Datasheet

AD7694
Rev. A | Page 14 of 16
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
05003-022
SAMPLING RATE (SPS)
OPERATING CURRENT (
μ
A)
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
05003-023
AD8031
AD7694
REF VDD
2.2
TO
10μF
1μF
10Ω
10kΩ
5V OR 3V
5V
OR
3V
5V OR 3V
1μF
(NOTE 1)
NOTES
1. OPTIONAL REFERENCE BUFFER AND FILTER.
Figure 23. Example of an Application Circuit