AD8220ARMZ-R7 datasheet(24/29 Pages) AD

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AD8220

Rev. B | Page 23 of 28

APPLICATIONS INFORMATION

AC-COUPLED INSTRUMENTATION AMPLIFIER

Measuring small signals that are in the noise or offset of the

amplifier can be a challenge. Figure 64 shows a circuit that

can improve the resolution of small ac signals. The large gain

reduces the referred input noise of the amplifier to 14 nV/√Hz.

Therefore, smaller signals can be measured because the noise

floor is lower. DC offsets that would have been gained by 100

are eliminated from the AD8220 output by the integrator

feedback network.

At low frequencies, the OP1177 forces the AD8220 output to

0 V. Once a signal exceeds fHIGH-PASS, the AD8220 outputs the

amplified input signal.

AD8220

OP1177

15.8kΩ

+VS

+IN

–IN

0.1µF

10µF

REF

1µF

–VS

+VS

–VS

499Ω

2πRC

fHIGH-PASS =

VREF

Figure 64. AC-Coupled Circuit

DIFFERENTIAL OUTPUT

In certain applications, it is necessary to create a differential

signal. New high resolution ADCs often require a differential

input. In other cases, transmission over a long distance can

require differential processing for better immunity to

interference.

Figure 65 shows how to configure the AD8220 to output a

differential signal. An OP1177 op amp is used to create a

differential voltage. Errors from the op amp are common to

both outputs and are thus common mode. Likewise, errors from

using mismatched resistors cause a common-mode dc offset

error. Such errors are rejected in differential signal processing

by differential input ADCs or instrumentation amplifiers.

When using this circuit to drive a differential ADC, VREF can be

set using a resistor divider from the reference of the ADC to

make the output ratiometric with the ADC as shown in Figure 66.

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