J
Jacques St-Pierre
- Jan 1, 1970
- 0
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
/<------|+ /---*-----* | | \--------*----- Vout
\ | / | | | / |
| |/LMC6482 | 100K 825 | | /LMC6482 |
| -----\/\--*-\/\------|+ /| |
\ 95K | | | / | |
/ | | | |
\ Vin ---||----- | GND |
| 1uF | |
| -------------\/\---
GND 5K
Thanks
Jacques
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
/<------|+ /---*-----* | | \--------*----- Vout
\ | / | | | / |
| |/LMC6482 | 100K 825 | | /LMC6482 |
| -----\/\--*-\/\------|+ /| |
\ 95K | | | / | |
/ | | | |
\ Vin ---||----- | GND |
| 1uF | |
| -------------\/\---
GND 5K
Thanks
Jacques