ADA4941-1YCPZ-R2 datasheet(22/24 Pages) AD

22 / 24 page

ADA4941-1

Rev. C | Page 22 of 24

DRIVING THE AD7687 ADC

The ADA4941-1 is an excellent driver for high resolution

ADCs, such as the AD7687, as shown in Figure 56. The Sallen-

Key, low-pass filter shown in Figure 54 is included in this

example but is not required. The circuit shown in Figure 56

accepts single-ended input signals that swing between 0 V and 3 V.

The ADR443 provides a stable, low noise, 3 V reference that is

buffered by one of the AD8032 amplifiers and applied to the

AD7687 REF input, providing a differential input full-scale level

of 6 V. The reference voltage is also divided by two and buffered

to supply the midsupply REF level of 1.5 V for the ADA4941-1.

GAIN OF −2 CONFIGURATION

The ADA4941-1 can be operated in a configuration referred

to as gain of −2. Clearly, a gain of −2 can be achieved by

simply swapping the outputs of a gain of +2 circuit, but the

configuration described here is different. The configuration is

referred to as having negative gain to emphasize that the input

amplifier, A1, is operated as an inverting amplifier instead of in

its usual noninverting mode. As implied in its name, the voltage

gain from VIN to VO, dm is −2 V/V. See Figure 57 for the gain

of −2 configuration on ±5 V supplies.

The gain of −2 configuration is most useful in applications that

have wide input swings because the input common-mode

voltages are held at constant levels. The signal size is therefore

constrained by the output swing limits. The gain of −2 has a low

input resistance that is equal to RG.

Ω

GND

500

Ω

ADA4941-1

REF

IN+

IN–

2.7nF

+5V

–5V

560pF

VS+

VS–

OUT+

OUT–

VIN

0V TO 3V

VOUT

562

Ω

562

Ω

562

Ω

1.69k

Ω

0.1µF

10µF

2.7nF

3.9nF

10nF

1/2

AD8032

ADR443

0.1µF

1/2

AD8032

AD7687

+5V

Ω

10µF

0.1µF

VDD

GND

REF

Figure 56. ADA4941-1 Driving the AD7687 ADC

Ω

500

Ω

500

Ω

REF

+5V

–5V

VS+

VS–

OUT+

OUT–

VIN

–

VO, dm

Figure 57. Gain of −2 Configuration

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