Why do DACs have output garbage at the sampling frequency?

[Bricolo]

Every DAC has some unwanted "noise" on its output, at the sampling frequency. (44kHz fof non os, more for the others)

But why is this garbage here? Aren't DACs limited in they frequency range from 0 to Fs/2? Is this some crosstalk between the digital inputs and the analog outputs?

[Steve Eddy]

Originally posted by Bricolo
Every DAC has some unwanted "noise" on its output, at the sampling frequency. (44kHz fof non os, more for the others)

But why is this garbage here?

It's not just at Fs, but at 2Fs, 3Fs, 4Fs and so on.

Aren't DACs limited in they frequency range from 0 to Fs/2?

That's what the anti-mage filter that's typically used after the DAC is for.

Is this some crosstalk between the digital inputs and the analog outputs?

No, it's just artifacts of the sampling signal.

When you sample a waveform, you create images centered at Fs, 2Fs, 3Fs and so on.

Remember, digital audio is sampled and quantized so the actual sampled waveform is a series of stairsteps instead of a smooth continuous waveform. It's the stairstep waveform which produces the imaging energy at Fs, 2Fs, 3Fs and so on.

When that energy is removed by way of the anti-image filter, then you get the smooth continuous waveform.

se

[jewilson]

Bricolo

Some of the noise you see on the DAC output is related to charge injection, which is due to the internal capacitances of the device. Other noise effects are related to over shoot of the sample, step, or settling time errors. In addition, DAC produces glitches and quantization noise or uncertainty errors. Another problem area for DAC’s is the ground, and power supply noise. Internally noise affects the voltage reference and the decision points. These are just of few of DAC related problems