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Data Sheet

AD8231

Rev. E | Page 21 of 24

APPLICATIONS INFORMATION

DIFFERENTIAL OUTPUT

Figure 53 shows how to create a differential output in-amp

using the AD8231 uncommitted op amp. Because this

configuration makes use of the reference terminal of the

in-amp, errors from the op amp and resistor mismatch result in

common-mode errors, rather than differential errors. Because

common-mode errors are typically rejected by the next device

in the signal chain, this circuit configuration adds almost no

extra error.

+IN

–IN

IN-AMP

VREF

4.99kΩ

–

OP AMP

+OUT

–OUT

4.99kΩ

REF

Figure 53. Differential Output Using Operational Amplifier

MULTIPLEXING

SDN0

SDN1

SDN2

SDN3

Figure 54. Four AD8231s in Multiplexing Configuration

The outputs of both the AD8231 in-amp and op amp are high

impedance in the shutdown state. This feature allows several

AD8231s to be multiplexed together without any external

switches. Figure 54 shows an example of such a configuration.

All the outputs are connected together and only one amplifier is

turned on at a time. This feature is analogous to the high-Z

mode of the digital tristate logic.

The resistors in the AD8231 instrumentation amplifier create a

resistive path from the output to the reference pin of about

100 kΩ. If a higher output impedance in shutdown mode is

desired, the reference pin can be driven with the op amp of

the AD8231. In this configuration, the output impedance in

shutdown is several GΩ, and many thousand AD8231s can

theoretically be multiplexed in such a way.

The AD8231 can enter and leave shutdown mode very quickly.

However, when the amplifier wakes up and reconnects its input

circuitry, the voltage at its internal input nodes changes dramati-

cally. It takes time for the output of the amplifier to settle. Refer

to Figure 28 through Figure 32 to determine the settling time

for different gains. This settling time limits how quickly the

AD8231 can be multiplexed with the SDN pin.

USING THE AD8231 WITH BIPOLAR SUPPLIES

The AD8231 can be used with bipolar supplies as long as the

maximum voltage drop between the supply rails is kept below

6 V and all input voltages are kept within the supply rails.

With bipolar supplies, the acceptable levels for the digital inputs

A0, A1, A2, CS, and SDN shift. Table 9 shows acceptable values

for low and high signals for both single and dual supplies.

Table 9. Digital Pin Thresholds

Supply Voltage (V)

Low

High

Min (V)

Max (V)

Min (V)

Max (V)

0 to 5

0 to 3

+0.8

2.2

−2.5 to +2.5

−2.5

−1.5

1.5

2.5

−1.5 to +1.5

−1.5

−0.7

0.7

1.5

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