Datasheet

Data Sheet AD7682/AD7689
Rev. D | Page 23 of 32
POWER SUPPLY SUPPLYING THE ADC FROM THE REFERENCE
The AD7682/AD7689 use two power supply pins: an analog
and digital core supply (VDD) and a digital input/output inter-
face supply (VIO). VIO allows direct interface with any logic
between 1.8 V and VDD. To reduce the supplies needed, the
VIO and VDD pins can be tied together. The AD7682/AD7689
are independent of power supply sequencing between VIO and
VDD. Additionally, it is very insensitive to power supply varia-
tions over a wide frequency range, as shown in Figure 34.
For simplified applications, the AD7682/AD7689, with their
low operating current, can be supplied directly using an
external reference circuit like the one shown in Figure 36. The
reference line can be driven by:
•
The system power supply directly
•
A reference voltage with enough current output capability,
such as the ADR43x/ADR44x
•
A reference buffer, such as the AD8605, which can also
filter the system power supply, as shown in Figure 36
75
70
65
60
55
50
45
40
35
30
1
10k10
PSSR (dB)
100
1k
FREQUENCY (kHz)
07353-034
AD8605
AD7689
VIOREF VDD
10µF
1µF
0.1µF
10Ω
10kΩ
5V
5V
5V
1µF
1
1
OPTIONAL REFERENCE BUFFER AND FILTER.
0.1µF
0
7353-035
Figure 36. Example of an Application Circuit
Figure 34. PSRR vs. Frequency
The AD7682/AD7689 power down automatically at the end of
each conversion phase; therefore, the operating currents and
power scale linearly with the sampling rate. This makes the part
ideal for low sampling rates (even of a few hertz) and low
battery-powered applications.
10,000
1000
100
10
1
0.1
0.010
0.001
10
1M100
OPERATING CURRENT (µA)
1k 10k
100k
SAMPLING RATE (SPS)
0
7353-040
VDD = 5V, INTERNAL REF
VDD = 5V, EXTERNAL REF
VDD = 2.5V, EXTERNAL REF
VIO
Figure 35. Operating Currents vs. Sampling Rate