Datasheet AD8476 Datasheet (ANALOG DEVICES)

Low Power, Unity Gain, Fully Differential

AD8476

10kΩ

10kΩ

10kΩ
10kΩ

INN

1

+V

S

2

VOCM

3

+OUT

4

INP8–V

S

7

NC
6

–OUT
5

NOTES

1. NC = NO CONNECT .
DO NOT CONNECT TO THIS PIN.

10195-001

AD8476

Data Sheet

FEATURES

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

Amplifier and ADC Driver

FUNCTIONAL BLOCK DIAGRAM

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.

Rev. A

Information furnished by Analog Devices is believed to be accurate and reliable. However, no
responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other
rights of third parties that may result from its use. Specifications subject to change without n otice. No
license is granted by implication or otherwise under any patent or patent rights of Analog Devices.
Trademarks and registered trademarks are the property of their respective owners.

One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: [phone]
Fax: [phone] ©2011 Analog Devices, Inc. All rights reserved.

www.analog.com

AD8476 Data Sheet

TABLE OF CONTENTS

Features .............................................................................................. 1

Applications ....................................................................................... 1

Functional Block Diagram .............................................................. 1

General Description ........................................................................... 1

Revision History ............................................................................... 2

Specifications ..................................................................................... 3

Absolute Maximum Ratings ............................................................ 5

Thermal Resistance ...................................................................... 5

Maximum Power Dissipation ..................................................... 5

ESD Caution .................................................................................. 5

Pin Configuration and Function Descriptions ............................. 6

Typical Performance Characteristics ............................................. 7

Terminology .................................................................................... 15

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

Theory of Operation ...................................................................... 16

Overview ..................................................................................... 16

Circuit Information .................................................................... 16

DC Precision ............................................................................... 16

Input Voltage Range ................................................................... 17

Driving the AD8476................................................................... 17

Power Supplies ............................................................................ 17

Applications Information .............................................................. 18

Typical Configuration ................................................................ 18

Single-Ended-to-Differential Conversion ............................... 18

Setting the Output Common-Mode Voltage .......................... 18

Outline Dimensions ....................................................................... 19

Ordering Guide .......................................................................... 19

Rev. A | Page 2 of 20

Data Sheet AD8476

SPECIFICATIONS

VS = +5 to ±5 V, VOCM = midsupply, V
otherwise noted.

Table 1.

Parameter Test Conditions/Comments

DYNAMIC PERFORMANCE

−3 dB Small Signal Bandwidth V

−3 dB Large Signal Bandwidth V
Slew Rate V
Settling Time to 0.01% V
Settling Time to 0.001% V

NOISE/DISTORTION1

THD + N f = 10 kHz, V

HD2 f = 10 kHz, V
HD3 f = 10 kHz, V
IMD3 f1 = 95 kHz, f2 = 105 kHz,

Output Voltage Noise f = 0.1 Hz to 10 Hz 6 6 µV p-p
Spectral Noise Density f = 10 kHz 39 39 nV/√Hz

GAIN 1 1 V/V

Gain Error RL = ∞ 0.02 0.04 %
Gain Drift −40°C ≤ TA ≤ +125°C 1 1 ppm/°C
Gain Nonlinearity V

OFFSET AND CMRR

Differential Offset2 50 200 50 500 µV

vs. Temperature −40°C ≤ TA ≤ +125°C 900 900 µV
Average TC −40°C ≤ TA ≤ +125°C 1 4 1 4 µV/°C

vs. Power Supply (PSRR) VS = ±2.5 V to ±9 V 90 90 dB
Common-Mode Offset2 50 50 µV
Common-Mode Rejection

Ratio

INPUT CHARACTERISTICS

Input Voltage Range3 Differential input −VS + 0.05 +VS − 0.05 −VS + 0.05 +VS − 0.05 V
Single-ended input 2(−VS + 0.05) 2(+V − 0.05) 2(−VS + 0.05)
Impedance4 Vcm = VS/2

Single-Ended Input 13.3 13.3 kΩ

Differential Input 20 20 kΩ

Common-Mode Input 10 10 kΩ

OUTPUT CHARACTERISTICS

Output Swing VS = +5 V −VS + 0.125 +VS − 0.14 −VS + 0.125 +VS − 0.14
VS = ±5 V −VS + 0.155 +VS − 0.18 −VS + 0.155 +VS − 0.18
Output Balance Error ∆V
Output Impedance 0.1 0.1 Ω
Capacitive Load Per output 20 20 pF
Short-Circuit Current Limit 35 35 mA

VOCM CHARACTERISTICS

VOCM Input Voltage Range −VS + 1 +VS − 1 −VS + 1 +VS − 1 V
VOCM Input Impedance 500 500 kΩ
VOCM Gain Error 0.05 0.05 %

22 kHz filter

V

V

OUT

= V

+OUT

− V

, RL = 2 kΩ differential, referred to output (RTO), TA = 25°C, unless

−OUT

B Grade A Grade

= 200 mV p-p 5 5 MHz

OUT

= 2 V p-p 1 1 MHz

OUT

= 2 V step 10 10 V/µs

OUT

= 2 V step 1.0 1.0 µs

OUT

= 2 V step 1.6 1.6 µs

OUT

= 2 V p-p,

OUT

= 2 V p-p −120 −120 dB

OUT

= 2 V p-p −122 −122 dB

OUT

−102 −102 dB

−82 −82 dBc

= 2 V p-p

OUT

= 4 V p-p 5 5 ppm

OUT

= ±5 V 90 80 dB

IN,cm

2(+VS − 0.05)

/∆V

OUT,cm

90 80 dB

OUT,dm

Unit Min Typ Max Min Typ Max

V

Rev. A | Page 3 of 20

AD8476 Data Sheet

B Grade A Grade

Parameter Test Conditions/Comments

POWER SUPPLY

Specified Supply Voltage ±5 ±5 V
Operating Supply Voltage

3 18 3 18 V

Range
Supply Current VS = +5 V, TA = 25°C 300 330 300 330 μA
VS = ±5 V, TA = 25°C 330 380 330 380 μA

Over Temperature −40°C ≤ TA ≤ +125°C 400 500 400 500 μA

TEMPERATURE RANGE

Specified Performance Range −40 +125 −40 +125 °C

1

Includes amplifier voltage and current noise, as well as noise of internal resistors.

2

Includes input bias and offset current errors.

3

The input voltage range is a function of the voltage supplies and ESD dio des.

4

Internal resistors are trimmed to be r atio matched but have ±20% absolute accuracy.

Unit Min Typ Max Min Typ Max

Rev. A | Page 4 of 20

Data Sheet AD8476

ABSOLUTE MAXIMUM RATINGS

Table 2.

Parameter Rating

Supply Voltage ±10 V

Maximum Voltage at Any Input Pin +VS + 18 V

Minimum Voltage at Any Input Pin −VS – 18 V

Storage Temperature Range −65°C to +150°C

Specified Temperature Range −40°C to +125°C

Package Glass Transition Temperature (TG) 150°C

ESD (Human Body Model) 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.

THERMAL RESISTANCE

The θJA values in Table 3 assume a 4-layer JEDEC standard
board with zero airflow.

Table 3. Thermal Resistance

Package Type θJA Unit

8-Lead MSOP 209.0 °C/W

MAXIMUM POWER DISSIPATION

The maximum safe power dissipation for the AD8476 is limited
by the associated rise in junction temperature (T
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.

) on the die. At

J

ESD CAUTION

Rev. A | Page 5 of 20

AD8476 Data Sheet

INN

1

+V

S

2

VOCM

3

+OUT

4

INP

8

–V

S

7

NC

6

–OUT

5

AD8476

TOP VIEW

(Not to S cale)

NOTES

1. NC = NO CONNECT.
DO NOT CO NNE CT 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. Mnemonic Description

1 INN Negative Input .
2 +VS Positive Supply.
3 VOCM Output Common-Mode Adjust.
4 +OUT Noninverting Output.
5 −OUT Inverting Output.
6 NC No Connect.
7 −VS Negative Supply.
8 INP Positive Input.

Rev. A | Page 6 of 20

Data Sheet AD8476

50

–50

–40

–30

–20

–10

0

10

20

30

40

–40 –25 –10 5 20 35 50 65 80 95 110 125

CMRR (µV/V)

TEMPERATURE (°C)

10195-005

NORMALIZED TO 25° C

–40 –25 –10 5 20 35 50 65 80 95 110 125

OFFSET VOLTAGE (µV)

TEMPERATURE (°C)

10195-006

–1500

–1300

–1100

–900

–700

–500

–300

–100

100

300

500

700

900

1100

1300

1500

NORMALIZED TO 25° C

–40 –25 –10 5 20 35 50 65 80 95 110 125

GAIN ERROR ( µ V /V)

TEMPERATURE (°C)

10195-007

–150

–100

–50

0

50

100

150

NORMALIZED TO 25° C

15

–15

–10

–5

0

5

10

–15 –10 –5 0 5 10 15

COMMON-MODE VOLTAGE (V)

OUTPUT VOLTAGE (V)

10195-008

V

S

= ±5V

V

S

= ±2.5V

115

65

70

75

80

85

90

95

100

105

110

10 100 1k 10k 100k 1M

CMRR (dB)

FREQUENCY (Hz)

V

S

= ±5V

V

S

= +5V

10195-010

–20

–100

–90

–80

–70

–60

–50

–40

–30

100 1k 10k 100k 1M 10M

PSRR (dB)

FREQUENCY ( Hz )

10195-011

VS = ±5V
VS = +5V

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.

Figure 3. CMRR vs. Temperature

Figure 4. System Offset Temperature Drift

Figure 6. Input Common-Mode Voltage vs. Output Voltage,

V

= ±5 V and ±2.5 V

S

Figure 7. Common-Mode Rejection vs. Frequency

Figure 5. Gain Error vs. Temperature

Figure 8. Power Supply Rejection vs. Frequency

Rev. A | Page 7 of 20

AD8476 Data Sheet

20

18

16

14

12

10

8

6

4

2

0

100 1k 10k 100k 10M1M

MAXIMUM OUTPUT VOLTAGE (V p-p)

FREQUENCY ( Hz )

2kΩ LOAD

NO LOAD

10195-012

1k 10k 100k 1M

OUTPUT VOLTAGE SWING (V)

REFERRED TO SUPPLY VOLTAGES

R

LOAD

(Ω)

10195-013

–55°C
–40°C
+25°C
+85°C
+125°C

+V

S

0.050

0.025

0.075

0.100

0.125

–V

S

0.025

0.050

0.075

0.100

0.125

0.150

0.175

0.150

0.175

–40 –25 –10 5 20 35 50 65 80 95 110 125

SLEW RATE (V/µS)

TEMPERATURE (°C)

10195-015

15

5

6

7

8

9

10

11

12

13

14

RISE

FALL

–40 –25 –10 5 20 35 50 65 80 95 110 125

CURRENT (mA)

TEMPERATURE (°C)

10195-016

50

5

10

15

20

25

30

35

40

45

V

S

= ±5V

V

S

= ±2.5V

10µA 100µA 1mA 10mA

OUTPUT VOLTAGE SWING (V)

REFERRED TO SUPPLY VOLTAGES

CURRENT (A)

10195-014

+V

S

0.050

0.025

0.075

0.100

0.125

–V

S

0.025

0.050

0.075

0.100

0.125

0.150

0.175

0.150

0.175

+125°C

+85°C

+25°C

–40°C

–55°C

2V/DIV

2µs/DIV

10195-051

V

IN

V

OUT

Figure 9. Maximum Output Voltage vs. Frequency

Figure 10. Output Voltage Swing vs. R

LOAD

vs. Temperature, VS = ±5 V

Figure 12. Short-Circuit Current vs. Temperature

Figure 13. Output Voltage Swing vs. Load Current vs. Temperature,

V

= ±5 V

S

Figure 11. Slew Rate vs. Temperature

Figure 14. Overdrive Recovery, V

= +5 V

S

Rev. A | Page 8 of 20

Data Sheet AD8476

10

–50

–45

–40

–35

–30

–25

–20

–15

–10

–5

0

5

100 1k 10k 100k 1M 10M

GAIN (dB)

FREQUENCY (Hz)

VS = ±5V
V

S

= +5V

10195-017

10

–50

–45

–40

–35

–30

–25

–20

–15

–10

–5

0

5

100 1k 10k 100k 1M 10M

GAIN (dB)

FREQUENCY (Hz)

R

L

= 10kΩ

RL = 2kΩ
R

L

= 200Ω

10195-018

10

–50

–40

–30

–20

–10

0

1k 10k 100k 1M 10M 100M

OUTPUT M AGNITUDE (d B)

FREQUENCY ( Hz )

10195-019

CL = 5pF
CL = 10pF
CL = 15pF

10

–50

–45

–40

–35

–30

–25

–20

–15

–10

–5

0

5

100 1k 10k 100k 1M 10M

GAIN (dB)

FREQUENCY (Hz)

VS = ±5V
VS = +5V

10195-020

10

–40

–35

–30

–25

–20

–15

–10

–5

0

5

100 1k 10k 100k 1M 10M

OUTPUT M AGNITUDE (d B)

FREQUENCY (Hz)

RL = 10kΩ
RL = 2kΩ
RL = 200Ω

10195-021

10

–40

–35

–30

–25

–20

–15

–10

–5

0

5

100 1k 10k 100k 1M 10M

OUTPUT M AGNITUDE (d B)

FREQUENCY ( Hz )

10195-101

CL = 5pF
CL = 10pF
CL = 15pF

Figure 15. Small Signal Frequency Response for Various Supplies

Figure 16. Small Signal Frequency Response for Various Loads

Figure 18. Large Signal Frequency Response for Various Supplies

Figure 19. Large Signal Frequency Response for Various Loads

Figure 17. Small Signal Frequency Response for Various Capacitive Loads

Figure 20. Large Signal Frequency Response for Various Capacitive Loads

Rev. A | Page 9 of 20

AD8476 Data Sheet

5

–25

–20

–15

–10

–5

0

1k 10k 100k 1M 10M

OUTPUT M AGNITUDE (d B)

FREQUENCY ( Hz )

VOCM = 1.0V
VOCM = 2.5V
VOCM = 4.0V

10195-024

5

–30

–25

–20

–15

–10

–5

0

1k 100k10k 1M 10M

OUTPUT M AGNITUDE (d B)

VOCM INPUT FREQUENCY ( Hz )

10195-056

POSITIVE OUTPUT (2kΩ LOAD) V

S

= 5V

NEGATIVE OUTPUT (2kΩ LOAD)

50mV/DIV

500ns/DIV

VS = ±5V
VS = +5V
VS = +3V

10195-029

5

–35

–20

–25

–30

–15

–10

–5

0

1k 10k 100k 1M 10M

OUTPUT M AGNITUDE (d B)

FREQUENCY (Hz)

CL = 5pF
CL = 10pF
CL = 15pF

10195-027

5

–30

–25

–20

–15

–10

–5

0

1k 100k10k 1M

OUTPUT M AGNITUDE (d B)

VOCM INPUT FREQUENCY ( Hz )

10195-055

POSITIVE OUTPUT
NEGATIVE OUTPUT

500mV/DIV

500ns/DIV

10195-032

VS = ±5V
VS = +5V
VS = +3V

Figure 21. Small Signal Frequency Response for Various VOCM Levels

Figure 22. VOCM Small Signal Frequency Response

Figure 24. Large Signal Frequency Response for Various VOCM Level

Figure 25. VOCM Large Signal Frequency Response

Figure 23. Small Signal Pulse Response for Various Supplies

Figure 26. Large Signal Pulse Response for Various Supplies

Rev. A | Page 10 of 20

Data Sheet AD8476

50mV/DIV

500ns/DIV

R

L

= 10kΩ

R

L

= 2kΩ

RL = 200Ω

10195-030

50mV/DIV

500ns/DIV

CL = 0pF
C

L

= 5pF

C

L

= 10pF

10195-031

20mV/DIV

500ns/DIV

10195-035

500mV/DIV

500ns/DIV

RL = 10kΩ
RL = 2kΩ
RL = 200Ω

10195-033

500mV/DIV

500ns/DIV

C

L

= 0pF
CL = 5pF
CL = 10pF

10195-034

500mV/DIV

10µs/DIV

10195-038

Figure 27. Small Signal Step Response for Various Resistive Loads, V

Figure 28. Small Signal Step Response for Various Capacitive Loads, V

= ±5 V

S

= ±5 V

S

Figure 30. Large Signal Step Response for Various Resistive Loads, V

Figure 31. Large Signal Step Response for Various Capacitive Loads, V

= ±5 V

S

= ±5 V

S

Figure 29. VOCM Small Signal Step Response

Figure 32. VOCM Large Signal Step Response

Rev. A | Page 11 of 20

AD8476 Data Sheet

3.0

–3.0

–2.5

–2.0

–1.5

–1.0

–0.5

0

0.5

1.0

1.5

2.0

2.5

0 100908070605040302010

OUTPUT VOLTAGE (µV)

TIME (Seconds)

10195-039

–20
–30
–40
–50
–60
–70
–80

–90
–100
–110
–120
–130
–140

100 1k 10k 100k 1M

HARMONIC DIS TORTIO N ( dBc)

FREQUENCY ( Hz )

10195-040

HD2, R

L

= NO LOAD

HD3, R

L

= NO LOAD
HD2, RL = 2kΩ LOAD
HD3, R

L

= 2kΩ LOAD

–30
–40
–50
–60
–70
–80

–90
–100
–110
–120
–130
–140

100 1k

10k 100k 1M

HARMONIC DIS TORTIO N ( dBc)

FREQUENCY ( Hz )

10195-042

HD2 (V

S

= ±5V, RL = 2kΩ)

HD3 (VS = ±5V, R

L

= 2kΩ)

HD2 (VS = +5V, R

L

= 2kΩ)

HD3 (VS = +5V, R

L

= 2kΩ)

140

130

120

110

100

90
80
70
60
50
40
30
20

1 10 100 1k 10k 100k

SPECTRAL NOISE DENSITY (nV/ Hz)

FREQUENCY ( Hz )

10195-036

–20

–30

–40

–50

–60

–70

–80

–90
–100
–110
–120
–130
–140

100 1k 10k 100k 1M

HARMONIC DIS TORTIO N ( dBc)

FREQUENCY ( Hz )

10195-046

HD2 (V

OUT

= 4V p-p)

HD3 (V

OUT

= 4V p-p)

HD2 (V

OUT

= 2V p-p)

HD3 (V

OUT

= 2V p-p)

–20

–30

–40

–50

–60

–70

–80

–90
–100
–110
–120
–130
–140

0 1 2 3 4 5 6 7 8 9 10

HARMONIC DIS TORTIO N ( dBc)

V

OUT

(V p-p)

10195-047

HD2, V

S

= 5V

HD3, VS = 5V

Figure 33. 0.1 Hz to 10 Hz Voltage Noise

Figure 34. Harmonic Distortion vs. Frequency at Various Loads

Figure 36. Voltage Noise Density vs. Frequency

Figure 37. Harmonic Distortion vs. Frequency at Various V

OUT,dm

Figure 35. Harmonic Distortion vs. Frequency at Various Supplies

Figure 38. Harmonic Distortion vs. V

OUT,dm

, f = 10 kHz

Rev. A | Page 12 of 20

Data Sheet AD8476

–140

–130

–120

–110

–100

–90

–80

–70

–60

–50

–40

–30

–20

–10

0

100 1k 10k 100k 1M

HARMONIC DISTORTION (dBc)

FREQUENCY (Hz)

HD2 (SINGL E - E NDE D INPUT)
HD3 (SINGL E - E NDE D INPUT)
HD2 (DIFFE RE NTIAL I NP UT)
HD3 (DIFFE RE NTIAL I NP UT)

10195-139

–80

–120

–115

–110

–105

–100

–95

–90

–85

10 1k100 10k 100k

THD + N (dB)

FREQUENCY ( Hz )

10195-053

V

OUT

= 2V p-p

V

OUT

= 4V p-p

V

OUT

= 8V p-p

1µs/DIV

10195-037

1V/DIV

200µV/DIV

0.01%/DIV

40

–40

–35

–30

–25

–20

–15

–10

–5

0

5

10

15

20

25

30

35

–1.0 –0.8 –0.6 –0.4 –0.2 0 0.2 0.4 0.6 0.8 1.0

ERROR (pp m)

OUTPUT VOLTAGE (V)

10195-200

VS = ±5V

–20
–30
–40
–50
–60
–70
–80

–90
–100
–110
–120
–130
–140

100 1k 10k 100k 1M

SPURIOUS- FREE DYNAMCI C RANGE (dBc)

FREQUENCY ( Hz )

10195-049

VS = 5V, RL = 2kΩ
VS = 5V, RL = NO LOAD

2µs/DIV

10195-100

1V/DIV

20µV/DIV

0.001%/DIV

Figure 39. Harmonic Distortion vs. Input Drive

Figure 40. Total Harmonic Distortion + Noise vs. Frequency

Figure 42. Gain Nonlinearity

Figure 43. Spurious-Free Dynamic Range vs. Frequency at Various Loads

Figure 41. Settling Time to 0.01% of 2 V Step

Figure 44. Settling Time to 0.001% of 2 V Step

Rev. A | Page 13 of 20

AD8476 Data Sheet

–30

–40

–50

–60

–70

–80

–90

–100

100 1k 10k 100k 1M 10M

OUTPUT BALANCE ERROR (dB)

FREQUENCY ( Hz )

10195-050

10

–100

–90

–80

–70

–60

–50

–40

–30

–20

–10

0

80 10090 110 1209585 105 115

NORMALIZED SPECT RUM ( dBc)

FREQUENCY ( Hz )

10195-054

1k

100

10

1

0.1
10k 100k 1M 10M

IMPEDANCE (Ω)

FREQUENCY ( Hz )

10195-052

POSITIVE OUTPUT
NEGATIVE OUTPUT

Figure 45. Output Balance Error vs. Frequency

Figure 47. Output Impedance vs. Frequency

Figure 46. 100 kHz Intermodulation Distortion

Rev. A | Page 14 of 20

Data Sheet AD8476

+IN

VOCM

–IN

+OUT

–OUT

V

OUT, dm

R

L, dm

AD8476

10kΩ

10kΩ

10kΩ

10kΩ

10195-057

dmOUT

cmOUT

V

V

ErrorBalanceOutput

,

,

∆

∆

=

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

V

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

V

where V

OUT, dm

+OUT

= (V

and V

+OUT

− V

−OUT

)

−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

V

= (V

− V

IN, dm

+IN

−IN

)

= (V

OUT, cm

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.

+OUT

+ V

−OUT

)/2

Rev. A | Page 15 of 20

AD8476 Data Sheet

10kΩ

10kΩ

10kΩ
10kΩ

INN

1

+V

S

2

VOCM

3

+OUT

4

INP
8

–V

S

7

NC
6

–OUT
5

NOTES

1. NC = NO CONNECT.
DO NOT CO NNE CT TO THIS PIN.

10195-058

AD8476

( )

( )

( )

( )

NP

dmOUT

NP

cmOUT

NPNP

dmIN

NPcmIN

RRVRRV

RRRRVRRV

+++−=

+++−

2

2

1

2

2

1

,

,

,

,

RGP

RFP

R

P

=

RGN

RFN

R

N

=

−=

)(

2

1

, NPcmIN

VVV +=

NP

NPNP

dmIN

dmOUT

RR

RRRR

V

V

++

++

=

2

2

,

,

( )

NP

NP

cmIN

dmOUT

RR

RR

V

V

++

−

=

2

2

,

,

RFP

RFN

RGP

RGN

V

ON

V

OP

VOCM

V

P

V

N

10195-059

RG

RF

v

v

dmIN

dmOUT

=

,

,

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).

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.

DC PRECISION

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:

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.

Figure 49. Block Diagram

,

VVV

,

dmIN

NP

The differential closed-loop gain of the amplifier is

and the common rejection of the amplifier is

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 R

and RN equal RF/RG, then the resistor network does not

P

generate any CMRR errors and the differential closed loop gain
of the amplifier reduces to

Rev. A | Page 16 of 20

Data Sheet AD8476










+

+

==

P

MINUSPLUS

V

RG

RF

VOCM

RGRF

RG

VV

2

1

RF

RF

RG

RG

V

ON

V

OP

VOCM

V

P

V

N

V

N

RF + RG

RF

V

P

− V

N

RG

RF

VOCM

RF + RG

RG

+

+

2

1

10195-060

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.

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.

Figure 51 shows the voltage at the internal summing nodes of
the amplifier, defined by the input voltage and internal resistor
network. If V

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
+V

− 1 V and above −VS.

S

The AD8476 provides overvoltage protection for excessive input
voltages beyond the supply rails. Integrated ESD protection diodes

is grounded, the expression shown reduces to

N

at the inputs prevent damage to the AD8476 up to +V
and −V

− 18 V.

S

+ 18 V

S

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.

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

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.

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.

Figure 51. Voltages at the Internal Op Amp Inputs of the AD8476

Rev. A | Page 17 of 20

AD8476 Data Sheet

10195-102

10kΩ

10kΩ

10kΩ
10kΩ

INN

1

+V

S

2

VOCM

3

+OUT

4

INP

8

–V

S

7

NC6–OUT

5

AD8476

LOAD

INPUT

SIGNAL

SOURCE

+5V

–V

OUT

+V

OUT

+

10µF

0.1µF

+

10µF

0.1µF

–5V

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.

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.

Because of the internal divider, the VOCM pin sources and sinks
current, depending on the externally applied voltage and its
associated source resistance.

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.

Table 5. Differential Input ADCs

1

ADC Resolution Throughput Rate Power Dissipation

AD7674 16 Bits 100 kSPS 25 mW
AD7684 16 Bits 100 kSPS 6 mW
AD7687 16 Bits 250 kSPS 12.5 mW
AD7688 16 Bits 500 kSPS* 21.5 mW

1

Depending on measurement/application type, check that the AD8476 meets

settling time requirements.

Figure 52. Typical Configuration—8-Lead MSOP

Rev. A | Page 18 of 20

Data Sheet AD8476

COMPLIANT TO JEDEC STANDARDS MO-187-AA

6°
0°

0.80

0.55

0.40

4

8

1

5

0.65 BSC

0.40

0.25

1.10 MAX

3.20

3.00

2.80

COPLANARITY

0.10

0.23

0.09

3.20

3.00

2.80

5.15

4.90

4.65

PIN 1

IDENTIFIER

15° MAX

0.95

0.85

0.75

0.15

0.05

10-07-2009-B

OUTLINE DIMENSIONS

Figure 53. 8-Lead Mini Small Outline Package [MSOP]

(RM-8)

Dimensions shown in millimeters

ORDERING GUIDE

Model1 Temperature Range Package Description Package Option Branding

AD8476BRMZ −40°C to +125°C 8-Lead Mini Small Outline Package [MSOP] RM-8 Y47
AD8476BRMZ-R7 −40°C to +125°C 8-Lead Mini Small Outline Package [MSOP] RM-8 Y47
AD8476BRMZ-RL −40°C to +125°C 8-Lead Mini Small Outline Package [MSOP] RM-8 Y47
AD8476ARMZ −40°C to +125°C 8-Lead Mini Small Outline Package [MSOP] RM-8 Y46
AD8476ARMZ-R7 −40°C to +125°C 8-Lead Mini Small Outline Package [MSOP] RM-8 Y46
AD8476ARMZ-RL −40°C to +125°C 8-Lead Mini Small Outline Package [MSOP] RM-8 Y46
AD8476-EVALZ Evaluation Board

1

Z = RoHS Compliant Part.

Rev. A | Page 19 of 20

AD8476 Data Sheet

NOTES

©2011 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
D10195-0-11/11(A)

Rev. A | Page 20 of 20