
ppm/°C
ppm
RL = ∞
−40°C ≤ TA ≤ +125°C
VOUT = 4 V p-p
0.02
1
0.04
1
5
50
−40°C ≤ TA ≤ +125°C
−40°C ≤ TA ≤ +125°C
VS = ±2.5 V to ±9 V
90
VIN,cm = ±5 V
90
Differential input
Single-ended input
Vcm = VS/2
−VS + 0.05
2(−VS + 0.05)
1
5
200
900
4
50
1
90
50
50
80
+VS − 0.05
−VS + 0.05
2(+V − 0.05) 2(−VS + 0.05)
13.3
20
10
VS = +5 V
VS = ±5 V
∆VOUT,cm/∆VOUT,dm
−VS + 0.125
−VS + 0.155
90
+VS − 0.14
+VS − 0.18
−VS + 1
+VS − 0.05
2(+VS − 0.05)
13.3
20
10
−VS + 0.125
−VS + 0.155
80
0.1
20
35
Per output
500
900
4
+VS − 1
V
V
kΩ
kΩ
kΩ
+VS − 0.14
+VS − 0.18
dB
Ω
pF
mA
0.1
20
35
500
µV
µV
µV/°C
dB
µV
dB
−VS + 1
+VS − 1
500
0.05
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0.05
V
kΩ
%
Parameter
POWER SUPPLY
Specified Supply Voltage
Operating Supply Voltage
Range
Supply Current
Test Conditions/Comments
Min
B Grade
Typ
Max
Min
±5
3
Over Temperature
TEMPERATURE RANGE
Specified Performance Range
VS = +5 V, TA = 25°C
VS = ±5 V, TA = 25°C
−40°C ≤ TA ≤ +125°C
−40
±5
18
300
330
400
A Grade
Typ
Max
3
330
380
500
+125
300
330
400
−40
1
Includes amplifier voltage and current noise, as well as noise of internal resistors.
Includes input bias and offset current errors.
The input voltage range is a function of the voltage supplies and ESD diodes.
4
Internal resistors are trimmed to be ratio matched but have ±20% absolute accuracy.
2
3
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Unit
18
V
V
330
380
500
μA
μA
μA
+125
°C
ABSOLUTE MAXIMUM RATINGS
THERMAL RESISTANCE
Table 2.
Parameter
Supply Voltage
Maximum Voltage at Any Input Pin
Minimum Voltage at Any Input Pin
Storage Temperature Range
Specified Temperature Range
Package Glass Transition Temperature (TG)
ESD (Human Body Model)
Rating
±10 V
+VS + 18 V
−VS – 18 V
−65°C to +150°C
−40°C to +125°C
150°C
2500 V
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.
The θJA values in Table 3 assume a 4-layer JEDEC standard
board with zero airflow.
Table 3. Thermal Resistance
Package Type
8-Lead MSOP
θJA
209.0
Unit
°C/W
MAXIMUM POWER DISSIPATION
The maximum safe power dissipation for the AD8476 is limited
by the associated rise in junction temperature (TJ) on the die. At
approximately 150°C, which is the glass transition temperature,
the properties of the plastic change. Even temporarily exceeding
this temperature limit may change the stresses that the package
exerts on the die, permanently shifting the parametric performance
of the amplifiers. Exceeding a temperature of 150°C for an
extended period may result in a loss of functionality.
ESD CAUTION
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INN 1
+VS 2
VOCM 3
8
INP
7
–VS
6
NC
5
–OUT
AD8476
TOP VIEW
(Not to Scale)
+OUT 4
NOTES
1. NC = NO CONNECT.
DO NOT CONNECT TO THIS PIN.
10195-004
PIN CONFIGURATION AND FUNCTION DESCRIPTIONS
Figure 2. 8-Lead MSOP Pin Configuration
Table 4. 8-Lead MSOP Pin Function Descriptions
Pin No.
1
2
3
4
5
6
7
8
Mnemonic
INN
+VS
VOCM
+OUT
−OUT
NC
−VS
INP
Description
Negative Input .
Positive Supply.
Output Common-Mode Adjust.
Noninverting Output.
Inverting Output.
No Connect.
Negative Supply.
Positive Input.
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TYPICAL PERFORMANCE CHARACTERISTICS
VS = +5 V, G = 1, VOCM connected to 2.5 V, RL = 2 kΩ differentially, TA = 25°C, referred to output (RTO), unless otherwise noted.
50
15
NORMALIZED TO 25°C
40
VS = ±5V
10
COMMON-MODE VOLTAGE (V)
30
CMRR (µV/V)
20
10
0
–10
–20
–30
VS = ±2.5V
5
0
–5
–10
–25
–10
5
20
35
50
65
80
95
110
125
TEMPERATURE (°C)
–15
–15
10195-005
–50
–40
0
5
10
15
Figure 6. Input Common-Mode Voltage vs. Output Voltage,
VS = ±5 V and ±2.5 V
1500
115
VS = ±5V
VS = +5V
NORMALIZED TO 25°C
110
1100
900
105
700
100
500
300
CMRR (dB)
100
–100
–300
–500
95
90
85
80
–700
–900
75
–1100
70
–1300
–25
–10
5
20
35
50
65
80
95
110
125
TEMPERATURE (°C)
65
10
10195-006
–1500
–40
100
1k
10k
100k
1M
FREQUENCY (Hz)
10195-010
OFFSET VOLTAGE (µV)
–5
OUTPUT VOLTAGE (V)
Figure 3. CMRR vs. Temperature
1300
–10
10195-008
–40
Figure 7. Common-Mode Rejection vs. Frequency
Figure 4. System Offset Temperature Drift
–20
150
VS = ±5V
NORMALIZED TO 25°C
VS = +5V
–30
100
PSRR (dB)
0
–50
–60
–70
–50
–80
–100
–150
–40
–25
–10
5
20
35
50
65
80
TEMPERATURE (°C)
Figure 5. Gain Error vs. Temperature
95
110
125
–100
100
1k
10k
100k
1M
FREQUENCY (Hz)
Figure 8. Power Supply Rejection vs. Frequency
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10M
10195-011
–90
10195-007
GAIN ERROR (µV/V)
–40
50
50
2kΩ LOAD
NO LOAD
18
45
16
40
VS = ±5V
14
35
CURRENT (mA)
12
10
8
30
25
20
6
VS = ±2.5V
15
4
1k
10k
100k
1M
10M
FREQUENCY (Hz)
5
–40
10195-012
0
100
OUTPUT VOLTAGE SWING (V)
REFERRED TO SUPPLY VOLTAGES
10195-013
OUTPUT VOLTAGE SWING (V)
REFERRED TO SUPPLY VOLTAGES
–55°C
–40°C
+25°C
+85°C
+125°C
100k
5
20
35
50
65
80
95
110
125
Figure 12. Short-Circuit Current vs. Temperature
+VS
0.025
0.050
0.075
0.100
0.125
0.150
0.175
10k
–10
TEMPERATURE (°C)
Figure 9. Maximum Output Voltage vs. Frequency
0.175
0.150
0.125
0.100
0.075
0.050
0.025
–VS
1k
–25
1M
RLOAD (Ω)
Figure 10. Output Voltage Swing vs. RLOAD vs. Temperature, VS = ±5 V
+VS
0.025
0.050
0.075
0.100
0.125
0.150
0.175
–55°C
–40°C
+25°C
+85°C
+125°C
0.175
0.150
0.125
0.100
0.075
0.050
0.025
–VS
10µA
100µA
1mA
10mA
CURRENT (A)
Figure 13. Output Voltage Swing vs. Load Current vs. Temperature,
VS = ±5 V
15
VIN
14
VOUT
12
2V/DIV
11
RISE
9
8
FALL
7
5
–40
–25
–10
5
20
35
50
65
80
TEMPERATURE (°C)
Figure 11. Slew Rate vs. Temperature
95
110
125
2µs/DIV
Figure 14. Overdrive Recovery, VS = +5 V
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10195-051
6
10195-015
SLEW RATE (V/µS)
13
10
10195-016
10
2
10195-014
MAXIMUM OUTPUT VOLTAGE (V p-p)
20
10
5
5
0
0
–5
–5
–10
–10
–15
–15
GAIN (dB)
–20
–25
–25
–30
–30
–35
–35
–40
–50
100
VS = ±5V
VS = +5V
1k
–45
10k
100k
1M
–50
100
10195-017
–45
10M
FREQUENCY (Hz)
10
5
5
0
100k
1M
10M
0
OUTPUT MAGNITUDE (dB)
–5
–10
–15
–20
–25
–30
–35
1k
–5
–10
–15
–20
–25
–30
RL = 10kΩ
RL = 2kΩ
RL = 200Ω
–35
10k
100k
1M
–40
100
10195-018
–50
100
10k
Figure 18. Large Signal Frequency Response for Various Supplies
10
–45
1k
FREQUENCY (Hz)
Figure 15. Small Signal Frequency Response for Various Supplies
–40
VS = ±5V
VS = +5V
10195-020
–40
GAIN (dB)
–20
10M
FREQUENCY (Hz)
RL = 10kΩ
RL = 2kΩ
RL = 200Ω
1k
10k
100k
1M
10M
FREQUENCY (Hz)
Figure 16. Small Signal Frequency Response for Various Loads
10195-021
GAIN (dB)
10
Figure 19. Large Signal Frequency Response for Various Loads
10
10
5
0
OUTPUT MAGNITUDE (dB)
–10
–20
–30
–50
1k
–15
–20
–25
–30
CL = 5pF
CL = 10pF
CL = 15pF
10k
–10
–35
100k
1M
FREQUENCY (Hz)
10M
100M
Figure 17. Small Signal Frequency Response for Various Capacitive Loads
–40
100
CL = 5pF
CL = 10pF
CL = 15pF
1k
10k
100k
FREQUENCY (Hz)
1M
10M
10195-101
–40
10195-019
OUTPUT MAGNITUDE (dB)
0
–5
Figure 20. Large Signal Frequency Response for Various Capacitive Loads
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5
5
0
–10
–15
–20
–25
1k
VOCM = 1.0V
VOCM = 2.5V
VOCM = 4.0V
10k
–10
–15
–20
–25
–30
100k
1M
10M
FREQUENCY (Hz)
Figure 21. Small Signal Frequency Response for Various VOCM Levels
–35
1k
100k
1M
10M
Figure 24. Large Signal Frequency Response for Various VOCM Level
5
POSITIVE OUTPUT (2kΩ LOAD)
NEGATIVE OUTPUT (2kΩ LOAD)
VS = 5V
POSITIVE OUTPUT
NEGATIVE OUTPUT
OUTPUT MAGNITUDE (dB)
0
–5
–10
–15
–20
–25
–5
–10
–15
–20
10k
100k
1M
10195-056
–25
10M
VOCM INPUT FREQUENCY (Hz)
–30
1k
10k
100k
1M
VOCM INPUT FREQUENCY (Hz)
Figure 22. VOCM Small Signal Frequency Response
Figure 25. VOCM Large Signal Frequency Response
VS = ±5V
VS = +5V
VS = +3V
Figure 23. Small Signal Pulse Response for Various Supplies
10195-029
500ns/DIV
500ns/DIV
Figure 26. Large Signal Pulse Response for Various Supplies
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10195-032
500mV/DIV
VS = ±5V
VS = +5V
VS = +3V
50mV/DIV
OUTPUT MAGNITUDE (dB)
10k
FREQUENCY (Hz)
0
–30
1k
CL = 5pF
CL = 10pF
CL = 15pF
10195-055
5
–5
10195-027
OUTPUT MAGNITUDE (dB)
–5
10195-024
OUTPUT MAGNITUDE (dB)
0
RL = 10kΩ
RL = 2kΩ
RL = 200Ω
500ns/DIV
Figure 27. Small Signal Step Response for Various Resistive Loads, VS = ±5 V
10195-033
10195-030
50mV/DIV
500mV/DIV
RL = 10kΩ
RL = 2kΩ
RL = 200Ω
500ns/DIV
Figure 30. Large Signal Step Response for Various Resistive Loads, VS = ±5 V
CL = 0pF
CL = 5pF
CL = 10pF
Figure 31. Large Signal Step Response for Various Capacitive Loads, VS = ±5 V
Figure 29. VOCM Small Signal Step Response
10µs/DIV
Figure 32. VOCM Large Signal Step Response
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10195-038
500ns/DIV
10195-035
20mV/DIV
500mV/DIV
Figure 28. Small Signal Step Response for Various Capacitive Loads, VS = ±5 V
500ns/DIV
10195-034
500ns/DIV
10195-031
50mV/DIV
500mV/DIV
CL = 0pF
CL = 5pF
CL = 10pF
140
2.5
130
SPECTRAL NOISE DENSITY (nV/ Hz)
3.0
1.5
1.0
0.5
0
–0.5
–1.0
–1.5
–2.0
120
110
100
90
80
70
60
50
40
10
20
30
40
50
60
70
80
90
100
TIME (Seconds)
20
1
–20
RL = NO LOAD
RL = NO LOAD
RL = 2kΩ LOAD
RL = 2kΩ LOAD
HARMONIC DISTORTION (dBc)
–40
–50
–60
–70
–80
–90
–100
–110
–70
–80
–90
–100
–110
–120
–130
10k
100k
1M
FREQUENCY (Hz)
–140
100
–40
–50
–20
HD2 (VS = ±5V, RL = 2kΩ)
HD3 (VS = ±5V, RL = 2kΩ)
HD2 (VS = +5V, RL = 2kΩ)
HD3 (VS = +5V, RL = 2kΩ)
–30
100k
1M
HD2,
HD3,
VS = 5V
VS = 5V
1
2
–40
–60
–70
–80
–90
–100
–110
–120
–50
–60
–70
–80
–90
–100
–110
–120
–130
–130
1k
10k
100k
1M
FREQUENCY (Hz)
Figure 35. Harmonic Distortion vs. Frequency at Various Supplies
10195-042
–140
100
10k
Figure 37. Harmonic Distortion vs. Frequency at Various VOUT,dm
HARMONIC DISTORTION (dBc)
–30
1k
FREQUENCY (Hz)
Figure 34. Harmonic Distortion vs. Frequency at Various Loads
HARMONIC DISTORTION (dBc)
–60
–130
1k
100k
–50
–120
–140
100
10k
HD2 (VOUT = 4V p-p)
HD3 (VOUT = 4V p-p)
HD2 (VOUT = 2V p-p)
HD3 (VOUT = 2V p-p)
–30
10195-040
HARMONIC DISTORTION (dBc)
–40
1k
Figure 36. Voltage Noise Density vs. Frequency
–20
–30
100
FREQUENCY (Hz)
Figure 33. 0.1 Hz to 10 Hz Voltage Noise
HD2,
HD3,
HD2,
HD3,
10
10195-046
0
10195-039
–3.0
10195-036
30
–2.5
–140
0
3
4
5
6
7
8
9
VOUT (V p-p)
Figure 38. Harmonic Distortion vs. VOUT,dm, f = 10 kHz
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10
10195-047
OUTPUT VOLTAGE (µV)
2.0
HD2 (SINGLE-ENDED INPUT)
HD3 (SINGLE-ENDED INPUT)
HD2 (DIFFERENTIAL INPUT)
HD3 (DIFFERENTIAL INPUT)
–40
–50
–60
–70
–80
–90
–100
–110
–120
1k
10k
100k
1M
FREQUENCY (Hz)
–20
SPURIOUS-FREE DYNAMCIC RANGE (dBc)
VOUT = 2V p-p
VOUT = 4V p-p
VOUT = 8V p-p
–95
–100
–105
–110
1k
10k
100k
FREQUENCY (Hz)
10195-053
–115
0.2
0.4
0.6
0.8
1.0
–30
VS = 5V, RL = 2kΩ
VS = 5V, RL = NO LOAD
–40
–50
–60
–70
–80
–90
–100
–110
–120
–130
–140
100
1k
10k
100k
1M
FREQUENCY (Hz)
Figure 43. Spurious-Free Dynamic Range vs. Frequency at Various Loads
Figure 40. Total Harmonic Distortion + Noise vs. Frequency
1V/DIV
1V/DIV
20µV/DIV
0.001%/DIV
200µV/DIV
0.01%/DIV
1µs/DIV
Figure 41. Settling Time to 0.01% of 2 V Step
10195-037
THD + N (dB)
–90
100
0
Figure 42. Gain Nonlinearity
–80
–120
10
–0.2
OUTPUT VOLTAGE (V)
Figure 39. Harmonic Distortion vs. Input Drive
–85
–0.4
10195-049
–140
100
10195-139
–130
2µs/DIV
Figure 44. Settling Time to 0.001% of 2 V Step
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10195-100
–30
ERROR (ppm)
HARMONIC DISTORTION (dBc)
–20
40
VS = ±5V
35
30
25
20
15
10
5
0
–5
–10
–15
–20
–25
–30
–35
–40
–1.0 –0.8 –0.6
10195-200
0
–10
–30
1k
POSITIVE OUTPUT
NEGATIVE OUTPUT
100
–50
IMPEDANCE (Ω)
OUTPUT BALANCE ERROR (dB)
–40
–60
–70
–80
10
1
1k
10k
100k
1M
10M
FREQUENCY (Hz)
Figure 45. Output Balance Error vs. Frequency
0.1
10k
100k
1M
FREQUENCY (Hz)
Figure 47. Output Impedance vs. Frequency
10
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
80
85
90
95
100
105
110
FREQUENCY (Hz)
115
120
10195-054
NORMALIZED SPECTRUM (dBc)
0
Figure 46. 100 kHz Intermodulation Distortion
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10M
10195-052
–100
100
10195-050
–90
TERMINOLOGY
Common-Mode Voltage
Common-mode voltage refers to the average of two node voltages
with respect to the local ground reference. The output commonmode voltage is defined as
10kΩ
10kΩ
VOCM
–IN
–OUT
RL, dm VOUT, dm
AD8476
10kΩ
VOUT, cm = (V+OUT + V−OUT)/2
+OUT
10kΩ
10195-057
+IN
Figure 48. Signal and Circuit Definitions
Differential Voltage
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)
where V+OUT and V−OUT refer to the voltages at the +OUT and
−OUT terminals with respect to a common ground reference.
Similarly, the differential input voltage is defined as
VIN, dm = (V+IN − V−IN)
Balance
Output balance is a measure of how close the output differential
signals are to being equal in amplitude and opposite in phase.
Output balance is most easily determined by placing a wellmatched resistor divider between the differential voltage nodes
and comparing the magnitude of the signal at the divider midpoint
with the magnitude of the differential signal. By this definition,
output balance is the magnitude of the output common-mode
voltage divided by the magnitude of the output differential
mode voltage.
Output Balance Error =
∆VOUT , cm
∆VOUT , dm
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THEORY OF OPERATION
OVERVIEW
The AD8476 is a fully differential amplifier, with integrated lasertrimmed resistors, that provides a precision gain of 1. The
internal differential amplifier of the AD8476 differs from
conventional operational amplifiers in that it has two outputs
whose voltages are equal in magnitude, but move in opposite
directions (180° out of phase).
5 –OUT
1
VIN ,cm = (VP + VN )
2
The differential closed-loop gain of the amplifier is
NOTES
1. NC = NO CONNECT.
DO NOT CONNECT TO THIS PIN.
10195-058
+OUT 4
VOCM 3
RFP
RFN
, RN =
RGP
RGN
VIN ,dm = VP − VN
AD8476
+VS 2
1
(2RP RN + RP + RN )
2
1
= VOUT ,cm (RP − RN ) + VOUT ,dm (2 + RP + RN )
2
VIN ,cm (RP − RN ) + VIN ,dm
RP =
10kΩ
INN 1
The dc precision of the AD8476 is highly dependent on the
accuracy of its integrated gain resistors. Using superposition to
analyze the circuit shown in Figure 50, the following equation
shows the relationship between the input and output voltages of
the amplifier:
where:
10kΩ
10kΩ
10kΩ
DC PRECISION
Figure 49. Block Diagram
CIRCUIT INFORMATION
The AD8476 amplifier uses a voltage feedback topology;
therefore, the amplifier exhibits a nominally constant gain
bandwidth product. Like a voltage feedback operational
amplifier, the AD8476 also has high input impedance at its
internal input terminals (the summing nodes of the internal
amplifier) and low output impedance.
The AD8476 employs two feedback loops, one each to control
the differential and common-mode output voltages. The differential feedback loop, which is fixed with precision laser-trimmed
on-chip resistors, controls the differential output voltage.
Output Common-Mode Voltage (VOCM)
The internal common-mode feedback controls the commonmode output voltage. This architecture makes it easy for the
user to set the output common-mode level to any arbitrary
value independent of the input voltage. The output commonmode voltage is forced by the internal common-mode feedback
loop to be equal to the voltage applied to the VOCM input. The
VOCM pin can be left unconnected, and the output commonmode voltage self-biases to midsupply by the internal feedback
control.
VOUT ,dm
VIN ,dm
=
2RP RN + RP + RN
2 + RP + RN
and the common rejection of the amplifier is
VOUT ,dm
VIN ,cm
=
2(RP − RN )
2 + RP + RN
VP
RGP
RFP
VON
VOCM
VOP
VN
RGN
RFN
10195-059
6 NC
8 INP
7 –VS
The AD8476 is designed to greatly simplify single-ended-todifferential conversion, common-mode level shifting and
precision driving of differential signals into low power,
differential input ADCs. The VOCM input allows the user to
set the output common-mode voltage to match with the input
range of the ADC. Like an operational amplifier, the VOCM
function relies on high open-loop gain and negative feedback to
force the output nodes to the desired voltages.
Due to the internal common-mode feedback loop and the fully
differential topology of the amplifier, the AD8476 outputs are
precisely balanced over a wide frequency range. This means that
the amplifier’s differential outputs are very close to the ideal of
being identical in amplitude and exactly 180° out of phase.
Figure 50. Functional Circuit Diagram of the AD8476 at a Given Gain
The preceding equations show that the gain accuracy and the
common-mode rejection (CMRR) of the AD8476 are determined primarily by the matching of the feedback networks
(resistor ratios). If the two networks are perfectly matched, that
is, if RP and RN equal RF/RG, then the resistor network does not
generate any CMRR errors and the differential closed loop gain
of the amplifier reduces to
v OUT ,dm
v IN ,dm
=
RF
RG
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at the inputs prevent damage to the AD8476 up to +VS + 18 V
and −VS − 18 V.
The AD8476 integrated resistors are precision wafer-lasertrimmed to guarantee a minimum CMRR of 90 dB (32 μV/V),
and gain error of less that 0.02%. To achieve equivalent precision
and performance using a discrete solution, resistors must be
matched to 0.01% or better.
DRIVING THE AD8476
Care should be taken to drive the AD8476 with a low
impedance source: for example, another amplifier. Source
resistance can unbalance the resistor ratios and, therefore,
significantly degrade the gain accuracy and common-mode
rejection of the AD8476. For the best performance, source
impedance to the AD8476 input terminals should be kept
below 0.1 Ω. Refer to the DC Precision section for details on
the critical role of resistor ratios in the precision of the AD8476.
INPUT VOLTAGE RANGE
The AD8476 can measure input voltages as large as the supply
rails. The internal gain and feedback resistors form a divider,
which reduces the input voltage seen by the internal input
nodes of the amplifier. The largest voltage that can be measured
properly is constrained by the output range of the amplifier and
the capability of the amplifier’s internal summing nodes. This
voltage is defined by the input voltage, and the ratio between the
feedback and the gain resistors.
POWER SUPPLIES
The AD8476 operates over a wide range of supply voltages. It
can be powered on a single supply as low as 3 V and as high as
18 V. The AD8476 can also operate on dual supplies from
±1.5 V to ±9 V
Figure 51 shows the voltage at the internal summing nodes of
the amplifier, defined by the input voltage and internal resistor
network. If VN is grounded, the expression shown reduces to
The internal amplifier of the AD8476 has rail-to-rail inputs.
To obtain accurate measurements with minimal distortion, the
voltage at the internal inputs of the amplifier must stay below
+VS − 1 V and above −VS.
Place a bypass capacitor of 0.1 μF between each supply pin and
ground, as close as possible to each supply pin. Use a tantalum
capacitor of 10 μF between each supply and ground. It can be
farther away from the supply pins and, typically, it can be
shared by other precision integrated circuits.
The AD8476 provides overvoltage protection for excessive input
voltages beyond the supply rails. Integrated ESD protection diodes
VP
RG
RF + RG
VOCM +
1 RF
2 RG
VP − VN
+
RF
RF + RG
RG
RF
VON
VN
VOCM
VOP
VN
RG
RF
10195-060
VPLUS = V MINUS
A stable dc voltage should be used to power the AD8476. Note
that noise on the supply pins can adversely affect performance.
For more information, see the PSRR performance curve in
Figure 8.
RG 
1 RF
=
VP 
 VOCM +
RF + RG 
2 RG

Figure 51. Voltages at the Internal Op Amp Inputs of the AD8476
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APPLICATIONS INFORMATION
TYPICAL CONFIGURATION
The AD8476 is designed to facilitate single-ended-to-differential
conversion, common-mode level shifting, and precision processing
of signals so that they are compatible with low voltage ADCs.
Figure 52 shows a typical connection diagram of the AD8476.
SINGLE-ENDED-TO-DIFFERENTIAL CONVERSION
Many industrial systems have single-ended inputs from input
sensors; however, the signals are frequently processed by high
performance differential input ADCs for higher precision. The
AD8476 performs the critical function of precisely converting
single-ended signals to the differential inputs of precision
ADCs, and it does so with no need for external components.
To convert a single-ended signal to a differential signal, connect
one input to the signal source and the other input to ground (see
Figure 52). Note that either input can be driven by the source
with the only effect being that the outputs have reversed polarity.
The AD8476 also accepts truly differential input signals in
precision systems with differential signal paths.
SETTING THE OUTPUT COMMON-MODE VOLTAGE
The VOCM pin of the AD8476 is internally biased by a
precision voltage divider comprising of two 1 MΩ resistors
between the supplies. This divider level shifts the output to
midsupply. Relying on the internal bias results in an output
common-mode voltage that is within 0.05% of the expected
value.
–5V
Table 5. Differential Input ADCs1
ADC
AD7674
AD7684
AD7687
AD7688
Depending on measurement/application type, check that the AD8476 meets
settling time requirements.
5
7
Resolution Throughput Rate Power Dissipation
16 Bits
100 kSPS
25 mW
16 Bits
100 kSPS
6 mW
16 Bits
250 kSPS
12.5 mW
16 Bits
500 kSPS*
21.5 mW
–OUT
10µF
6 NC
–VS
INP
It is also possible to connect the VOCM input to the commonmode level output of an ADC; however, care must be taken to
ensure that the output has sufficient drive capability. The input
impedance of the VOCM pin is 500 kΩ. If multiple AD8476
devices share one ADC reference output, a buffer may be necessary to drive the parallel inputs.
+
8
Because of the internal divider, the VOCM pin sources and sinks
current, depending on the externally applied voltage and its
associated source resistance.
1
0.1µF
10kΩ
–VOUT
AD8476
LOAD
+VOUT
0.1µF
10µF
4
+OUT
VOCM 3
+VS
1
2
10kΩ
+
+5V
10195-102
10kΩ
10kΩ
INN
INPUT
SIGNAL
SOURCE
In cases where control of the output common-mode level is
desired, an external source or resistor divider can be used to
drive the VOCM pin. If driven directly from a source, or with a
resistor divider of unequal resistor values, the resistance seen by
the VOCM pin should be less than 1 kΩ. If an external voltage
divider consisting of equal resistor values is used to set VOCM
to midsupply, higher values can be used because the external
resistors are placed in parallel with the internal resistors. The
output common-mode offset listed in the Specifications section
assumes that the VOCM input is driven by a low impedance
voltage source.
Figure 52. Typical Configuration—8-Lead MSOP
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OUTLINE DIMENSIONS
3.20
3.00
2.80
8
3.20
3.00
2.80
1
5
5.15
4.90
4.65
4
PIN 1
IDENTIFIER
0.65 BSC
0.95
0.85
0.75
15° MAX
1.10 MAX
0.40
0.25
6°
0°
0.23
0.09
COMPLIANT TO JEDEC STANDARDS MO-187-AA
0.80
0.55
0.40
10-07-2009-B
0.15
0.05
COPLANARITY
0.10
Figure 53. 8-Lead Mini Small Outline Package [MSOP]
(RM-8)
Dimensions shown in millimeters
ORDERING GUIDE
Model1
AD8476BRMZ
AD8476BRMZ-R7
AD8476BRMZ-RL
AD8476ARMZ
AD8476ARMZ-R7
AD8476ARMZ-RL
AD8476-EVALZ
1
Temperature Range
−40°C to +125°C
−40°C to +125°C
−40°C to +125°C
−40°C to +125°C
−40°C to +125°C
−40°C to +125°C
Package Description
8-Lead Mini Small Outline Package [MSOP]
8-Lead Mini Small Outline Package [MSOP]
8-Lead Mini Small Outline Package [MSOP]
8-Lead Mini Small Outline Package [MSOP]
8-Lead Mini Small Outline Package [MSOP]
8-Lead Mini Small Outline Package [MSOP]
Evaluation Board
Package Option
RM-8
RM-8
RM-8
RM-8
RM-8
RM-8
Z = RoHS Compliant Part.
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Branding
Y47
Y47
Y47
Y46
Y46
Y46
NOTES
©2011 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
D10195-0-11/11(A)
www.BDTIC.com/ADI

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