R
rickman
- Jan 1, 1970
- 0
I have an audio I/O board that I test by looping back the analog I/Os
externally, stimulating the digital input and examining the digital
output. The functional test shows a -0.9 dB drop in gain at 20 Hz and a
-2.5 dB drop in gain at 20 kHz. After a 24 Hr burn in the 20 kHz gain
is the same within measurement error, but the 20 Hz gain is always down
to -1.0 dB.
I don't know what could be causing this. Do capacitors age have an
initial rapid aging of some sort? The low end response is controlled by
a couple of X5R caps on each channel. I can't think of anything else
that would affect the frequency response.
I suppose there could be a small effect due to a drift in the DC bias
point which is set by zener diodes. This would modify the capacitance
of the coupling caps a bit. Do zener diodes shift characteristics in
the initial 24 Hrs of operation? I guess I could measure the DC set
point on some before and after boards.
externally, stimulating the digital input and examining the digital
output. The functional test shows a -0.9 dB drop in gain at 20 Hz and a
-2.5 dB drop in gain at 20 kHz. After a 24 Hr burn in the 20 kHz gain
is the same within measurement error, but the 20 Hz gain is always down
to -1.0 dB.
I don't know what could be causing this. Do capacitors age have an
initial rapid aging of some sort? The low end response is controlled by
a couple of X5R caps on each channel. I can't think of anything else
that would affect the frequency response.
I suppose there could be a small effect due to a drift in the DC bias
point which is set by zener diodes. This would modify the capacitance
of the coupling caps a bit. Do zener diodes shift characteristics in
the initial 24 Hrs of operation? I guess I could measure the DC set
point on some before and after boards.