AD8220BRMZ-R7 datasheet(23/28 Pages) AD

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AD8220

Rev. 0 | Page 23 of 28

APPLICATIONS

AC-COUPLED INSTRUMENTATION AMPLIFIER

Measuring small signals that are in the amplifier’s noise or offset

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. Thus,

smaller signals can be measured since 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.

DIFFERENTIAL OUTPUT

In certain applications, it is necessary to create a differential

signal. New high resolution analog-to-digital converters 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 ADC’s reference to make

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

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

AD8220BRMZ-R7 数据表(PDF) 23 Page - Analog Devices

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