Am 24.08.19 um 23:49 schrieb Jan-Derk Bakker:
there may be some flicker noise present, I've not personally seen the poor
DC performance in many S/D audio converters either (apart as artefacts from
the front-end electrolytic coupling capacitors).
Yes, these electrolytic capacitors can produce ugly effects.
purple = DigiKey 493-4582-1-ND organic polymer Alus 4700u/2.5V
blue = Digikey 1189-2962-ND 2200u/25V
green = DigiKey 1189-1056-ND 3300u/10V Rubycon, cheap
The winner is clearly the cheap
< https://www.digikey.de/products/de?keywords=1189-1056-ND >
The test circuit is an Interfet IF3602 based low noise baseband amplifier.
I definitely wanted no bias loop via the gate and also for some reasons no
feedback into the source, so I enforced the drain current with a current
mirror.
That is ugly, because the source must be AC-grounded with huge
electrolytics.
In the test setup, there were 4 in parallel. There is only 500 mV across
the capacitors.
Even the 2V5 organic polymer is not challenged voltage-wise.
The traces are the own input voltage noise of the amplifier. The
horizontal part
of the traces is about 250 pV/rtHz noise density. The bad capacitors
both show a 1/f**3 rise,
in textbook quality. At 1Hz is the lower passband corner of the
following stages.
I wonder what process produces that. Generation/recombination and most
others do only 1/f**2.
A 4700u/25V wet slug tantal showed no weakness. But at $100 a pop I like
to avoid them.
Vishay costs even 3 dB more than AVX. :-(
regards, Gerhard
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