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 ADCís 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.

 

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