Help with INA103 mic preamp

[error401]

I've built the circuit as attached and I'm having trouble with output DC offset. I haven't actually attached a real source or load to the circuit, but it is generating about -13V DC offset on the output. I have verified that the common and differential input is 0V, and all power supply voltages seem correct. Phantom power is not enabled. The gain resistor I choose doesn't seem to matter.

Strangely (to me, anyway), the REF pin is only at 0.9V; wouldn't the opamp swing further than that due to the huge offset?

Any suggestions?

[Shaun]

Hi

The circuit looks fine. The next thing to suspect (other than a blown chip) is whether there is any oscillation going on. I built a similar circuit (see link), but I used a TL071 with an offset adjust pot. What chip do you use here?

Mic Amp

[error401]

I am using OPA134 in U2. Oscillation is a possibility, is there any easy way to test? I don't have access to a scope at the moment.

[Shaun]

First off, I'm not familiar with the INA103, so some of this is fairly speculative.

1) Do check that it is OK to leave those unconnected pins floating. Sometimes chips behave unpredictably when they are not correctly loaded.
2) Dial in the lowest gain, then re-check the DC voltages around the circuit. Also disconnect R102 and R202 (just to set up quiescent DC conditions).
3) Disconnect the integrator circuit and connect the REF input straight to ground; see if this rectifies the large DC output. Either the integrator is giving 0.9 volt to compensate for the high DC output, or it is doing so because it is not working correctly (and thus the huge voltage swing). The 0.9V (limit) is either due to clamping internal to the INA103 or it is how your meter interprets a signal that is actually oscillating.

There is no easy way to check for oscillation without an oscilloscope, as oscillation is typically high frequency, but there are free oscilloscope programs available on the internet that use your sound card. With this you will at least be able to see lower frequency oscillation.

HTH

[error401]

Thanks for the suggestions, but still no closer:

1) These pins aren't really mentioned at all in the datasheet as to what their use is. The only mention is a basic block diagram that shows the gain drive pins being the output of the internal first stage. The offset null pins are used in one example schematic with a 10K variable resistor between them, with a note that the circuit is 'optional'. I assume that means they're to be left unconnected...

2) Do you think it's really necessary to disconnect R101, R102? This is entirely surface mount and that'd be pretty tricky. There's no input connected at all, and I've verified that the DC at the input to the INA103 is negligible (19mV when I'm set up for ~0dB gain).

3) I lifted its output pin and that did change things. A few things I observed - the output DC offset is now positive 12.6V. The output of the integrator is positive 13V - it should be negative, but either way I'm fairly sure the output DC shouldn't be that high. Other DC values are essentially identical. I didn't measure any AC across the gain setting feedback resistor, nor was I able to measure any AC output of either the integrator or INA103. I'm sure my meter is only good up to 100khz (spec) or so at best though. Current draw is a fair bit higher than expected - 110mArms at 18VAC.

At this point I'm kinda stumped. The power supply is verifiable and it's fine for sure. The INA103 circuit is very simple and I'm pretty sure I built it correctly - but it's not working. I'm tempted to just build another one, but I only have two boards and rework damages them very easily, so I'd really rather figure out what's wrong first. Plus I'd like two preamps if I can!

Thanks for any further advice.