AD8295BCPZ-R7 datasheet(23/28 Pages) AD

23 / 28 page

AD8295

Rev. 0 | Page 23 of 2

APPLICATIONS INFORMATION

CREATING A REFERENCE VOLTAGE AT MIDSCALE

HIGH ACCURACY G = −1 CONFIGURATION WITH

LOW-PASS FILTER

A reference voltage other than ground is often useful, for

example, when driving a single-supply ADC. Creating a

reference voltage derived from a voltage divider is straight-

forward with the AD8295 (see Figure 61). In this configuration,

Op Amp A2 is used to provide a buffered VS/2 reference for the

in-amp section. This configuration is very similar to the one

described in the Reference Terminal section.

The circuit in Figure 62 uses Op Amp A1 and the resistor string

to provide a precise G = −1 configuration. Because no external

resistors are used to set the gain, gain accuracy and gain drift

depend only on the internally matched resistors, yielding excel-

lent performance.

Adding a capacitor across Resistor R2 is a simple way to provide

a single-pole low-pass filter that rolls off at 20 dB per decade.

This capacitor is shown as C1 in Figure 62.

Note that the internal resistors of Op Amp A1 are not used

to provide VS/2. Instead, external 1% (or better) resistors are

used. Because the negative input of Op Amp A1 is permanently

connected to the junction of internal resistors R1 and R2,

Op Amp A1 operates as a low voltage clamp, preventing the

resistor string from providing a convenient VS/2 voltage.

OUT

+IN

–IN

–INPUT

+INPUT

+VS

–VS

OUT

+IN

20kΩ

OUT

AD8295

VREF INPUT

LP FILTERED

OUTPUT

VREF

BUFFERED

–IN

NOTES

fLOW PASS = 1/(2π 20kΩ C1).

Noise at the reference feeds directly to the output, so if the

reference voltage is derived from a noisy source, filtering is

required. In Figure 61, Capacitor C1 has been added to filter

out high frequency noise on the positive power supply line.

The 10 uF capacitor and the 100 kΩ resistors shown in Figure 61

roll off noise starting at 0.3 Hz. The filter frequency is a trade-

off between noise rejection and start-up time.

OUT

+IN

–IN

+VS

OUT

REF

20kΩ

100kΩ

OUT

10µF

AD8295

+IN

–IN

–INPUT

OUTPUT

+INPUT

+IN

+VS

0.1µF

VS/2

BUFFERED

Figure 62. Single-Pole Output Filter Using a Single External Capacitor

If the connections to Pin 10 and Pin 11 in Figure 62 are changed

so that Pin 10 connects to ground and Pin 11 connects to the

in-amp output, the result is a G = 2 circuit, also with excellent

gain accuracy and drift. In the G = 2 configuration, Capacitor C1

lowers the gain from 2 to 1 at higher frequencies.

Figure 61. Single-Supply Connection with Buffered Reference

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