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.