Does
a S/H followed by a Nyquist-rate
ADC still experience the Sinc response discussed in
Sec. 2.1.3?
No,
the sampling process used in the Nyquist-rate ADC and
the associated aliasing result in the response
of the cascade having a constant
magnitude (= 1) frequency response. The Sinc response
associated with
the S/H (see page 36) is repeated at
multiples of the ADC’s sampling frequency. When we add the Sinc
responses up (see
similar, but in the time-domain, summation of Sinc
responses in Fig. 2.10) the aliased
signals add to the
desired signal resulting in a constant amplitude response.
Note
that a simple example of where it is important to know that the S/H has a Sinc response is found in
using a S/H on the
output of a DAC (for, example, deglitching).
In
other terms if the output of the circuit (e.g. a DAC, a switched-capacitor, or a S/H) is analog (continuous-
time) then the
signal experiences the Sinc response seen in Fig. 2.17. However, if the output
of the circuit is
digital (a number, or
point, at the sampling instance [discrete-time]) then the Sinc
responses add together,
as mentioned above, and the signal
does not experience a Sinc response.