A project log for TOSLINK DAC

Instead of a $10 box from Amazon, go above and beyond

Nick SayerNick Sayer 09/29/2021 at 07:340 Comments

I took some measurements rather than go to bed on time.

I'm not happy with them, but I am not sure whether it's my measurement technique or the equipment or the device under test.

I sort of have the feeling that the device isn't to blame. If I lift the probe off the output pin, I don't see the noise floor change any, so that tells me that I'm not seeing the device create extra noise on its output.

This screenshot is an FFT with my mac playing a 1 kHz sine wave at full amplitude (I hope) through the AirPlay device I have and into the DAC. AY is at the noise floor and BY is at the peak of the fundamental. The delta between the two is just shy of 90 dB. The specs claim that on a good day this DAC should be able to do 113 dB. But as I said, I don't see the noise floor drop when I take the probe off the output, so maybe the noise floor of the scope is higher than the noise floor of the DAC. Either that, or perhaps the 1 kHz fundamental peak is lower than it is in reality (that seems unlikely).

That said, figure 19 on the PCM1793 datasheet shows a noise floor at -140 dB and a signal peak at -60 dB. That's a difference of... 80 dB, so maybe this is measuring correctly? I'm not that experienced with this sort of measurement, so I can't really be sure. Anyone want to comment?