AD8271

Rev. 0 | Page 18 of 20

Many signal gains have more than one configuration choice, which

allows freedom in choosing the op amp closed-loop gain. In

general, for designs that need to be stable with a large capacitive

load on the output, choose a configuration with high loop gain.

Otherwise, choose a configuration with low loop gain, because

these configurations typically have lower noise, lower offset,

and higher bandwidth.

The AD8271 Specifications section and Typical Performance

Characteristics section show the performance of the part primarily

when it is in the difference amplifier configuration. To estimate

the performance of the part in a single-ended configuration, refer

to the difference amplifier configuration with the corresponding

closed-loop gain (see Table 10).

Table 10. Closed-Loop Gain of the Difference Amplifiers

Difference Amplifier Gain

Closed-Loop Gain

0.5

1.5

1

2

2

3

Gain of 1 Configuration

The AD8271 is designed to be stable for loop gains of 1.5 and

greater. Because a typical voltage follower configuration has

a loop gain of 1, it may be unstable. Several stable configurations

for gain of 1 are listed in Table 9.

KELVIN MEASUREMENT

In the case where the output load is located remotely or at

a distance from the AD8271, as shown in Figure 51, wire

resistance can actually cause significant errors at the load.

–IN

10k

Ω

10k

Ω

10k

Ω

10k

Ω

+IN

(WIRE RESISTANCE)

1k

Ω

Figure 51. Wire Resistance Causes Errors at Load Voltage

Since the output of the AD8271 is not internally tied to any of

the feedback resistors, Kelvin type measurements are possible

because the op amp output and feedback can both be connected

closer to the load (Figure 52). The Kelvin sensing on the feedback

minimizes error at the load caused by voltage drops across the

wire resistance. This technique is most effective in reducing errors

for loads less than 10 kΩ. As the load resistance increases, the

error due to the wire resistance becomes less significant.

Because it adds the sense wire resistance to the feedback resistor, a

trade-off of the Kelvin connection is that it can degrade common-

mode rejection, especially over temperature. For sense wire

resistance less than 1 Ω, it is typically not an issue. If common-

mode performance is critical, two amplifier stages can be used:

the first stage removes common-mode interference, and the

second stage performs the Kelvin drive.

10k

Ω

SENSE

FORCE

–IN

10k

Ω

10k

Ω

10k

Ω

+IN

1k

Ω

Figure 52. Connecting Both the Output and Feedback at the Load Minimizes

Error Due to Wire Resistance

INSTRUMENTATION AMPLIFIER

The AD8271 can be used as a building block for high performance

instrumentation amplifiers. For example, Figure 53 shows how

to build an ultralow noise instrumentation amplifier using the

therefore, the output is

()

()

8271

2

1

AD

G

Fx

IN

IN

OUT

G

R

R

V

V

V

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

+

−

=

−

+

–IN

+IN

10k

Ω

10k

Ω

10k

Ω

10k

Ω

REF

AD8599

A2

AD8599

A2

20

Ω

2k

Ω

AD8271

OUT

2k

Ω

Figure 53.Ultralow Noise Instrumentation Amplifier Using AD8599

Configured for Gain = 201

For optimal noise performance, it is desirable to have a high

gain at the input stage using low value gain-setting resistors, as

shown in this particular example. With less than 2 nV/√Hz

input-referred noise (see Figure 54) at ~10 mA supply current,

the AD8271 and AD8599 combination offers an in-amp with a

fine balance of critical specifications: a gain bandwidth product

of 10 MHz, low bias current, low offset drift, high CMRR, and

high slew rate.

0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

10.0

1

10

100

1k

10k

100k

FREQUENCY (Hz)

G = 201

BANDWIDTH

LIMIT

Figure 54. Ultralow Noise In-Amp Voltage Noise Spectral Density vs.

Frequency, Referred to Input