[sdiy] Summing op amp calculation fail

[Justin Owen]

Hello, time to come asking for help again I'm afraid.

I'm trying to set up an inverting summing amp to add (subtract) two DC voltages/inputs.

I've used the inverting/summing config with AC Coupled audio signals loads of times with no problems - but I cannot, for the life of me - get it to play nice with DC signals.

I'm running on a 9 Volt, single supply. Here's a schematic - http://www.sdiy.org/juz/summing_opamp.png

The first oddity was trying to subtract/invert one input at 1V. If you look at the schematic (at the top) you'll see that running the non-inverting input at VCC/2 (4.5V - as per just about every text book example I've ever seen) gives me an output of 7.8V. I don't know why.

To fix this - I ran the non-inverting input at VCC/4 (2.25V). Then I got 3.5V - which is correct (4.5V i.e Ground - 1V = 3.5V). This seems to work for just about every voltage within the range of the op-amp. I don't understand this - but it works.

So then trying to sum 2 inputs, no matter what resistor combos I tried, I was still getting outputs of 7 or more Volts.

The closest I got was to drop the non-inverting input back down to VCC/4 and set the total parallel resistance of the inverting input resistors to the same value as the resistor in the feedback loop (10K).

Then my output was 3V. Which is 50% of what the output value should be (4.5V i.e Ground - (1V + 2V) = 1.5V).

All the complex things I set out to do last week - all done. The one simple thing? Fail. What am I doing wrong here?

Thanks in advance.

Justin