28 May
2010
28 May
'10
7:24 p.m.
Hi,
nice quiz. Can't wait for the solution!
On Friday 28 May 2010 17:37:47 fons(a)kokkinizita.net wrote:
On Fri, May 28, 2010 at 08:30:54AM -0700, Niels Mayer
wrote:
I would say it is an effect of the power-supply. As others stated, 50Hz
switching power supplies produce distortions of 100Hz (and of much higher
harmonics).
Here I think these affect the op-amp of the input by modulating the supply
voltage and/or the reference-voltage. This could probably be fixed by more or
better capacitors stabilizing the +5V (or +12V).
My guess why the development engineers didn't catch it:
a) They used a high-quality lab-power supply during development which is
stabilized far better then the consumer psu that is shipped.
b) Someone swapped the high-quality capacitors for lower-spec ones at the end
of the development cycle to cut costs.
c) The capacitors in your special device just broke their smoke-seal. Better
refill them soon:-)
BTW: The graphs would be better to interpret if both had the same x-axis-
scaling.
Have fun,
Arnold
Audio ADCs and DACs have three important inputs;
the signal input, the voltage reference, and the clock.
Noise and interference on the voltage reference causes
amplitude modulation, and jitter on the clock causes
phase modulation. The resulting modulation products
look very similar in the frequency domain. One of the
authors once spent several days trying to track down a
low-frequency jitter problem, only to find that it was
in fact a problem of LF noise on the voltage reference.
You're close. See the
extra hint in a previous post.