Strategies for reducing THD in ADCs/DACs

[ezalys]

I’m curious how Audio Precision reaches such low THD in their signal chains. Has anyone disassembled an apx555 to see what ADC or DAC is used? Can you put multiple ADCs/DACs together in some configuration to cancel second or third order distortion somehow? Can you perhaps characterize the distortion and compensate for it? How stable is the distortion with process, voltage, and temperature?  I know ESS and Cirus make some very well specced chips which seem like they might fit the requirements of the AP units... maybe it’s “just” these? Any additional thoughts?

[David Hess]

Part of it is removing common mode distortion from any non-inverting amplifiers by using inverting amplifiers instead.  This is a big problem when trying to make a Wein bridge oscillator pure enough to check the performance of an audio ADC.

[udok]

The low THD generator is an anlog state variable oscillator.

The analyzer uses a notch filter before the ADC.  The ADC convertes the residual after the notch filter.

Older devices use standard audio DACs (AK5354A) and ADCs (AD1955), but i do not know
which parts are in the APx555.

[Marco]

There is actually a way to remove distortion from DACs to generate near pure tones. You switch a low distortion low pass filter into or out of the chain and loop it back to an ADC. The harmonics caused by the ADC stay the same, the harmonics from the DAC change according to the transfer function of the filter. Thus you can calculate them and correct them in the signal you send to the DAC, iterate that a couple of times for a near pure tone. Of course the resolution of the DAC sets a lower limit, so you should do this with a high resolution DAC, though it can be a pretty shitty one, as long as it's consistently shitty.

See the paper "Low Cost Ultra-Pure Sine Wave Generation with Self Calibration", they used two 16 bit DACs resistively combined for a 140 dB THD sine generator AFAIR.