input frequency, progressively more and more energy in the output signal resides in those higher frequency image components instead – the muck that we usually try to filter out to get a nice clean output signal. Hence the droop.
Note that modern DACs designed for the audio market don't exhibit this problem. That's because they are not just updating once per sample and holding the signal. Deep in the bowels of the converter they are running much faster, and then applying digital filter techniques to produce an output that magically does not appear to have been sampled at all on casual inspection. The fact that it's so easy to get an audio DAC with a super-flat frequency response also makes it easy for engineers to forget that 'old school' sampling DACs don't have this flat response property.
That's the 'why' for the output part of our system. Can the input path also cause droop that might be detectable when we analyze the data in the digital domain without even going back to analog? Well, yes, sometimes it can; let's look at the circumstances under which it does.
If you are using a sampling ADC, the answer is generally 'don't worry'. Such an ADC takes a snapshot of the input signal over a brief 'aperture' of time. This aperture is usually far narrower than the time between samples, so it has insignificant effect on the frequency response. However, if you are using the kind of delta-sigma ADC that is targeted at industrial instrumentation applications, you'll probably get much more droop than you bargained for. (Do people actually bargain for droop? I suppose it's just a figure of speech.)
The reason why delsig (to use the industry vernacular) ADCs have a droopy frequency response is that the averaging filters, used to smooth out the fast pulse streams from their front-end 'modulators', have analogous impulse responses to that of