Circuit Note

CN-0122

Circuit Designs Using Analog Devices Products

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Devices Connected/Referenced

AD8271/AD8274 Difference Amplifier

ADA4627-1

High Speed JFET Input Operational

Amplifier

AD8599 Low Noise Operational Amplifier

High Speed Instrumentation Amplifier Using the AD8271 Difference Amplifier and

the ADA4627-1 JFET Input Op Amp

Rev.A

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CIRCUIT FUNCTION AND BENEFITS

A traditional method for building an instrumentation amplifier is

to use three op amps and seven resistors as shown in Figure 1.

This approach requires four precision matched resistors for a good

common-mode rejection ratio (CMRR). Errors in matching will

produce errors at the final output. An imbalance of one or two

picofarads on certain nodes will drastically degrade the high

frequency CMRR, a fact often overlooked.

This circuit uses a monolithic difference amplifier with laser

trimmed thin film resistors for the output amplifier, thereby

providing good dc and ac accuracy with fewer components

than the traditional approach.

CIRCUIT DESCRIPTION

This circuit utilizes the AD8271 difference amplifier and two

ADA4627-1 amplifiers, which have low noise, low drift, low

offset, and high speed. For high impedance sources, the

ADA4627-1 is an ideal choice for the input stage amplifiers due

to the extremely low input bias current of their JFET inputs.

The op amps selected for the input stage must also have low

offset voltage and low offset voltage drift with temperature. They

also need to have good drive characteristics. This allows the use

of low value resistors to minimize resistor thermal noise.

Headroom issues relating to the op amp must be considered in

this circuit for proper operation.

When working with any op amp having a gain-bandwidth

product greater than a few MHz, careful layout and bypassing

are essential. A typical decoupling network consists of a 1 µF

to 10 µF electrolytic capacitor in parallel with a 0.01 µF to

0.1 µF low inductance ceramic MLCC type.

For the lowest noise with low impedance sources only, low

voltage noise is important. The AD8599 has lower noise, lower

offset voltage drift, and lower supply current; but the input bias

currents are much higher, and the bandwidth will be lower than

that obtained with the ADA4627-1. The measured −3 dB points

–IN

+IN

10kΩ

ADA4627-1

20Ω

2kΩ

AD8271

OUT

2kΩ

= ±15V

08517-001

–V

NOTES

1. 10kΩ THIN FILM TRIMMED RESISTOR

ARE INTERNAL TO THE AD8271.

Figure 1. In Amp with Gain = 201

(Simplified Schematic: Decoupling and All Connections Not Shown)

are 56.6 kHz and 87.6 kHz for the AD8599 and ADA4627-1,

respectively. (See Figure 2).

With high impedance sources, the input bias current and the

input noise current of a bipolar op amp can result in errors. The

bias current creates an I × R drop, which will be multiplied by

the overall circuit gain. This can result in several volts of offset

at the output. The input noise current is also multiplied by the

source impedances, creating an additional noise voltage. To

avoid this, a JFET input op amp, such as the ADA4627-1,

should be used. Even though the voltage noise is slightly higher

than the AD8599, the current noise is significantly lower,

resulting in lower overall noise when used with high impedance

sources.

As Figure 3 and Figure 4 show, the AD8599 is the proper choice

with low source impedances, and the ADA4627-1 is better with

higher source impedances. There is a trade-off: the input

capacitance of JFET op amps is higher than bipolar op amps, so

the RC time constant must be considered.

CN-0122 Circuit Note

Rev. A | Page 2 of 3

100 1k 10k 100k 1M

GAIN (dB)

FREQUENCY (Hz)

08517-002

ADA4627-1

AD8599

Figure 2. Bandwidth of Circuit Shown in Figure 1 Comparing the ADA4627-1

to the AD8599 as the Input Stage.

0.1

0.02

0.1 1 10 100 200

RTO NOISE (µV/√Hz)

FREQUENCY (kHz)

08517-003

ADA4627-1

SOURCE

= 0Ω

AD8599

Figure 3. Noise Spectral Density (RTO) of Circuit Shown in Figure 1

Comparing the ADA4627-1 to the AD8599 as the Input Stage:

Low Impedance Source (0 Ω)

10k

100

0.02 0.1 1 10 100 200

RTO NOISE (µV/√Hz)

FREQUENCY (kHz)

08517-004

AD8599

ADA4627-1

SOURCE

= 66kΩ

Figure 4. Noise Spectral Density (RTO) of Circuit Shown in Figure 1

Comparing the ADA4627-1 to the AD8599 as the Input Stage:

High Impedance Source (66 kΩ)

COMMON VARIATIONS

The AD8271 or AD8274 can be used with a variety of op amps

to optimize the overall performance with respect to supply

current, signal bandwidth, temperature drift, and noise.

For the lowest possible drift over temperature, one of the auto-

zero amplifiers, such as the AD8539, can be used, but the band-

width will be reduced and wideband noise increased. This would

be an excellent choice for bandwidths less than 10 Hz, however.

When selecting op amp and difference amplifier combinations

for this circuit, always ensure that the input common-mode

voltage range of each amplifier is not violated. This is com-

monly overlooked but is the subject of a fair number of

application questions.

If the first stage gain is greater than about five, consider using a

decompensated op amp, such as the OP37, to get a higher slew rate

and signal bandwidth with less supply current. To avoid common-

mode oscillation, the circuit must be modified slightly as described

in "Phase Compensation of the Three Op Amp Instrumentation

Amplifier." White, D. Rod. IEEE Transactions on Instrumentation

and Measurement. Vol. IM-36, No. 3, Sept. 1987.

With microvolt-level input signals and a gain of 1000, the first

stage can be operated on ±2.5 V, saving power and giving more

choices of op amps, such as the AD8539 auto-zero amplifier.

However, if the input common-mode voltage range is high, an op

amp with a higher supply voltage must be chosen for the first stage.

LEARN MORE

Jung, Walter G. 2005. Op Amp Applications Handbook.

Elsevier/Newnes. 2005. ISBN 0-7506-7844-5. Chapter 2.

Kitchin, Charles and Lew Counts. 2006. A Designer’s Guide to

Instrumentation Amplifiers, 3rd Edition. Analog Devices.

MT-031 Tutorial, Grounding Data Converters and Solving the

Mystery of AGND and DGND. Analog Devices.

MT-055 Tutorial, Chopper Stabilized (Auto-Zero) Precision

Op Amps. Analog Devices.

MT-061 Tutorial, Instrumentation Amplifiers (In-Amp) Basics.

Analog Devices.

MT-063 Tutorial, Basic Three Op Amp In-Amp Configuration.

Analog Devices.

MT-064 Tutorial, In-Amp DC Error Sources. Analog Devices.

MT-065 Tutorial, In-Amp Noise. Analog Devices.

MT-068 Tutorial, Difference Amplifiers. Analog Devices.

MT-101 Tutorial, Decoupling Techniques. Analog Devices.

White, D. Rod, "Phase Compensation of the Three Op Amp

Instrumentation Amplifier." IEEE Transactions on

Instrumentation and Measurement. Vol IM-36, No. 3,

Sept. 1987.