Having ADC output as DAC input

[alphacat]

I learned that one way for an ADC to sample its input is using a Sample and Hold circuit.

In order to compute the digital output, the ADC can use a DAC and a comprator, in the following way:
**broken link removed**

Say that the ADC samples a sine wave.
I read that:
1. if i start increasing the sine wave's frequency from zero Hz, then at some point, the output will be a straight line,
2. and that the first frequency that the output becomes a staight line is the ADC's sampling frequency.

What is the reason for these 2 phenomenons?

Thank you.

 

[crutschow]

When the sampling frequency is exactly equal to the frequency of the input AC signal, the sample will always occur at the same point on the AC waveform, giving a constant sample voltage. The sampled output will thus be a DC level. The value of this DC will depend at what point on the AC waveform the samples are being made. It can vary from 0V to the peak AC value.

 

[alphacat]

Thank you, got it.

And what happens if i increase the sine wave frequency?
I read that the output will remain a straight line.
Why is that?

 

[smanches]

It would not remain a straight line if you vary one of the frequencies. It's only going to be a straight line when one of the frequencies is a direct harmonic of the other, where it's sampling the same place in the sine wave every cycle, as crutschow mentioned.

 

[alphacat]

Thanks a lot!

I understand it now.