Can you please put together a digest for the unwashed masses? I (an admitted skeptical) cannot find anything coherent in that thread, other than about the oscillator (which is, in itself, a heroical effort somewhere around -140dB.
How you got to -150dB which a) is about 20dB under the self advertised ESS THD @1kHz (see the application note regarding THD correction) b) includes analog stages with op amps specified at -140dB in ideal best conditions (unity gain, unloaded, etc…) c) includes metal film resistors distortions; not even the metal foil resistors are guaranteed in that range, and your board doesn’t use them, anyway d) includes the measurements uncertainties from the EM radiation around (last time I’ve checked you were not located in the middle of the Mato Grosso jungle). Even in differential mode, the CMMR of any implementation is, in the best case, defined by the resistors precision, and then you would need 0.0001% matching.
Side note, I see your board has 10 cents electrolytic capacitors in the signal path. As much as I respect the distortion performance of aluminum electrolytic capacitors in audio, expecting such devices to have a voltage coefficient practically zero, in particular when driven at 10V, is simply not realistic.
-130dB, yes, that’s possible, I get it with my own ES9822PRO implementation, after a painful tuning, which is valid strictly @1kHz and 10V input. I could imagine a lookup table in the controller, and an interpolation routine, to extend the calibration, but for production purposes that would be a PITA (read: expensive) to implement. Otherwise, anything outside the calibration frequency and level brings the ES9822PRO in the familiar -120… -125dB range, which is still a very respectable performance.
So I must ask the logical questions about your claimed -150dB performance: a) what are you smoking? and b) can I have some? Or should I assume you are in advertising mode looking to promote your sales?
P.S.1. I just noticed PMAs loop measurements. Yep, that makes a lot of sense.
P.S.2. here’s the Vishay bulk metal foil resistor spec and a brief discussion, straight from the horse’s mouth:
http://www.vishaypg.com/docs/49788/10reasns.pdf Bottom line 0.1ppm/V voltage coefficient, which is -120dB at 10V excursion. Probably better in practice, but some rather simple algebra for an inverting amplifier will show -150dB at the output is, ahem, in the realm of bull chips. Now consider the best metal film resistors are at least 10dB worse… and the bulk foil thingies are $100 a pop.
