AD8222-EVAL 查看數據表（PDF） - Analog Devices

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AD8222-EVAL

Precision, Dual-Channel Instrumentation Amplifier

Analog Devices 

APPLICATIONS

DIFFERENTIAL OUTPUT

The differential configuration of the AD8222 has the same

excellent dc precision specifications as the single-ended output

configuration and is recommended for applications in the

frequency range of dc to 100 kHz.

The circuit configuration is shown in Figure 49. The differential

output specification in Table 2 and Table 4 refer to this

configuration only. The circuit includes an RC filter that maintains

the stability of the loop.

The transfer function for the differential output is:

VDIFF_OUT = V+OUT − V−OUT = (V+IN − V−IN) × G

where

49.4 kΩ

G =1+

RG

+IN

+

RG AD8222

–IN

–

+OUT

10kΩ

AD8222

100pF

+IN2

REF2

–OUT

Figure 49. Differential Circuit Schematic

AD8222

Setting the Common-Mode Voltage

The output common-mode voltage is set by the average of +IN2

and REF2. The transfer function is

VCM_OUT = (V+OUT + V−OUT)/2 = (V+IN2 + VREF2)/2

+IN2 and REF2 have different properties that allow the

reference voltage to be easily set for a wide variety of applications.

+IN2 has high impedance but cannot swing to the supply rails

of the part. REF2 must be driven with a low impedance but can

go 300 mV beyond the supply rails.

A common application sets the common-mode output voltage

to the midscale of a differential ADC. In this case, the ADC

reference voltage would be sent to the +IN2 terminal, and

ground would be connected to the REF2 terminal. This would

produce a common-mode output voltage of half the ADC

reference voltage.

2-Channel Differential Output Using a Dual Op Amp

Another differential output topology is shown in Figure 50.

Instead of a second in-amp, ½ of a dual OP2177 op amp creates

the inverted output. Because the OP2177 comes in an MSOP,

this configuration allows the creation of a dual channel,

precision differential output in-amp with little board area.

Errors from the op amp are common to both outputs and are

thus common mode. Errors from mismatched resistors also

create a common-mode dc offset. Because these errors are

common mode, they will likely be rejected by the next device

in the signal chain.

+IN

AD8222

–IN

REF 4.99kΩ

4.99kΩ

+OUT

VREF

–

+

OP2177

–OUT

Figure 50. Differential Output Using Op Amp

Rev. 0 | Page 19 of 24

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