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AD7357YRUZ Datasheet(PDF) 17 Page - Analog Devices

Part No. AD7357YRUZ
Description  Differential Input, Dual, Simultaneous Sampling, 4.2 MSPS, 14-Bit, SAR ADC
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
Logo AD - Analog Devices

AD7357YRUZ Datasheet(HTML) 17 Page - Analog Devices

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The AD7357 has two power-down modes: partial power-
down and full power-down. There are described in detail in
the Partial Power-Down Mode and Full Power-Down Mode
sections. This section deals with the power-up time required
when coming out of either of these modes. It should be noted
that the power-up times apply with the recommended decoupling
capacitors in place on the REFA and REFB pins.
To power up from partial power-down mode, one dummy cycle
is required. The device is fully powered up after approximately
200 ns from the falling edge of CS has elapsed. Once the partial
power-up time has elapsed, the ADC is fully powered up and
the input signal is acquired properly. The quiet time, tQUIET, 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.
To power up from full power-down, approximately 6 ms should
be allowed from the falling edge of CS, shown in
Figure 28
Note that during power-up from partial power-down mode, the
track-and-hold, which is in hold mode while the part is powered
down, returns to track mode after the first SCLK edge that the
part receives after the falling edge of CS.
When power supplies are first applied to the AD7357, the ADC
can power up in either of the power-down modes or in normal
mode. Because of this, it is best to allow a dummy cycle to elapse
to ensure that the part is fully powered up before attempting
a valid conversion. Likewise, if the part is to be kept in partial
power-down mode immediately after the supplies are applied,
then two dummy cycles must be initiated. The first dummy
cycle must hold CS low until after the 10th SCLK falling edge;
in the second cycle, CS must be brought high between the
second and 10th SCLK falling edges (see
Figure 25
Alternatively, if the part is to be placed into full power-down
mode when the supplies are applied, three dummy cycles must
be initiated. The first dummy cycle must hold CS low until after
the 10th SCLK falling edge; the second and third dummy cycles
place the part into full power-down mode (see
Figure 27
Modes of Operation
The power consumption of the AD7357 varies with the
throughput rate. When using very slow throughput rates
and as fast an SCLK frequency as possible, the various power-
down options can be used to make significant power savings.
However, the AD7357 quiescent current is low enough that
even without using the power-down options, there is a noticeable
variation in power consumption with sampling rate. This is true
whether a fixed SCLK value is used or if it is scaled with the sam-
pling rate. Figure 29 shows a plot of power vs. throughput rate
when operating in normal mode for a fixed maximum SCLK
frequency and an SCLK frequency that scales with the sampling
rate. The internal reference was used for Figure 29.
Figure 29. Power vs. Throughput Rate

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