Datasheet

AD7679
Rev. A | Page 19 of 28
Power Supply
The AD7679 uses three sets of power supply pins: an analog 5 V
supply (AVDD), a digital 5 V core supply (DVDD), and a digital
output interface supply (OVDD). The OVDD supply defines
the output logic level and allows direct interface with any logic
working between 2.7 V and DVDD + 0.3 V. To reduce the
number of supplies needed, the digital core (DVDD) can be
supplied through a simple RC filter from the analog supply, as
shown in Figure 25. The AD7679 is independent of power
supply sequencing once OVDD does not exceed DVDD by
more than 0.3 V, and is therefore free from supply voltage
induced latch-up. Additionally, it is very insensitive to power
supply variations over a wide frequency range (see Figure 30).
FREQUECY (kHz)
65
PSRR (dB)
40
100 1000 10000110
60
55
50
45
03085-0-032
Figure 30. PSRR vs. Frequency
POWER DISSIPATION VERSUS THROUGHPUT
The AD7679 automatically reduces its power consumption at
the end of each conversion phase. During the acquisition phase,
the operating currents are very low, which allows for a
significant power savings when the conversion rate is reduced,
as shown in Figure 31. This feature makes the AD7679 ideal for
very low power battery applications.
It should be noted that the digital interface remains active even
during the acquisition phase. To reduce the operating digital
supply currents even further, the digital inputs need to be
driven close to the power rails (DVDD and DGND), and
OVDD should not exceed DVDD by more than 0.3 V.
SAMPLING RATE (SPS)
1000000
POWER DISSAPATION (
μ
W)
1M
100000
10000
1000
100
10
1
0.1
100k10k1k1001
10
PDBUF HIGH
03085-0-033
Figure 31. Power Dissipation vs. Sample Rate
CONVERSION CONTROL
Figure 32 shows the detailed timing diagrams of the conversion
process. The AD7679 is controlled by the
CNVST
signal, which
initiates conversion. Once initiated, it cannot be restarted or
aborted, even by PD, until the conversion is complete. The
CNVST
signal operates independently of
CS
and
RD
.
CNVST
t
1
t
2
MODE ACQUIRE CONVERT ACQUIRE CONVERT
t
7
t
8
BUSY
t
4
t
3
t
5
t
6
03085-0-034
Figure 32. Basic Conversion Timing
Although
CNVST
is a digital signal, it should be designed with
special care with fast, clean edges and levels with minimum
overshoot and undershoot or ringing.
For applications where SNR is critical, the
CNVST
signal should
have very low jitter. This may be achieved by using a dedicated
oscillator for
CNVST
generation, or to clock it with a high
frequency low jitter clock, as shown in . Figure 25
For other applications, conversions can be automatically
initiated. If
CNVST
is held low when BUSY is low, the AD7679
controls the acquisition phase and automatically initiates a new
conversion. By keeping
CNVST
low, the AD7679 keeps the
conversion process running by itself. It should be noted that the
analog input has to be settled when BUSY goes low. Also, at
power-up,
CNVST
should be brought low once to initiate the
conversion process. In this mode, the AD7679 could sometimes
run slightly faster than the guaranteed limits of 570 kSPS.