AD7694

Data Sheet

Table 9. Recommended Driver Amplifiers

Amplifier

Typical Application

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 and low power

AD8519

Small, low power, and low frequency

AD8031

High frequency and low power

VOLTAGE REFERENCE INPUT

The AD7694 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, an unbuffered reference voltage like the low

temperature drift ADR43x reference or a reference buffer using

the AD8031 or the AD8605), a 10 µF (X5R, 0805 size) ceramic

chip capacitor is appropriate for optimum performance.

If desired, smaller reference decoupling capacitor values down

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

especially DNL.

POWER SUPPLY

The AD7694 powers down automatically at the end of each

conversion phase and, therefore, the power scales linearly with

the sampling rate, as shown in Figure 22. This makes the part

ideal for a low sampling rate (even a few Hz) and low battery-

powered applications.

10,000

1,000

100

10

1

0.1

0.01

10

100

1k

10k

100k

1M

SAMPLING RATE (SPS)

VDD = 5V

VDD = 2.7V

Figure 22. Operating Current vs. Sampling Rate

SUPPLYING THE ADC FROM THE REFERENCE

For simplified applications, the AD7694, with its low operating

current, can be supplied directly using the reference circuit, as

shown in Figure 23. The reference line can be driven by either

•

The system power supply directly

•

A reference voltage with enough current output capability,

such as the ADR43x

•

A reference buffer, such as the AD8031, that can also filter

the system power supply, as shown in Figure 23

AD8031

AD7694

REF

VDD

2.2

TO

10

µF

1

µF

10

Ω

10k

Ω

5V OR 3V

5V

OR

3V

5V OR 3V

1

µF

(NOTE 1)

NOTES

1. OPTIONAL REFERENCE BUFFER AND FILTER.

Figure 23. Example of an Application Circuit

Rev. B | Page 14 of 16