Datasheet AD8656 Datasheet (ANALOG DEVICES)

Low Noise,

–

A

Data Sheet

FEATURES

Low noise: 2.7 nV/√Hz @ f = 10 kHz
Low offset voltage: 250 μV max over V
Offset voltage drift: 0.4 μV/°C typ and 2.3 μV/°C max
Bandwidth: 28 MHz
Rail-to-rail input/output
Unity gain stable

2.7 V to 5.5 V operation

−40°C to +125°C operation
Qualified for automotive applications (AD8656)

APPLICATIONS

ADC and DAC buffers
Audio
Industrial controls
Precision filters
Digital scales
Automotive collision avoidance
PLL filters

GENERAL DESCRIPTION

CM

1

NC

IN

2

+IN

3

(Not to Scale)

V–

4

NC = NO CONNECT

Figure 1. AD8655

8-Lead MSOP (RM-8)

NC

1

–IN

2
3

+IN

(Not to Scale)

4

V–

NC = NO CONNECT

Figure 3. AD8655

8-Lead SOIC (R-8)

Precision CMOS Amplifier

AD8655/AD8656

PIN CONFIGURATIONS

8

AD8655

TOP VIEW

AD8655

TOP VIEW

NC
V+

7

OUT

6

NC

5

05304-048

NC

8

V+

7
6

OUT

5

NC

05304-049

1

OUT

–IN A

2

+IN A

3

V–

4

Figure 2. AD8656

8-Lead MSOP (RM-8)

OUT A

1
2

–IN A
+IN A

3

V–

4

Figure 4. AD8656
8-Lead SOIC (R-8)

AD8656

TOP VIEW

(Not to Scale)

AD8656

TOP VIEW

(Not to Scale)

8
7
6
5

8
7
6
5

V+
OUT B
–IN B
+IN B

V+
OUT B
–IN B
+IN B

05304-059

05304-060

The AD8655/AD8656 are the industry’s lowest noise, precision
CMOS amplifiers. They leverage the Analog Devices DigiTrim®
technology to achieve high dc accuracy.

The AD8655/AD8656 provide low noise (2.7 nV/√Hz @ 10 kHz),
low THD + N (0.0007%), and high precision performance
(250 μV max over V

) to low voltage applications. The ability

CM

to swing rail-to-rail at the input and output enables designers
to buffer analog-to-digital converters (ADCs) and other wide
dynamic range devices in single-supply systems.

The high precision performance of the AD8655/AD8656
improves the resolution and dynamic range in low voltage
applications. Audio applications, such as microphone pre-amps
and audio mixing consoles, benefit from the low noise, low
distortion, and high output current capability of the AD8655/
AD8656 to reduce system level noise performance and maintain
audio fidelity. The high precision and rail-to-rail input and
output of the AD8655/AD8656 benefit data acquisition, process
controls, and PLL filter applications.

The AD8655/AD8656 are fully specified over the −40°C to
+125°C temperature range. The AD8655/AD8656 are available
in Pb-free, 8-lead MSOP and SOIC packages.

Rev. B

Information furnished by Analog Devices is believed to be accurate and reliable. However, no
responsibility is assumed by Analog Devi ces 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 notice. 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] www.analog.com
Fax: [phone] ©2005–2011 Analog Devices, Inc. All rights reserved.

AD8655/AD8656

TABLE OF CONTENTS

Specifications..................................................................................... 3

Data Sheet

Driving Capacitive Loads.......................................................... 16

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

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

Typical Performance Characteristics............................................. 6

Theory of Operation ...................................................................... 15

Applications..................................................................................... 16

Input Overvoltage Protection................................................... 16

Input Capacitance....................................................................... 16

REVISION HISTORY

9/11—Rev. A to Rev. B

Changes to Features Section............................................................ 1

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

Changes to Ordering Guide.......................................................... 19

Added Automotive Products Section .......................................... 19

6/05—Rev. 0 to Rev. A

Added AD8656 ...................................................................Universal

Added Figure 2 and Figure 4........................................................... 1

Changes to Specifications................................................................ 3

Changed Caption of Figure 12 and Added Figure 13.................. 7

Replaced Figure 16 ...........................................................................7

Changed Caption of Figure 37 and Added Figure 38................ 11

Replaced Figure 47 .........................................................................13

Added Figure 55.............................................................................. 14

Changes to Ordering Guide.......................................................... 18

Layout, Grounding, and Bypassing Considerations.................. 18

Power Supply Bypassing............................................................ 18

Grounding................................................................................... 18

Leakage Currents........................................................................ 18

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

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

Automotive Products ................................................................. 19

4/05—Revision 0: Initial Version

Rev. B | Page 2 of 20

Data Sheet

AD8655/AD8656

SPECIFICATIONS

VS = 5.0 V, VCM = VS/2, TA = 25°C, unless otherwise specified.

Table 1.

Parameter Symbol Conditions Min Typ Max Unit

INPUT CHARACTERISTICS

Offset Voltage VOS V

−40°C ≤ TA ≤ +125°C 550 μV
Offset Voltage Drift

ΔV

OS

/ΔT

Input Bias Current IB 1 10 pA

−40°C ≤ TA ≤ +125°C 500 pA
Input Offset Current IOS 10 pA

−40°C ≤ TA≤ +125°C 500 pA
Input Voltage Range 0 5 V
Common-Mode Rejection Ratio CMRR VCM = 0 V to 5 V 85 100 dB
Large Signal Voltage Gain AVO V

−40°C ≤ TA ≤ +125°C 95 dB
OUTPUT CHARACTERISTICS

Output Voltage High VOH I
Output Voltage Low VOL I
Output Current I

V

OUT

POWER SUPPLY

Power Supply Rejection Ratio PSRR VS = 2.7 V to 5.0 V 88 105 dB
Supply Current/Amplifier ISY V

−40°C ≤ TA ≤ +125°C 5.3 mA

INPUT CAPACITANCE CIN

Differential 9.3 pF
Common-Mode 16.7 pF

NOISE PERFORMANCE

Input Voltage Noise Density en f = 1 kHz 4
f = 10 kHz 2.7
Total Harmonic Distortion + Noise THD + N G = 1, RL = 1 kΩ, f = 1 kHz, VIN = 2 V p-p 0.0007 %

FREQUENCY RESPONSE

Gain Bandwidth Product GBP 28 MHz
Slew Rate SR RL = 10 kΩ 11 V/μs
Settling Time ts To 0.1%, VIN = 0 V to 2 V step, G = +1 370 ns
Phase Margin CL = 0 pF 69 degrees

= 0 V to 5 V 50 250 μV

CM

−40°C ≤ T

= 0.2 V to 4.8 V, RL = 10 kΩ, VCM = 0 V 100 110 dB

O

= 1 mA; −40°C ≤ TA ≤ +125°C 4.97 4.991 V

L

= 1 mA; −40°C ≤ TA ≤ +125°C 8 30 mV

L

OUT

= 0 V 3.7 4.5 mA

O

≤ +125°C 0.4 2.3 μV/°C

A

= ±0.5 V ±220 mA

nV/√Hz
nV/√Hz

Rev. B | Page 3 of 20

AD8655/AD8656

Data Sheet

VS = 2.7 V, VCM = VS/2, TA = 25°C, unless otherwise specified.

Table 2.

Parameter Symbol Conditions Min Typ Max Unit

INPUT CHARACTERISTICS

Offset Voltage VOS V

= 0 V to 2.7 V 44 250 μV

CM

−40°C ≤ TA ≤ +125°C 550 μV
Offset Voltage Drift

ΔV

OS

/ΔT

−40°C ≤ T

≤ +125°C 0.4 2.0 μV/°C

A

Input Bias Current IB 1 10 pA

−40°C ≤ TA ≤ +125°C 500 pA
Input Offset Current IOS 10 pA

−40°C ≤ TA ≤ +125°C 500 pA
Input Voltage Range 0 2.7 V
Common-Mode Rejection Ratio CMRR VCM = 0 V to 2.7 V 80 98 dB
Large Signal Voltage Gain AVO V

= 0.2 V to 2.5 V, RL = 10 kΩ, VCM = 0 V 98 dB

O

−40°C ≤ TA ≤ +125°C 90 dB
OUTPUT CHARACTERISTICS

Output Voltage High VOH I
Output Voltage Low VOL I
Output Current I

V

OUT

= 1 mA; −40°C ≤ TA ≤ +125°C 2.67 2.688 V

L

= 1 mA; −40°C ≤ TA ≤ +125°C 10 30 mV

L

= ±0.5 V ±75 mA

OUT

POWER SUPPLY

Power Supply Rejection Ratio PSRR VS = 2.7 V to 5.0 V 88 105 dB
Supply Current/Amplifier ISY V

= 0 V 3.7 4.5 mA

O

−40°C ≤ TA ≤ +125°C 5.3 mA

INPUT CAPACITANCE CIN

Differential 9.3 pF
Common-Mode 16.7 pF

NOISE PERFORMANCE

Input Voltage Noise Density en f = 1 kHz 4.0
f = 10 kHz 2.7

nV/√Hz
nV/√Hz

Total Harmonic Distortion + Noise THD + N G = 1, RL = 1kΩ, f = 1 kHz, VIN = 2 V p-p 0.0007 %

FREQUENCY RESPONSE

Gain Bandwidth Product GBP 27 MHz
Slew Rate SR RL = 10 kΩ 8.5 V/μs
Settling Time ts To 0.1%, VIN = 0 to 1 V step, G = +1 370 ns
Phase Margin CL = 0 pF 54 degrees

Rev. B | Page 4 of 20

Data Sheet

ABSOLUTE MAXIMUM RATINGS

Table 3.

Parameter Rating

Supply Voltage 6 V
Input Voltage VSS − 0.3 V to VDD + 0.3 V
Differential Input Voltage ±6 V
Output Short-Circuit Duration

to GND
Electrostatic Discharge (HBM) 3.0 kV
Storage Temperature Range

R, RM Packages
Junction Temperature Range

R, RM Packages
Lead Temperature

(Soldering, 10 sec)

Indefinite

−65°C to +150°C

−65°C to +150°C

260°C

AD8655/AD8656

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.

Table 4.

Package Type θ

8-Lead MSOP (RM) 210 45 °C/W
8-Lead SOIC (R) 158 43 °C/W

1

θJA is specified for worst-case conditions; that is, θJA is specified for a device

soldered in the circuit board for surface-mount packages.

1

θJC Unit

JA

ESD 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 this product 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. B | Page 5 of 20

AD8655/AD8656

TYPICAL PERFORMANCE CHARACTERISTICS

60

VS = ±2.5V

50

40

30

20

NUMBER OF AMPLIFIERS

10

Data Sheet

20

VS = ±2.5V

10

0

(μV)

OS

V

–10

–20

0

–150 –100 –50 0 50 100 150

(μV)

V

OS

Figure 5. Input Offset Voltage Distribution

150.0

100.0

50.0

(μV)

0.0

OS

V

–50.0

–100.0

–150.0

–50 0 50

TEMPERATURE (°C)

Figure 6. Input Offset Voltage vs. Temperature

60

50

40

30

VS = ±2.5V

100 150

VS = ±2.5V

05304-001

05304-002

–30

01234

COMMON-MODE VOLTAGE (V)

Figure 8. Input Offset Voltage vs. Common-Mode Voltage

250

VS = ±2.5V

200

150

IB (pA)

100

50

0

0 20 40 60 80 100 120 140

TEMPERATURE (°C)

Figure 9. Input Bias Current vs. Temperature

4.0

VS = ±2.5V

3.5

3.0

2.5

2.0

56

05304-004

05304-005

20

NUMBER OF AMPLIFIERS

10

0

0 0.2 0.4 0.6 0.8 1.0 1.2

|TCV

Figure 7. |TC V

| (μV/°C)

OS

| Distribution

OS

1.4 1.6

05304-003

Rev. B | Page 6 of 20

1.5

SUPPLY CURRENT (mA)

1.0

0.5

0

012 34

SUPPLY VOLTAGE (V)

Figure 10. Supply Current vs. Supply Voltage

56

05304-006

Data Sheet

4.5
VS = ±2.5V

4.0

3.5

3.0

(V)

OH

V

4.996

4.994

4.992

4.990

4.988

AD8655/AD8656

VS = ±2.5V
LOAD CURRENT = 1mA

SUPPLY CURRENT (mA)

2.5

2.0

–50 0 50

TEMPERATURE (°C)

100 150

Figure 11. Supply Current vs. Temperature

2500

VS = ±2.5V

2000

1500

V

OH

1000

V

OL

500

DELTA SWING FROM SUPPLY (mV)

0

0 50 100 150 200

CURRENT LOAD (mA)

250

Figure 12. AD8655 Output Voltage to Supply Rail vs. Current Load

10000

1000

VS = ±2.5V

05304-007

05304-008

4.986

4.984

4.982
–50 0 50

TEMPERATURE (°C)

100 150

Figure 14. Output Voltage Swing High vs. Temperature

12

LOAD CURRENT = 1mA
V

= ±2.5V

S

10

8

(mV)

OL

V

6

4

2

–50 0 50

TEMPERATURE (°C)

100 150

Figure 15. Output Voltage Swing Low vs. Temperature

120

100

80

VS = ±2.5V

= 28mV

V

IN

= 1MΩ

R

L

= 47pF

C

L

05304-009

05304-010

100

10

V

OL

DELTA SWING FROM SUPPLY (mV)

V

OH

1

0.1 1 10
CURRENT LOAD (mA)

Figure 13. AD8656 Output Swing vs. Current Load

100 1000

05304-056

Rev. B | Page 7 of 20

60

CMRR (dB)

40

20

0

100 1k 10k

Figure 16. CMRR vs. Fre quency

FREQUENCY (Hz)

100k 10M

1M

05304-011

AD8655/AD8656

110.00

107.00

104.00

VS = ±2.5V
V

= 0V

CM

100

Hz 1/2)

√

Data Sheet

VS = ±2.5V

101.00

CMRR (dB)

98.00

95.00

92.00
–50 0 50

TEMPERATURE (°C)

Figure 17. Large Signal CMRR vs. Temperature

100

80

60

PSRR (dB)

40

20

0

100 1k 10k 100k 1M

+PSRR

–PSRR

FREQUENCY (Hz)

Figure 18. Small Signal PSSR vs. Frequency

110.00

108.00

VS = ±2.5V

100 150

VS = ±2.5V
V

= 50mV

IN

R

= 1M

Ω

L

CL = 47pF

10M 100M

05304-012

05304-013

10

VOLTAGE NOISE DENSITY (nV/

1

1 10 100 1k

FREQUENCY (Hz)

Figure 20. Voltage Noise Density vs. Frequency

VS = ±2.5V
Vn (p-p) = 1.23μV

1

500nV/DIV

1s/DIV

Figure 21. Low Frequency Noise (0.1 Hz to 10 Hz).

T

V

IN

V

OUT

VS = ±2.5V
C

= 50pF

L

GAIN = +1

10k 100k

05304-019

05304-020

106.00

2

PSRR (dB)

104.00

102.00

100.00
–50 0 50

Figure 19. Large Signal PSSR vs. Temperature

TEMPERATURE (°C)

100 150

05304-014

1V/DIV

20μs/DIV

Figure 22. No Phase Reversal

05304-021

Rev. B | Page 8 of 20

Data Sheet

120

100

80

60

VS = ±2.5V

= 11.5pF

C

LOAD

PHASE MARGIN = 69°

–45

–90

AD8655/AD8656

6

5

4

VS = ±2.5V
V

= 5V

IN

G = +1

40

GAIN (dB)

20

0

–20

–40

10k 100k 1M

FREQUENCY (Hz)

10M 100M

Figure 23. Open-Loop Gain and Phase vs. Frequency

140.00

130.00

120.00

(dB)

VO

A

100.00

VS = ±2.5V
R

= 10kΩ

L

110.00

90.00
–50 0 50

TEMPERATURE (°C)

Figure 24. Large Signal Open-Loop Gain vs. Temperature

50

40

30

–225

100 150

VS = ±2.5V
R

= 1M

Ω

L

CL = 47pF

–135

–180

PHASE SHIFT (Degrees)

05304-015

05304-016

3

OUTPUT (V)

2

1

0

10k 100k 1M

FREQUENCY (Hz)

Figure 26. Maximum Output Swing vs. Frequency

VS = ±2.5V
C

= 100pF

L

GAIN = +1
V

= 4V

IN

2

(1V/DIV)

OUT

V

TIME (10μs/DIV)

Figure 27. Large Signal Response

T

VS = ±2.5V
C

= 100pF

L

G = +1

05304-018

10M

T

05304-022

20

10

0

CLOSED-LOOP GAIN (dB)

–10

–20

1k 10k 100k 1M

FREQUENCY (Hz)

Figure 25. Closed-Loop Gain vs. Frequency

2

(100mV/DIV)

OUT

V

10M 100M

05304-017

TIME (1μs/DIV)

05304-023

Figure 28. Small Signal Response

Rev. B | Page 9 of 20

AD8655/AD8656

–

30

VS = ±2.5V
V

= 200mV

IN

25

20

15

OVERSHOOT %

10

5

–OS

+OS

100

VS = ±2.5V

G = +100

10

1

OUTPUT IMPEDANCE (Ω)

G = +10

Data Sheet

G = +1

0

0 50 100 150 200 250 300 350

CAPACITANCE (pF)

Figure 29. Small Signal Overshoot vs. Load Capacitance

300mV

0V

0V

–2.5V

T

V

IN

1

2

V

OUT

VS = ±2.5V

= 300mV

V

IN

GAIN = –10
RECOVERY TIME = 240ns

400ns/DIV

Figure 30. Negative Overload Recovery Time

T

1

0V

300mV

2.5V

V

IN

V

OUT

V

= ±2.5V

S

= 300mV

V

IN

GAIN = –10
RECOVERY TIME = 240ns

05304-024

05304-025

0.1
100 1k 10k 100k 1M 10M 100M

FREQUENCY (Hz)

Figure 32. Output Impedance vs. Frequency

80

70

60

50

40

30

NUMBER OF AMPLIFIERS

20

10

0

–150 –125 –100 –75 –50 –25 0

25 50 75 100 125 150

V

(μV)

OS

VS = ±1.35V

Figure 33. Input Offset Voltage Distribution

60

40

20

(μV)

OS

V

0

VS = ±1.35V

05304-027

05304-028

2

0V

400ns/DIV

05304-026

Figure 31. Positive Overload Recovery Time

–20

–40

–50 0 50 100 150

TEMPERATURE (°C)

Figure 34. Input Offset Voltage vs. Temperature

05304-029

Rev. B | Page 10 of 20

Data Sheet

80

70

VS = ±1.35V

AD8655/AD8656

10000

VS =±1.35V

60

50

40

30

NUMBER OF AMPLIFIERS

20

10

0

0 0.2 0.4 0.6 0.8

4.5
VS = ±1.35V

4.0

3.5

3.0

SUPPLY CURRENT (mA)

2.5

|TCV

Figure 35. |TCV

1.0 1.2 1.4 1.6

| (μV/°C)

OS

| Distribution

OS

05304-030

1000

100

V

10

OL

DELTA OUTPUT FROM SUPPLY (mV)

1

0.1 1 10

V

OH

CURRENT LOAD (mA)

Figure 38. AD8656 Output Swing vs. Current Load

2.698

2.694

2.690

(V)

2.686

OH

V

2.682

2.678

VS = ±1.35V
LOAD CURRENT = 1mA

05304-057

100

2.0

–50 0 50 100 150

TEMPERATURE (°C)

Figure 36. Supply Current vs. Temperature

1400

VS = ±1.35V

1200

1000

800

) (mV)

OUT

-V

600

SY

(V

400

200

0

020406080

LOAD CURRENT (mA)

V

OH

V

OL

100 120

Figure 37. AD8655 Output Voltage to Supply Rail vs. Load Current

05304-031

05304-050

2.674
–50 0 50 100 150

TEMPERATURE (°C)

Figure 39. Output Voltage Swing High vs. Temperature

14

VS = ±1.35V
LOAD CURRENT = 1mA

12

10

8

(mV)

OL

V

6

4

2

–50 0 50 100 150

TEMPERATURE (°C)

Figure 40. Output Voltage Swing Low vs. Temperature

05304-032

05304-033

Rev. B | Page 11 of 20

AD8655/AD8656

–

T

V

IN

V

OUT

2

1V/DIV

35

VS = ±1.35V
G = +1
C

= 50pF

L

30

25

20

15

OVERSHOOT %

10

5

VS = ±1.35V
V

= 200mV

IN

–OS

+OS

Data Sheet

(500mV/DIV)

V

OUT

2

Figure 41. No Phase Reversal

VS = ±1.35V

= 50pF

C

L

GAIN = +1

TIME (10μs/DIV)

Figure 42. Large Signal Response

T

20μs/DIV

T

VS = ±1.35V

= 100pF

C

L

GAIN = +1

05304-047

05304-042

0

0 50 100 150 200 250 300 350

CAPACITANCE (pF)

Figure 44. Small Signal Overshoot vs. Load Capacitance

T

200mV

0V

0V

–1.35V

1

2

V

IN

V

OUT

VS = ±1.35V
V

= 200mV

IN

GAIN = –10
RECOVERY TIME = 180ns

400ns/DIV

Figure 45. Negative Overload Recovery Time

T

1

0V

200mV

V

IN

VS = ±1.35V

= 200mV

V

IN

GAIN = –10
RECOVERY TIME = 200ns

05304-044

05304-045

(100mV/DIV)

V

OUT

2

TIME (1μs/DIV)

05304-043

Figure 43. Small Signal Response

1.35V

0V

V

2

OUT

400ns/DIV

Figure 46. Positive Overload Recovery Time

05304-046

Rev. B | Page 12 of 20

Data Sheet

120

100

80

VS = ±1.35V

= 28mV

V

IN

= 1MΩ

R

L

= 47pF

C

L

120

100

AD8655/AD8656

VS = ±1.35V
C

= 11.5pF

LOAD

PHASE MARGIN = 54°

80

60

–45

–90

60

CMRR (dB)

40

20

0

100 1k 10k

Figure 47. CMRR vs. Fre quency

102.00

98.00

94.00

CMRR (dB)

90.00

86.00
–50 0 50

Figure 48. Large Signal CMRR vs. Temperature

100

80

60

+PSRR

–PSRR

FREQUENCY (Hz)

TEMPERATURE (°C)

100k 1M

VS = ±1.35V

100 150

VS = ±1.35V

= 50mV

V

IN

= 1MΩ

R

L

= 47pF

C

L

05304-034

05304-035

40

GAIN (dB)

20

0

–20

–40

10k 100k 1M

FREQUENCY (Hz)

10M 100M

Figure 50. Open-Loop Gain and Phase vs. Frequency

130.00

120.00

110.00

(dB)

VO

A

100.00

90.00

80.00
–50 0 50

TEMPERATURE (°C)

Figure 51. Large Signal Open-Loop Gain vs. Temperature

50

40

30

20

–135

–180

–225

VS = ±1.35V
R

= 10kΩ

L

100 150

VS = ±1.35V
R

= 1M

Ω

L

CL = 47pF

PHASE SHIFT (Degrees)

05304-036

05304-037

PSRR (dB)

40

20

0

100 1k 10k 100k

Figure 49. Small Signal PSSR vs. Frequency

FREQUENCY (Hz)

1M 100M10M

05304-040

Rev. B | Page 13 of 20

10

0

CLOSED-LOOP GAIN (dB)

–10

–20

1k 10k 100k 1M

FREQUENCY (Hz)

Figure 52. Closed-Loop Gain vs. Frequency

10M 100M

05304-038

AD8655/AD8656

Data Sheet

3.0

2.5
VS = 1.35V
V

= 2.7V

IN

2.0

G = +1
NO LOAD

1.5

OUTPUT (V)

1.0

0.5

0

10k 100k 1M

FREQUENCY (Hz)

Figure 53. Maximum Output Swing vs. Frequency

1000

VS = ±1.35V

100

G = +100

10

G = +10

G = +1

10M

05304-039

0

–20

50mV p-p

–40

–60

–80

–100

CHANNEL SEPERATION (dB)

–120

–140

10 100 1k

+2.5V

V+

+

V

A

IN

V–

–

–2.5V

Figure 55. Channel Separation vs. Frequency

R1

10k

Ω

R2

100

V–

B

V

OUT

V+

10k

FREQUENCY (Hz)

100k 1M 10M 100M

VS =±2.5V
V

= 50mV

Ω

IN

05304-058

OUTPUT IMPEDANCE (Ω)

1

0.1
100 1k 10k 100k 1M 100M10M

FREQUENCY (Hz)

05304-041

Figure 54. Output Impedance vs. Frequency

Rev. B | Page 14 of 20

Data Sheet

THEORY OF OPERATION

The AD8655/AD8656 amplifiers are voltage feedback, rail-to­rail input and output precision CMOS amplifiers, which operate
from 2.7 V to 5.0 V of power supply voltage. These amplifiers
use the Analog Devices DigiTrim technology to achieve a
higher degree of precision than is available from most CMOS
amplifiers. DigiTrim technology, used in a number of ADI
amplifiers, is a method of trimming the offset voltage of the
amplifier after it is packaged. The advantage of post-package
trimming is that it corrects any offset voltages caused by the
mechanical stresses of assembly.

AD8655/AD8656

The AD8655/AD8656 can be used in any precision op amp
application. The amplifier does not exhibit phase reversal for
common-mode voltages within the power supply. The
AD8655/AD8656 are great choices for high resolution data
acquisition systems with voltage noise of 2.7 nV/√Hz and
THD + Noise of –103 dB for a 2 V p-p signal at 10 kHz. Their
low noise, sub-pA input bias current, precision offset, and high
speed make them superb preamps for fast filter applications.
The speed and output drive capability of the AD8655/AD8656
also make them useful in video applications.

The AD8655/AD8656 are available in standard op amp pinouts,
making DigiTrim completely transparent to the user. The input
stage of the amplifiers is a true rail-to-rail architecture, allowing
the input common-mode voltage range of the amplifiers to
extend to both positive and negative supply rails. The open­loop gain of the AD8655/AD8656 with a load of 10 kΩ is
typically 110 dB.

Rev. B | Page 15 of 20

AD8655/AD8656

APPLICATIONS

INPUT OVERVOLTAGE PROTECTION

The internal protective circuitry of the AD8655/AD8656 allows
voltages exceeding the supply to be applied at the input. It is
recommended, however, not to apply voltages that exceed the
supplies by more than 0.3 V at either input of the amplifier. If a
higher input voltage is applied, series resistors should be used to
limit the current flowing into the inputs. The input current
should be limited to less than 5 mA.

The extremely low input bias current allows the use of larger
resistors, which allows the user to apply higher voltages at the
inputs. The use of these resistors adds thermal noise, which
contributes to the overall output voltage noise of the amplifier.
For example, a 10 kΩ resistor has less than 12.6 nV/√Hz of
thermal noise and less than 10 nV of error voltage at room
temperature.

INPUT CAPACITANCE

Along with bypassing and ground, high speed amplifiers can be
sensitive to parasitic capacitance between the inputs and ground.
For circuits with resistive feedback network, the total capacitance,
whether it is the source capacitance, stray capacitance on the
input pin, or the input capacitance of the amplifier, causes a
breakpoint in the noise gain of the circuit. As a result, a
capacitor must be added in parallel with the gain resistor to
obtain stability. The noise gain is a function of frequency and
peaks at the higher frequencies, assuming the feedback capaci­tor is selected to make the second-order system critically
damped. A few picofarads of capacitance at the input reduce
the input impedance at high frequencies, which increases the
amplifier’s gain, causing peaking in the frequency response or
oscillations. With the AD8655/AD8656, additional input
damping is required for stability with capacitive loads greater
than 200 pF with direct input to output feedback. See the
Driving Capacitive Loads section.

DRIVING CAPACITIVE LOADS

Although the AD8655/AD8656 can drive capacitive loads up to
500 pF without oscillating, a large amount of ringing is present
when operating the part with input frequencies above 100 kHz.
This is especially true when the amplifiers are configured in
positive unity gain (worst case). When such large capacitive
loads are required, the use of external compensation is highly
recommended. This reduces the overshoot and minimizes
ringing, which, in turn, improves the stability of the
AD8655/AD8656 when driving large capacitive loads.

Data Sheet

One simple technique for compensation is a snubber that
consists of a simple RC network. With this circuit in place,
output swing is maintained, and the amplifier is stable at all
gains. Figure 57 shows the implementation of a snubber, which
reduces overshoot by more than 30% and eliminates ringing.
Using a snubber does not recover the loss of bandwidth
incurred from a heavy capacitive load.

VS = ±2.5V
A

= 1

V

C

= 500pF

L

VOLTAGE (100mV/DIV)

TIME (2μs/DIV)

Figure 56. Driving Heavy Capacitive Loads Without Compensation

V

CC

+

V–
V+

–

+
–

200mV

Figure 57. Snubber Network

VS = ±2.5V
A

= 1

V

R

= 200Ω

S

= 500pF

C

S

C

= 500pF

L

VOLTAGE (100mV/DIV)

Figure 58. Driving Heavy Capacitive Loads Using a Snubber Network

500pF

V

EE

TIME (10μs/DIV)

200Ω

500pF

05304-052

05304-051

05304-053

Rev. B | Page 16 of 20

Data Sheet

THD Readings vs. Common-Mode Voltage

Total harmonic distortion of the AD8655/AD8656 is well below

0.0007% with a load of 1 kΩ. This distortion is a function of the
circuit configuration, the voltage applied, and the layout, in
addition to other factors.

+2.5V

–

AD8655

+

–2.5V

V

IN

Figure 59. THD + N Test Circuit

V

OUT

R

L

05304-054

AD8655/AD8656

1.0

0.5

SWEEP 1:

= 2V p-p

V

IN

0.2

0.1

0.05

0.02

0.01

%

0.005

0.002

0.001

0.0005

0.0002

0.0001

= 10kΩ

R

L

SWEEP 2

SWEEP 1

20 100 1k 10k 80k50 500 5k 50k200 2k 20k

Hz

Figure 60. THD + Noise vs. Frequency

SWEEP 2:
V

= 2V p-p

IN

= 1kΩ

R

L

05304-055

Rev. B | Page 17 of 20

AD8655/AD8656

LAYOUT, GROUNDING, AND BYPASSING CONSIDERATIONS

POWER SUPPLY BYPASSING

Power supply pins can act as inputs for noise, so care must be
taken to apply a noise-free, stable dc voltage. The purpose of
bypass capacitors is to create low impedances from the supply
to ground at all frequencies, thereby shunting or filtering most
of the noise. Bypassing schemes are designed to minimize the
supply impedance at all frequencies with a parallel combination
of capacitors with values of 0.1 μF and 4.7 μF. Chip capacitors
of 0.1 μF (X7R or NPO) are critical and should be as close as
possible to the amplifier package. The 4.7 μF tantalum capacitor
is less critical for high frequency bypassing, and, in most cases,
only one is needed per board at the supply inputs.

GROUNDING

A ground plane layer is important for densely packed PC
boards to minimize parasitic inductances. This minimizes
voltage drops with changes in current. However, an under­standing of where the current flows in a circuit is critical to
implementing effective high speed circuit design. The length
of the current path is directly proportional to the magnitude
of parasitic inductances, and, therefore, the high frequency
impedance of the path. Large changes in currents in an
inductive ground return create unwanted voltage noise.

LEAKAGE CURRENTS

Poor PC board layout, contaminants, and the board insulator
material can create leakage currents that are much larger than
the input bias current of the AD8655/AD8656. Any voltage
differential between the inputs and nearby traces creates leakage
currents through the PC board insulator, for example, 1 V/100
GΩ = 10 pA. Similarly, any contaminants on the board can
create significant leakage (skin oils are a common problem).

To significantly reduce leakage, put a guard ring (shield) around
the inputs and input leads that are driven to the same voltage
potential as the inputs. This ensures there is no voltage potential
between the inputs and the surrounding area to create any
leakage currents. To be effective, the guard ring must be driven
by a relatively low impedance source and should completely
surround the input leads on all sides, above and below, by using
a multilayer board.

The charge absorption of the insulator material itself can also
cause leakage currents. Minimizing the amount of material
between the input leads and the guard ring helps to reduce the
absorption. Also, using low absorption materials, such as
Teflon® or ceramic, may be necessary in some instances.

Data Sheet

The length of the high frequency bypass capacitor leads is
critical, and, therefore, surface-mount capacitors are recom­mended. A parasitic inductance in the bypass ground trace
works against the low impedance created by the bypass
capacitor. Because load currents flow from the supplies, the
ground for the load impedance should be at the same physical
location as the bypass capacitor grounds. For larger value
capacitors intended to be effective at lower frequencies, the
current return path distance is less critical.

Rev. B | Page 18 of 20

Data Sheet

OUTLINE DIMENSIONS

5.00(0.1968)

4.80(0.1890)

4.00 (0.1574)

3.80 (0.1497)

0.25 (0.0098)

0.10 (0.0040)

COPLANARITY

0.10

CONTROLLING DIMENSIONS ARE IN MILLIMETERS; INCH DIMENSIONS
(IN PARENTHESES)ARE ROUNDED-OFF MILLIMETER EQUIVALENTS FOR
REFERENCE ONLY AND ARE NOT APPROPRIATE FOR USE IN DESIGN.

85

1

1.27 (0.0500)

SEATING

PLANE

COMPLIANT TO JEDEC STANDARDS MS-012-AA

Figure 61. 8-Lead Standard Small Outline Package [SOIC_N]

Dimensions shown in millimeters and (inches)

6.20 (0.2441)

5.80 (0.2284)

4

BSC

0.51 (0.0201)

0.31 (0.0122)

Narrow Body (R-8)

1.75 (0.0688)

1.35 (0.0532)
8°

0°

0.25 (0.0098)

0.17 (0.0067)

0.50 (0.0196)

0.25 (0.0099)

1.27 (0.0500)

0.40 (0.0157)

45°

012407-A

3.20

3.00

2.80

PIN 1

IDENTIFIER

0.95

0.85

0.75

0.15

0.05

COPLANARITY

AD8655/AD8656

3.20

3.00

2.80

8

5

5.15

4.90

1

0.65 BSC

0.10

COMPLIANT TO JEDEC STANDARDS MO-187-AA

Figure 62. 8-Lead Mini Small Outline Package [MSOP]

4.65

4

15° MAX

6°
0°

0.23

0.09

0.40

0.25

1.10 MAX

(RM-8)

Dimensions shown in millimeters

0.80

0.55

0.40

10-07-2009-B

ORDERING GUIDE

1, 2

Model

AD8655ARZ −40°C to +125°C 8-Lead SOIC_N R-8
AD8655ARZ-REEL −40°C to +125°C 8-Lead SOIC_N R-8
AD8655ARZ-REEL7 −40°C to +125°C 8-Lead SOIC_N R-8
AD8655ARMZ-REEL −40°C to +125°C 8-Lead MSOP RM-8 A0D
AD8655ARMZ −40°C to +125°C 8-Lead MSOP RM-8 A0D
AD8656ARZ −40°C to +125°C 8-Lead SOIC_N R-8
AD8656ARZ-REEL −40°C to +125°C 8-Lead SOIC_N R-8
AD8656ARZ-REEL7 −40°C to +125°C 8-Lead SOIC_N R-8
AD8656ARMZ −40°C to +125°C 8-Lead MSOP RM-8 A0S
AD8656ARMZ-REEL −40°C to +125°C 8-Lead MSOP RM-8 A0S
AD8656WARMZ-REEL −40°C to +125°C 8-Lead MSOP RM-8 A0S

1

Z = RoHS Compliant Part.

2

W = Qualified for Automotive Applications.

Temperature
Range Package Description Package Option Branding

AUTOMOTIVE PRODUCTS

The AD8656W model is available with controlled manufacturing to support the quality and reliability requirements of automotive
applications. Note that these automotive models may have specifications that differ from the commercial models; therefore, designers
should review the Specifications section of this data sheet carefully. Only the automotive grade product shown is available for use in
automotive applications. Contact your local Analog Devices account representative for specific product ordering information and to
obtain the specific Automotive Reliability reports for this model.

.

Rev. B | Page 19 of 20

AD8655/AD8656

NOTES

Data Sheet

©2005–2011 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.

D05304-0-9/11(B)

Rev. B | Page 20 of 20