I would be very grateful for any suggestions on this strange problem (maybe not strange to someone who is not a newbie!)
I am building a sound-level measuring stage that feeds the ADC of a PIC.
The attached circuit works fine when run from a 12V battery. But if I use a 12V plugpack (I've tried a number of them), the opamp output often starts to drift high - approaching maximum sometimes. If I remove the microphone (electret) from the circuit, the problem disappears (so doesn't seem to be power supply noise simply punching right through to the opamp). Not shown - the opamp power supply is also bypassed by 0.1uF.
Very weirdly - if I touch a single point of the circuit with my finger - problem also disappears. Lots of different points work - 0v, opamp output... you name it, if I touch it, problem fixed!
The 12v and 5v power rails look very clean, although my scope is low bandwidth so I wouldn't see RF stuff. Even attaching my scope seems to often fix the problem.
The choice of opamp I've used etc has a lot to do with what I had lying about.
The PIC seems perfectly happy with either plugpack or battery supply.
I have tried ferrite beads, more bypass caps, etc, no luck. Any ideas would be fantastic!
I am building a sound-level measuring stage that feeds the ADC of a PIC.
The attached circuit works fine when run from a 12V battery. But if I use a 12V plugpack (I've tried a number of them), the opamp output often starts to drift high - approaching maximum sometimes. If I remove the microphone (electret) from the circuit, the problem disappears (so doesn't seem to be power supply noise simply punching right through to the opamp). Not shown - the opamp power supply is also bypassed by 0.1uF.
Very weirdly - if I touch a single point of the circuit with my finger - problem also disappears. Lots of different points work - 0v, opamp output... you name it, if I touch it, problem fixed!
The 12v and 5v power rails look very clean, although my scope is low bandwidth so I wouldn't see RF stuff. Even attaching my scope seems to often fix the problem.
The choice of opamp I've used etc has a lot to do with what I had lying about.
The PIC seems perfectly happy with either plugpack or battery supply.I have tried ferrite beads, more bypass caps, etc, no luck. Any ideas would be fantastic!
