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Data Sheet

AD7690

Rev. C | Page 15 of 24



For multichannel multiplexed applications, the driver

amplifier and the AD7690 analog input circuit must settle

for a full-scale step onto the capacitor array at an 18-bit

level (0.0004%, 4 ppm). In the amplifier’s data sheet, settling

at 0.1% to 0.01% is more commonly specified. This may

differ significantly from the settling time at an 18-bit level

and should be verified prior to driver selection.

Table 8. Recommended Driver Amplifiers

Amplifier

Typical Application

ADA4941-1

Very low noise, low power single to differential

ADA4841-x

Very low noise, small, and low power

AD8655

5 V single supply, low noise

AD8021

Very low noise and high frequency

AD8022

Low noise and high frequency

OP184

Low power, low noise, and low frequency

AD8605, AD8615

5 V single supply, low power

SINGLE-TO-DIFFERENTIAL DRIVER

For applications using a single-ended analog signal, either bipolar

or unipolar, the ADA4941-1 single-ended-to-differential driver

allows for a differential input into the part. The schematic is

shown in Figure 30.

R1 and R2 set the attenuation ratio between the input range and

the ADC range (VREF). R1, R2, and CF are chosen depending on

the desired input resistance, signal bandwidth, antialiasing, and

noise contribution. For example, for the ±10 V range with a 4 kΩ

impedance, R2 = 1 kΩ and R1 = 4 kΩ.

R3 and R4 set the common mode on the IN− input, and R5 and

R6 set the common mode on the IN+ input of the ADC. The

common mode should be set close to VREF/2; however, if single

supply is desired, it can be set slightly above VREF/2 to provide

some headroom for the ADA4941-1 output stage. For example,

for the ±10 V range with a single supply, R3 = 8.45 kΩ, R4 =

11.8 kΩ, R5 = 10.5 kΩ, and R6 = 9.76 kΩ.

Figure 30. Single-Ended-to-Differential Driver Circuit

VOLTAGE REFERENCE INPUT

The AD7690 voltage reference input, REF, has a dynamic input

impedance and should therefore be driven by a low impedance

source with efficient decoupling between the REF and GND

pins, as explained in the Layout section.

When REF is driven by a very low impedance source (for

example, a reference buffer using the AD8031 or the AD8605),

a 10 μF (X5R, 0805 size) ceramic chip capacitor is appropriate

for optimum performance.

If an unbuffered reference voltage is used, the decoupling value

depends on the reference used. For instance, a 22 μF (X5R,

1206 size) ceramic chip capacitor is appropriate for optimum

performance using a low temperature drift ADR43x reference.

If desired, a reference-decoupling capacitor with a value as small

as 2.2 μF can be used with a minimal impact on performance,

especially DNL.

Regardless, there is no need for an additional lower value

ceramic decoupling capacitor (for example, 100 nF) between the

REF and GND pins.

AD7690

REF

GND

VDD

IN+

2.7nF

100nF

2.7nF

IN–

+5V REF

±10V, ±5V, ...

+5.2V

15Ω

10µF

15Ω

ADA4941

100nF

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