AD8231

Rev. 0 | Page 17 of 20

APPLICATIONS INFORMATION

DIFFERENTIAL OUTPUT

Figure 34 shows how to create a differential output in-amp

using the AD8231 uncommitted op amp. 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

–IN

IN-AMP

4.99kΩ

+

–

OP AMP

+OUT

–OUT

4.99kΩ

REF

9

10

76

8

3

2

Figure 34. Differential Output Using Op Amp

MULTIPLEXING

SDN0

SDN1

SDN2

SDN3

Figure 35.

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 35 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 digital tristate logic. Because the output impedance in

shutdown is multiple megaohms, several 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 will take time for the output of the amplifier to settle.

Refer to Figure 16 through Figure 20 to determine the settling

time for different gains. This settling time limits how quickly

the user can multiplex the AD8231 with the SDN pin.