Datasheet

AD7621
Rev. 0 | Page 20 of 32
Power-Up
At power-up, or returning to operational mode from the power-
down mode (PD = high), the AD7621 engages an initialization
process. During this time, the first 128 conversions should be
ignored or the RESET input could be pulsed to engage a faster
initialization process. Refer to the
Digital Interface section for
RESET and timing details.
A simple power-on reset circuit, as shown in
Figure 23, can be
used to minimize the digital interface. As OVDD powers up, the
capacitor is shorted and brings RESET high; it is then charged
returning RESET to low. However, this circuit only works when
powering up the AD7621 because the power down mode (PD =
high) does not power down any of the supplies. As a result,
RESET is low.
POWER DISSIPATION VS. THROUGHPUT
In impulse mode, the AD7621 automatically reduces its power
consumption at the end of each conversion phase. During the
acquisition phase, the operating currents are very low which
allows a significant power saving when the conversion rate is
reduced (see
Figure 30). This feature makes the AD7621 ideal
for very low power, battery-operated 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, drive the digital inputs close to the
power rails (that is, OVDD and OGND).
04565-032
SAMPLING RATE (SPS)
POWER DISSIPATION (μW)
100
100k
10k
1k
100 1k 10k 100k 1M 10M
WARP MODE POWER
IMPULSE MODE POWER
PDREF = PDBUF = HIGH
Figure 30. Power Dissipation vs. Sample Rate
CONVERSION CONTROL
The AD7621 is controlled by the
CNVST
input. A falling edge
on
CNVST
is all that is necessary to initiate a conversion.
Detailed timing diagrams of the conversion process are shown
in
Figure 31. Once initiated, it cannot be restarted or aborted,
even by the power-down input, PD, until the conversion is
complete. The
CNVST
signal operates independently of
CS
and
RD
signals.
04565-034
BUSY
MODE
CONVERT ACQUIREACQUIRE CONVERT
CNVST
t
1
t
1
t
2
t
4
t
3
t
5
t
6
t
7
t
8
Figure 31. Basic Conversion Timing
For optimal performance, the rising edge of
CNVST
should not
occur after the maximum
CNVST
low time, t
1
, or until the end
of conversion.
Although
CNVST
is a digital signal, it should be designed with
special care with fast, clean edges, and levels with minimum
overshoot, undershoot, or ringing.
The
CNVST
trace should be shielded with ground and a low
value (such as 50 Ω) serial resistor termination should be added
close to the output of the component that drives this line. Also,
a 60 pF capacitor is recommended to further reduce the effects
of overshoot and undershoot as shown in
Figure 23.
For applications where SNR is critical, the
CNVST
signal should
have very low jitter. This can be achieved by using a dedicated
oscillator for
CNVST
generation, or by clocking
CNVST
with a
high frequency, low jitter clock, as shown in
Figure 23.