Hello,
I have been searching for a bug with a simple analog circuit for weeks without results, so now I am trying to get help from you people.
The final design is obviously more complex than that, but I have been able to isolate the problem to that particular circuit, build around a single LMC6282 chip. In fact, that section is part of an amplifier/filter/A/D circuit for a sound recording system.
The audio signal is feed to Vin, a 2.5Vdc reference from the A/D chip is feed to Vref and the circuit Vout is feed to the A/D input. The circuit is use to add a 2Vdc offset to the audio signal that is couple with the 1uF capacitor. The circuit is supply via the same +5Vdc used for the digital section, but that +5Vdc is cleaned using a T filter using a couple of 1mH coils & 10uF capacitors (not on the schematic) and the 10 Ohms & 1uF filter on the supply of the LMC6282 Opamp.
That circuit work fine for years until now. Until we had been force to use larger compact flash memory card, in our case 4gb and more. We suspect that those newer cards used more energy on the +5Vdc supply when they are in write mode and that put glitch on the supply rail. We have not been able to prove this, but the card speck sheets confirm that they use a lot more current than our older card. Nothing can be detected on the supply using our current instrumentations.
In turn, those supposed supply glitches appear amplified at the output of the circuit (Vout) and are digitalize by the A/D. They arrear has single random value sample in the data stream. By example, when all samples should be +2Vdc, we have isolated sample at 1 or 3Vdc, during the write data phase to the compact flash card. We have been able to prove the presence of those glitches at Vout, but since they are very occasional, they are quite hard to catch on our scope, they are easily spotted on the recorded digital audio stream.
We try to power the circuit via an different supply, the problem disappear, so it's clear it come from the supply, but I have no idea why that particular design is so sensitive to glitches on the supply rail.
Any ideas?
Here an ASCII representation of the circuit.
+5Vdc Supply | \Vref 2.5Vdc / 10
| \
\ |
/ 20K --------- *-----|
\ | | |\ | --- 1uF
| | |\ | -----\/\---------*---|-\ | ---
| |-|-\ | | 1.0K | | \| |
\ 50K | \ | | | | \ GND
/