AD8295ACPZ-R7 datasheet(22/28 Pages) AD

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AD8295

Rev. 0 | Page 22 of 2

AD8295

IN-AMP

+VS

+IN

–IN

0.1µF

10µF

0.1µF

REF

VOUT

–VS

10nF

1nF

4.02kΩ

Figure 57. RFI Suppression

Lower cutoff frequencies improve RFI robustness. Accuracy of

the CC capacitors is important, because any mismatch between

the R × CC at the positive input and the R × CC at the negative

input degrades the CMRR of the AD8295. Keeping CD at least

10 times larger than CC is recommended.

DIFFERENTIAL OUTPUT

The AD8295 can be pin-strapped to provide a differential

output; the simplified schematic is shown in Figure 58 and the

full pin connection is shown in Figure 59. This configuration

uses the instrumentation amplifier to maintain the differential

voltage, while the op amp maintains the common-mode voltage.

Because the in-amp precisely controls the output relative to its

reference pin, this circuit has the same excellent dc performance

as the single-ended output configuration. The transfer function

for the differential and common-mode outputs are as follows:

VDIFF_OUT = VOUT+ − VOUT− = G × (VIN+ − VIN−)

VCM_OUT = (VOUT+ + VOUT−)/2 = VREF

where:

kΩ

1 +

This configuration is fully specified (see Table 2, Figure 49, and

Figure 50). DC performance is the same as for the single-ended

configuration; ac performance is slightly different.

+IN

–IN

REF

IN-AMP

VREF INPUT

20kΩ

–

+OUT

–OUT

20kΩ

–

Figure 58. Differential Output Using an Op Amp

OUT

+IN

–IN

–INPUT

+INPUT

+VS

0.1µF

–VS

OUT

REF

+IN

20kΩ

OUT

AD8295

+IN VREF

INPUT

+OUT

–OUT

–IN

0.1µF

NOTES

1. CONNECT AS SHOWN IF A2 IS NOT BEING USED.

Figure 59. Minimum Component Connections for Differential Output

An alternative differential output configuration, which also

requires no external components, is shown in Figure 60. Unlike

the previous circuit, this configuration uses an inverting op amp

configuration to double the gain from the instrumentation

amplifier. Because this configuration requires less gain from

the instrumentation amplifier, it can have a wider frequency

response and a wider input common-mode range vs. output

voltage. However, because it does not take advantage of feed-

back at the reference pin of the instrumentation amplifier, dc

performance includes the errors from the op amp and the

resistor network. When using the internal precision components

of the AD8295, these errors have a minimal effect on overall

accuracy. This configuration is not specified in this data sheet.

REF

VREF INPUT

+IN

–IN

20kΩ

+OUT

–OUT

–

IN-AMP

–

20kΩ

Figure 60. Alternative Differential Output Configuration

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