[sdiy] Summing op amp calculation fail

[George Hearn]

Don't forget your (-) terminal is no longer a virtual earth but is actually
at the same potential as the (+) terminal.  This is the basic rule of opamps
with negative feedback.  
  For this reason your 1V input in the first example looks like a -3.5V
input.  This input is multiplied by the gain (-1) to give +3.5V and then
added to your bias voltage (4.5V at the (+) input) to give 8V out.  The 7.8
is probably the opamp saturating.  The same rules apply to all your other
examples.  For example Vcc/4=2.25V, 1V input looks like -1.25V (2.25-1).
This is then multiplied by your gain (-1) to give +1.25V and this is added
to your 2.25V bias at the (+) terminal to give +3.5V.
  The current in both resistors must be the same, and the negative feedback
makes the two opamp inputs look like a virtual short circuit.  This is how
you approach this sort of design.  George

-----Original Message-----
From: synth-diy-bounces at dropmix.xs4all.nl
[mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of Justin Owen
Sent: 28 September 2009 19:03
To: SDIY List
Subject: [sdiy] Summing op amp calculation fail

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



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