–18–REV. PrB
PRELIMINARY TECHNICAL DATA
AD7273/AD7274
Preliminary Technical Data
Power-up Time
The power-up time of the AD7273/AD7274 is TBD ns,
which means that with any frequency of SCLK up to 52
MHz, one dummy cycle will always be sufficient to allow
the device to power up. Once the dummy cycle is com-
plete, the ADC will be fully powered up and the input
signal will be acquired properly. The quite time t
QUIET
must still be allowed from the point where the bus goes
back into three-state after the dummy conversion, to the
next falling edge of CS. When running at 3 MSPS
throughput rate, the AD7273/AD7274 will power up and
acquire a signal within ±0.5 LSB in one dummy cycle,
i.e. TBD ns.
When powering up from the Power-Down mode with a
dummy cycle, as in Figure 14, the track and hold which
was in hold mode while the part was powered down,
returns to track mode after the first SCLK edge the part
receives after the falling edge of CS. This is shown as
point A in Figure 14. Although at any SCLK frequency
one dummy cycle is sufficient to power the device up and
acquire V
IN
, it does not necessarily mean that a full
dummy cycle of 16 SCLKs must always elapse to power
up the device and acquire V
IN
fully; TBD ns will be suffi-
cient to power the device up and acquire the input signal.
If, for example, a 25 MHz SCLK frequency was applied
to the ADC, the cycle time would be 640 ns. In one
dummy cycle, 640 ns, the part would be powered up and
V
IN
acquired fully. However after TBD ns with a 25 MHz
SCLK only TBD SCLK cycles would have elapsed. At
this stage, the ADC would be fully powered up and the
signal acquired. So, in this case the CS can be brought
high after the 10th SCLK falling edge and brought low
again after a time t
QUIET
to initiate the conversion.
When power supplies are first applied to the AD7273/
AD7274, the ADC may either power up in the Power-
Down mode or in Normal mode. Because of this, it is best
to allow a dummy cycle to elapse to ensure the part is fully
powered up before attempting a valid conversion. Like-
wise, if it is intended to keep the part in the Power-Down
mode while not in use and the user wishes the part to
power up in Power-Down mode, then the dummy cycle
may be used to ensure the device is in Power-Down by
executing a cycle such as that shown in Figure 13. Once
supplies are applied to the AD7273/AD7274, the power
up time is the same as that when powering up from the
Power-Down mode. It takes approximately TBD ns to
power up fully if the part powers up in Normal mode. It is
not necessary to wait TBD ns before executing a dummy
cycle to ensure the desired mode of operation. Instead, the
dummy cycle can occur directly after power is supplied to
the ADC. If the first valid conversion is then performed
directly after the dummy conversion, care must be taken to
ensure that adequate acquisition time has been allowed. As
mentioned earlier, when powering up from the Power-
Down mode, the part will return to track upon the first
SCLK edge applied after the falling edge of CS.
However, when the ADC powers up initially after supplies
are applied, the track and hold will already be in track.
This means, assuming one has the facility to monitor the
ADC supply current, if the ADC powers up in the desired
mode of operation and thus a dummy cycle is not required
to change mode, then neither is a dummy cycle required
to place the track and hold into track.
POWER VERSUS THROUGHPUT RATE
By using the Power-Down mode on the AD7273/AD7274
when not converting, the average power consumption of
the ADC decreases at lower throughput rates. Figure 15
shows how as the throughput rate is reduced, the device
remains in its Power-Down state longer and the average
power consumption over time drops accordingly.
For example, if the AD7273/AD7274 is operated in a
continuous sampling mode with a throughput rate of
500KSPS and a SCLK of 52MHz (V
DD
= 3V), and the
device is placed in the Power-Down mode between
conversions, then the power consumption is calculated as
follows. The power dissipation during normal operation is
13.5 mW (V
DD
= 3V). If the power up time is one dummy
cycle, i.e. 333ns, and the remaining conversion time is
another cycle, i.e. 333ns, then the AD7273/AD7274 can
be said to dissipate 13.5mW for 666ns during each conver-
sion cycle.If the throughput rate is 500KSPS, the cycle
time is 2µs and the average power dissipated during each
cycle is (666/2000) x (13.5 mW)= 4.5mW.
Figure 15 shows the Power vs. Throughput Rate when
using the Power-Down mode between conversions at 3V.
The Power-Down mode is intended for use with
throughput rates of approximately TBD MSPS and under
as at higher sampling rates there is no power saving made
by using the Power-Down mode.
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Figure 15. Power vs Throughput