Book Analog Interfacing to Embedded Microprocessors
ified in millivolts per second, microvolts per microsecond, or millivolts per
microsecond.
A real S/H also has finite input impedance, because the electronic switch
isn’t perfect. This means that, in sample mode, the hold capacitor is charged
through some resistance. This limits the speed with which the S/H can
acquire an input. The time that the S/H must remain in sample mode in
order to acquire a full-scale input is called the acquisition time, and is specified
in nanoseconds or microseconds.
Since there is some impedance in series with the hold capacitor when sam-
pling, the effect is the same as a low-pass R-C filter. This limits the maximum
frequency that the S/H can acquire. This is called the full power bandwidth,
specified in kHz or MHz.
As mentioned, the electronic switch is imperfect and some of the input
signal appears at the output, even in hold mode. This is called feedthrough, and
is typically specified in db.
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