ANALOG DEVICES AD8421 Service Manual

3 nV/√Hz, Low Power

Data Sheet

FEATURES

Low power

2.3 mA maximum supply current

Low noise

3.2 nV/√Hz maximum input voltage noise at 1 kHz 200 fA/√Hz current noise at 1 kHz

Excellent ac specifications

10 MHz bandwidth (G = 1) 2 MHz bandwidth (G = 100)

0.6 μs settling time to 0.001% (G = 10) 80 dB CMRR at 20 kHz (G = 1) 35 V/μs slew rate

High precision dc performance (AD8421BRZ)

94 dB CMRR minimum (G = 1)

0.2 μV/°C maximum input offset voltage drift 1 ppm/°C maximum gain drift (G = 1)

500 pA maximum input bias current Inputs protected to 40 V from opposite supply ±2.5 V to ±18 V dual supply (5 V to 36 V single supply) Gain set with a single resistor (G = 1 to 10,000)

Instrumentation Amplifier

AD8421

PIN CONNECTION DIAGRAM

AD8421

1

–IN

2

R

G

3

R

G

4

+IN

TOP VIEW

(Not to Scale)

Figure 1.

10µ

G = 100

1µ

100n

8

+V

S

7

V

OUT

6

REF

5

–V

S

10123-001

BEST AVAILABLE 7mA LOW NOISE IN-AMP

APPLICATIONS

Medical instrumentation Precision data acquisition Microphone preamplification Vibration analysis Multiplexed input applications ADC driver

GENERAL DESCRIPTION

The AD8421 is a low cost, low power, extremely low noise, ultralow bias current, high speed instrumentation amplifier that is ideally suited for a broad spectrum of signal conditioning and data acquisition applications. This product features extremely high CMRR, allowing it to extract low level signals in the presence of high frequency common-mode noise over a wide temperature range.

The 10 MHz bandwidth, 35 V/µs slew rate, and 0.6 µs settling time to 0.001% (G = 10) allow the AD8421 to amplify high speed signals and excel in applications that require high channel count, multiplexed systems. Even at higher gains, the current feedback architecture maintains high performance; for example, at G = 100, the bandwidth is 2 MHz and the settling time is 0.8 µs. The

AD8421 has excellent distortion performance, making it suitable

for use in demanding applications such as vibration analysis.

Rev. 0

Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Anal og Devices for its use, nor for any infringements of patents or ot her 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.

BEST AVAILABLE 1mA LOW POWER IN-AMP

10n

AD8421

TOTAL NOISE DENSITY AT 1kHz (V/√Hz)

1n

100 1M

Figure 2. Noise Density vs. Source Resistance

R

NOISE ONLY

S

1k 10k 100k

SOURCE RESISTANCE, RS (Ω)

The AD8421 delivers 3 nV/√Hz input voltage noise and 200 fA/√Hz current noise with only 2 mA quiescent current, making it an ideal choice for measuring low level signals. For applications with high source impedance, the AD8421 employs innovative process technology and design techniques to provide noise performance that is limited only by the sensor.

The AD8421 uses unique protection methods to ensure robust inputs while still maintaining very low noise. This protection allows input voltages up to 40 V from the opposite supply rail without damage to the part.

A single resistor sets the gain from 1 to 10,000. The reference pin can be used to apply a precise offset to the output voltage.

The AD8421 is specified from −40°C to +85°C and has typical performance curves to 125°C. It is available in 8-lead MSOP and SOIC packages.

10123-078

AD8421 Data Sheet

REVISION HISTORY

5/12—Revision 0: Initial Version

Rev. 0 | Page 2 of 28

Data Sheet AD8421

SPECIFICATIONS

VS = ±15 V, V

AR AND BR GRADES

Table 1.

Parameter

COMMON-MODE REJECTION

RATIO (CMRR) CMRR DC to 60 Hz with 1 kΩ

Source Imbalance G = 1 86 94 dB G = 10 106 114 dB G = 100 126 134 dB G = 1000 136 140 dB Over Temperature, G = 1 T = −40°C to +85°C 80 93 dB

CMRR at 20 kHz VCM = −10 V to +10 V

G = 1 80 80 dB G = 10 90 100 dB G = 100 100 110 dB G = 1000 110 120 dB

NOISE

Voltage Noise, 1 kHz1 V

Input Voltage Noise, eni 3 3.2 3 3.2 nV/√Hz Output Voltage Noise, eno 60 60 nV/√Hz

Peak to Peak, RTI f = 0.1 Hz to 10 Hz

G = 1 2 2 2.2 μV p-p G = 10 0.5 0.5 μV p-p G = 100 to 1000 0.07 0.07 0.09 μV p-p

Current Noise

Spectral Density f = 1 kHz 200 200 fA/√Hz Peak to Peak, RTI f = 0.1 Hz to 10 Hz 18 18 pA p-p

VOLTAGE OFFSET2

Input Offset Voltage, V

Over Temperature TA = −40°C to +85°C 86 45 μV Average TC 0.4 0.2 μV/°C

Output Offset Voltage, V

Over Temperature TA = −40°C to +85°C 0.66 0.45 mV Average TC 6 5 μV/°C

Offset RTI vs. Supply (PSR) VS = ±2.5 V to ±18 V

G = 1 90 120 100 120 dB G = 10 110 120 120 140 dB G = 100 124 130 140 150 dB G = 1000 130 140 140 150 dB

INPUT CURRENT

Input Bias Current 1 2 0.1 0.5 nA

Over Temperature TA = −40°C to +85°C 8 6 nA Average TC 50 50 pA/°C

Input Offset Current 0.5 2 0.1 0.5 nA

Over Temperature TA = −40°C to +85°C 2.2 0.8 nA Average TC 1 1 pA/°C

= 0 V, TA = 25°C, G = 1, RL = 2 k, unless otherwise noted.

REF

Test Conditions/ Comments

Min Typ Max Min Typ Max

V

= −10 V to +10 V

CM

+, VIN− = 0 V

IN

V

OSI

350 250 μV

OSO

= ±5 V to ±15 V 60 25 μV

S

AR Grade BR Grade

Unit

Rev. 0 | Page 3 of 28

AD8421 Data Sheet

Test Conditions/

Parameter

Comments

Min Typ Max Min Typ Max

DYNAMIC RESPONSE

Small Signal Bandwidth −3 dB

G = 1 10 10 MHz G = 10 10 10 MHz G = 100 2 2 MHz G = 1000 0.2 0.2 MHz

Settling Time to 0.01% 10 V step

G = 1 0.7 0.7 μs G = 10 0.4 0.4 μs G = 100 0.6 0.6 μs G = 1000 5 5 μs

Settling Time to 0.001% 10 V step

G = 1 1 1 μs G = 10 0.6 0.6 μs G = 100 0.8 0.8 μs G = 1000 6 6 μs

Slew Rate

G = 1 to 100 35 35 V/μs

GAIN3 G = 1 + (9.9 kΩ/RG)

Gain Range 1 10,000 1 10,000 V/V

Gain Error V

= ±10 V

OUT

G = 1 0.02 0.01 % G = 10 to 1000 0.2 0.1 %

Gain Nonlinearity V

= −10 V to +10 V

OUT

G = 1 RL ≥ 2 kΩ 1 1 ppm R

= 600 Ω 1 3 1 3 ppm

L

G = 10 to 1000 RL ≥ 600 Ω 30 50 30 50 ppm V

= −5 V to +5 V 5 10 5 10 ppm

OUT

Gain vs. Temperature3

G = 1 5 0.1 1 ppm/°C G > 1 −50 −50 ppm/°C

INPUT

Input Impedance

Differential 30||3 30||3 GΩ||pF Common Mode 30||3 30||3 GΩ||pF

Input Operating Voltage Range4 VS = ±2.5 V to ±18 V −VS + 2.3 +VS − 1.8 −VS + 2.3 +VS − 1.8 V

Over Temperature TA = −40°C −VS + 2.5 +VS − 2.0 −VS + 2.5 +VS − 2.0 V T

= +85°C −VS + 2.1 +VS − 1.8 −VS + 2.1 +VS − 1.8 V

A

OUTPUT RL = 2 kΩ

Output Swing VS = ±2.5 V to ±18 V −VS + 1.2 +Vs − 1.6 −VS + 1.2 +VS − 1.6 V

Over Temperature TA = −40°C to +85°C −VS + 1.2 +Vs − 1.6 −VS + 1.2 +VS − 1.6 V

Short-Circuit Current 65 65 mA REFERENCE INPUT

RIN 20 20 kΩ

IIN V

+, VIN− = 0 V 20 24 20 24 μA

IN

Voltage Range −VS +VS −VS +VS V

Reference Gain to Output

AR Grade BR Grade

1 ±

0.0001

1 ±

0.0001

Unit

V/V

Rev. 0 | Page 4 of 28

Data Sheet AD8421

Test Conditions/

Parameter

Comments

Min Typ Max Min Typ Max

POWER SUPPLY

Operating Range Dual supply ±2.5 ±18 ±2.5 ±18 V Single supply 5 36 5 36 V Quiescent Current 2 2.3 2 2.3 mA

Over Temperature TA = −40°C to +85°C 2.6 2.6 mA

TEMPERATURE RANGE

For Specified Performance −40 +85 −40 +85 °C Operational5 −40 +125 −40 +125 °C

1

Total voltage noise = √(e

2

Total RTI VOS = (V

3

These specifications do not include the tolerance of the external gain setting resistor, RG. For G > 1, add RG errors to the specifications given in this table.

4

Input voltage range of the AD8421 input stage only. The input range can depend on the common-mode voltage, differential voltage, gain, and reference voltage.

See the section for more details. Input Voltage Range

5

See the section for expected operation between 85°C and 125°C. Typical Performance Characteristics

OSI

) + (V

2

+ (eno/G)2 + e

ni

/G).

OSO

2

). See the Th section for more information. eory of Operation

RG

ARM AND BRM GRADES

Table 2.

Test Conditions/

Parameter

COMMON-MODE REJECTION

RATIO (CMRR) CMRR DC to 60 Hz with 1 kΩ

Source Imbalance G = 1 84 92 dB G = 10 104 112 dB G = 100 124 132 dB G = 1000 134 140 dB Over Temperature, G = 1 TA = −40°C to +85°C 80 90 dB

CMRR at 20 kHz VCM = −10 V to +10 V

G = 1 80 80 dB G = 10 90 90 dB G = 100 100 100 dB G = 1000 100 100 dB

NOISE

Voltage Noise, 1 kHz1 V

Input Voltage Noise, eni 3 3.2 3 3.2 nV/√Hz Output Voltage Noise, eno 60 60 nV/√Hz

Peak to Peak, RTI f = 0.1 Hz to 10 Hz

G = 1 2 2 2.2 μV p-p G = 10 0.5 0.5 μV p-p G = 100 to 1000 0.07 0.07 0.09 μV p-p

Current Noise

Spectral Density f = 1 kHz 200 200 fA/√Hz Peak to Peak, RTI f = 0.1 Hz to 10 Hz 18 18 pA p-p

VOLTAGE OFFSET2

Input Offset Voltage, V

V

OSI

Over Temperature TA = −40°C to +85°C 135 135 μV Average TC 0.9 0.9 μV/°C

Output Offset Voltage, V

OSO

Over Temperature TA = −40°C to +85°C 1 1 mV Average TC 9 9 μV/°C

Comments

Min Typ Max Min Typ Max

= −10 V to +10 V

V

CM

+, VIN− = 0 V

IN

= ±5 V to ±15 V 70 50 μV

S

600 400 μV

AR Grade BR Grade

Unit

ARM Grade BRM Grade

Unit

Rev. 0 | Page 5 of 28

AD8421 Data Sheet

Test Conditions/

Parameter

Comments

Min Typ Max Min Typ Max

Offset RTI vs. Supply (PSR) VS = ±2.5 V to ±18 V

G = 1 90 120 100 120 dB G = 10 110 120 120 140 dB G = 100 124 130 140 150 dB G = 1000 130 140 140 150 dB

INPUT CURRENT

Input Bias Current 1 2 0.1 1 nA

Over Temperature TA = −40°C to +85°C 8 6 nA Average TC 50 50 pA/°C

Input Offset Current 0.5 2 0.1 1 nA

Over Temperature TA = −40°C to +85°C 3 1.5 nA Average TC 1 1 pA/°C

DYNAMIC RESPONSE

Small Signal Bandwidth −3 dB

G = 1 10 10 MHz G = 10 10 10 MHz G = 100 2 2 MHz G = 1000 0.2 0.2 MHz

Settling Time 0.01% 10 V step

G = 1 0.7 0.7 μs G = 10 0.4 0.4 μs G = 100 0.6 0.6 μs G = 1000 5 5 μs

Settling Time 0.001% 10 V step

G = 1 1 1 μs G = 10 0.6 0.6 μs G = 100 0.8 0.8 μs G = 1000 6 6 μs

Slew Rate

G = 1 to 100 35 35 V/μs

GAIN3 G = 1 + (9.9 kΩ/RG)

Gain Range 1 10,000 1 10,000 V/V Gain Error V

= ±10 V

OUT

G = 1 0.05 0.02 % G = 10 to 1000 0.3 0.2 %

Gain Nonlinearity V

= −10 V to +10 V

OUT

G = 1 RL ≥ 2 kΩ 1 1 ppm R

= 600 Ω 1 3 1 3 ppm

L

G = 10 to 1000 RL ≥ 600 Ω 30 50 30 50 ppm V

= −5 V to +5 V 5 10 5 10 ppm

OUT

Gain vs. Temperature3

G = 1 5 0.1 1 ppm/°C G > 1 −50 −50 ppm/°C

INPUT

Input Impedance

Differential 30||3 30||3 GΩ||pF Common Mode 30||3 30||3 GΩ||pF

Input Operating Voltage

Range

4

VS = ±2.5 V to ±18 V −VS + 2.3 +VS − 1.8 −VS + 2.3 +VS − 1.8 V

Over Temperature TA = −40°C −VS + 2.5 +VS − 2.0 −VS + 2.5 +VS − 2.0 V T

= +85°C −VS + 2.1 +VS − 1.8 −VS + 2.1 +VS − 1.8 V

A

ARM Grade BRM Grade

Unit

Rev. 0 | Page 6 of 28

Data Sheet AD8421

Test Conditions/

Parameter

Comments

Min Typ Max Min Typ Max

OUTPUT RL = 2 kΩ

Output Swing VS = ±2.5 V to ±18 V −VS + 1.2 +VS − 1.6 −VS + 1.2 +Vs − 1.6 V

Over Temperature TA = −40°C to +85°C −VS + 1.2 +VS − 1.6 −VS + 1.2 +Vs − 1.6 V

Short-Circuit Current 65 65 mA

REFERENCE INPUT

RIN 20 20 kΩ IIN V

+, VIN− = 0 V 20 24 20 24 μA

IN

Voltage Range −VS +VS −VS +VS V Reference Gain to Output

POWER SUPPLY

Operating Range Dual supply ±2.5 ±18 ±2.5 ±18 V Single supply 5 36 5 36 V Quiescent Current 2 2.3 2 2.3 mA

Over Temperature TA = −40°C to +85°C 2.6 2.6 mA

TEMPERATURE RANGE

For Specified Performance −40 +85 −40 +85 °C Operational5 −40 +125 −40 +125 °C

1

Total voltage noise = √(e

2

Total RTI VOS = (V

3

These specifications do not include the tolerance of the external gain setting resistor, RG. For G > 1, add RG errors to the specifications given in this table.

4

Input voltage range of the AD8421 input stage only. The input range can depend on the common-mode voltage, differential voltage, gain, and reference voltage.

See the section for more information. Input Voltage Range

5

See the section for expected operation between 85°C and 125°C. Typical Performance Characteristics

OSI

) + (V

2

+ (eno/G)2 + e

ni

/G).

OSO

2

). See the Th section for more information. eory of Operation

RG

ARM Grade BRM Grade

1 ±

0.0001

1 ±

0.0001

Unit

V/V

Rev. 0 | Page 7 of 28

AD8421 Data Sheet

ABSOLUTE MAXIMUM RATINGS

Table 3.

Parameter Rating

Supply Voltage ±18 V Output Short-Circuit Current Duration Indefinite Maximum Voltage at −IN or +IN1 −VS + 40 V Minimum Voltage at −IN or +IN +VS − 40 V Maximum Voltage at REF2 +VS + 0.3 V Minimum Voltage at REF −VS − 0.3 V Storage Temperature Range −65°C to +150°C Operating Temperature Range −40°C to +125°C Maximum Junction Temperature 150°C ESD

Human Body Model 2 kV Charged Device Model 1.25 kV Machine Model 0.2 kV

1

For voltages beyond these limits, use input protection resistors. See the

Theory of Operation section for more information.

2

There are ESD protection diodes from the reference input to each supply, so

REF cannot be driven beyond the supplies in the same way that +IN and −IN can. See the Reference Terminal section for more information.

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

θJA is specified for a device in free air using a 4-layer JEDEC printed circuit board (PCB).

Table 4.

Package θJA Unit

8-Lead SOIC 107.8 °C/W 8-Lead MSOP 138.6 °C/W

ESD CAUTION

Rev. 0 | Page 8 of 28

Data Sheet AD8421

PIN CONFIGURATION AND FUNCTION DESCRIPTIONS

–IN

R

+IN

G

1

2

3

4

(Not to Scale)

AD8421

TOP VIEW

8

+V

S

7

V

OUT

6

REF

5

–V

S

10123-002

Figure 3. Pin Configuration

Table 5. Pin Function Descriptions

Pin No. Mnemonic Description

1 −IN Negative Input Terminal. 2, 3 RG Gain Setting Terminals. Place resistor across the RG pins to set the gain. G = 1 + (9.9 kΩ/RG). 4 +IN Positive Input Terminal. 5 −VS Negative Power Supply Terminal. 6 REF Reference Voltage Terminal. Drive this terminal with a low impedance voltage source to level shift the output. 7 V

Output Terminal.

OUT

8 +VS Positive Power Supply Terminal.

Rev. 0 | Page 9 of 28

ANALOG DEVICES AD8421 Service Manual

FEATURES

PIN CONNECTION DIAGRAM

APPLICATIONS

GENERAL DESCRIPTION

TABLE OF CONTENTS

REVISION HISTORY

SPECIFICATIONS

AR AND BR GRADES

ARM AND BRM GRADES

ABSOLUTE MAXIMUM RATINGS

THERMAL RESISTANCE

ESD CAUTION

PIN CONFIGURATION AND FUNCTION DESCRIPTIONS