Datasheet

Data Sheet AD7949
Rev. D | Page 21 of 32
POWER SUPPLY
The AD7949 uses two power supply pins: an analog and digital
core supply (VDD) and a digital input/output interface 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 AD7949 is independent of power
supply sequencing between VIO and VDD. Additionally, it is
very insensitive to power supply variations over a wide
frequency range, as shown in Figure 33.
75
70
65
60
55
50
45
40
35
30
1
10k10
PSSR (dB)
100
1k
FREQUENCY (kHz)
07351-033
Figure 33. PSRR vs. Frequency
The AD7949 powers 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)
VDD = 5V, INTERNAL REF
VDD = 5V, EXTERNAL REF
VDD = 2.5V, EXTERNAL REF
VIO
0
7351-034
Figure 34. Operating Currents vs. Sampling Rate
SUPPLYING THE ADC FROM THE REFERENCE
For simplified applications, the AD7949, with its low operating
current, can be supplied directly using an external reference
circuit like the one shown in Figure 35. The reference line can
be driven by:
 The system power supply directly
 A reference voltage with enough current output capability,
such as the ADR43x or ADR44x
 A reference buffer, such as the AD8605, which can also
filter the system power supply, as shown in Figure 35
AD8605
AD7949
VIOREF VDD
10µF
1µF
0.1µF
10Ω
10kΩ
5V
5V
5V
1µF
1
1
OPTIONAL REFERENCE BUFFER AND FILTER.
0.1µF
07351-035
Figure 35. Example of an Application Circuit