Datasheet

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 V
O
, 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 R
G
.
1kΩ
1kΩ
GND
500Ω
A2
A1
ADA4941-1
REF
IN
IN+
IN–
2
8
4
2.7nF
5
FB
+5V
+5V
–5V560pF
V
S+
V
S–
OUT+
1
3
OUT–
V
IN
VIN
0V TO 3V
V
OUT
562Ω
562Ω
562Ω
1.69kΩ
33Ω
33Ω
6
05704-060
0.1µF
0.1µF
10µF
2.7nF
3.9nF
10nF
1/2
AD8032
ADR443
4
8
1
4
0.1µF
0.1µF
3
3
4
2 6
2
1/2
AD8032
AD7687
7
5
6
+5V
1kΩ
1kΩ
10µF
10µF 0.1µF
0.1µF
VDD
GND
5
REF
1
2
Figure 56. ADA4941-1 Driving the AD7687 ADC
1kΩ
1kΩ
R
F
1kΩ
R
G
1kΩ
500Ω
500Ω
A2
A1
REF
IN
2
8
4
5
FB
+5V
–5V
V
S+
V
S–
OUT+
1
3
6
OUT–
VIN
05704-061
+
–
V
O, dm
Figure 57. Gain of −2 Configuration