CDS-1402
6
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®
As soon as the transients/noise associated with the charge
transport begins to decay, S/H2 can be driven into the sample
mode. S/H2 can then be left in the sample mode until just
before the reset pulse for the output capacitor.
In Figure 2, S/H's 1 and 2 both have the same acquisition time.
If the pixel-to-pixel amplitude variation of offset signals is much
less than that of video signals, it may not be necessary for the
allocated acquisition time of S/H1 to be as long as that of S/H2.
As shown in the plot (Figure 4) of acquisition times vs. input
signal step size, the S/H's internal to the CDS-1402 acquire
smaller-amplitude signals quicker than they acquire larger-
amplitude signals. In "maximum-throughput" applications,
assuming "asymmetric" timing can be accommodated, each
S/H should only be given the time it requires, and no more, to
acquire its input signal. Leaving a S/H amp in the sample
mode for a longer period of time has little added benefit.
As an example, the graph shows that it takes 32ns to acquire a
500mV step to within 10mV of accuracy and 73ns to acquire a
500mV step to within 0.5mV of accuracy. The figures in this
graph are typical values at room temperature.
The CDS-1402 brings out 4 control lines that can be used to
trigger an A/D converter connected to its output. If the A/D is a
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1mV accuracy
2mV accuracy
5mV accuracy
10mV accuracy
Input Step Size (Volts)
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±0.5mV accuracy
±1mV accuracy
±2mV accuracy
±5mV accuracy
±10 mV accuracy
Figure 4. Acquisition Time versus Accuracy and Step Size
sampling type, system timing should be such that the A/D's
input S/H amplifier is acquiring the output of the CDS-1402 at
the same time the output is settling to its final value.
For most sampling A/D's, the rising edge of the start-convert
pulse drives the internal S/H into the hold mode under the
assumption the S/H has already fully acquired and is tracking
the input signal. In this case, the same edge can not be used
to drive S/H2 into the hold mode and simultaneously initiate
the A/D conversion. The output of S/H2 needs time to settle its
sample-to-hold switching transient, and the input S/H of the
A/D needs time to fully acquire its new input signal.
As shown in Figure 1, output line A/D CLOCK1 (pin 18) is a
slightly delayed version of the signal applied to pin 11 (S/H1
COMMAND), and A/D CLOCK1 (pin 17) is its complement.
A/D CLOCK2 (pin 19) is a delayed version of the signal applied
to pin 12 (S/H2 COMMAND), and A/D CLOCK2 (pin 20) is its
complement. Any one of these signals, as appropriate, may be
used to trigger the A/D conversion.
Figure 3 is a typical timing diagram for a CDS-1402 in front of
DATEL's 14-bit, 5MHz sampling A/D, the ADS-944.
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