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I have the following differential amplifier in simulation. According to the theory I know, the output should be 0.2V - the difference between the two inputs at 2V and 1.8V. What am I misunderstanding here?

enter image description here

EDIT:.... Added some more probes:

enter image description here

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    \$\begingroup\$ You're power connections are correct. Don't mind Olin. A few extra seconds to look at the pin would have confirmed that the power rails are connected correctly even though they are not in the typical top to bottom configuration. \$\endgroup\$
    – efox29
    Feb 26, 2015 at 18:19
  • \$\begingroup\$ Looks like something wrong with your simulation or connections. How much is the positive voltage supply? \$\endgroup\$
    – Eugene Sh.
    Feb 26, 2015 at 18:32
  • \$\begingroup\$ Try probing the two inputs. They should both be at 1 volt. \$\endgroup\$ Feb 26, 2015 at 19:57
  • \$\begingroup\$ @WhatRoughBeast Check Edit \$\endgroup\$
    – Jolle
    Feb 26, 2015 at 20:04

4 Answers 4

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From your updated schematic, it is clear that there is a problem with R9/R19. Either R9 is essentially shorted, or R19 is essentially open. With pin 3 held at 2 volts, the op amp is otherwise working just fine.

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Looks like the positive supply is not going to a voltage (eg. 5V) that will allow it to work.

Put a marker on it to display the voltage.

Even if that arrow (looks like an inverted chassis ground) is correct (I doubt it), the line looks like it might be off-grid.

enter image description here

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  • \$\begingroup\$ I tried probing it. The VCC is OK at 3V :/ \$\endgroup\$
    – Jolle
    Feb 26, 2015 at 18:52
  • \$\begingroup\$ Try increasing it. \$\endgroup\$
    – Eugene Sh.
    Feb 26, 2015 at 19:12
  • \$\begingroup\$ Same output.... \$\endgroup\$
    – Jolle
    Feb 26, 2015 at 19:31
  • \$\begingroup\$ When you say that the VCC is OK, which part of this did you check? The right side (your 3V input) or the left side (the power pin of the amp)? \$\endgroup\$
    – Greg d'Eon
    Feb 26, 2015 at 19:59
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This was said by the OP under Spehro's answer: -

The VCC is OK at 3V

What Microchip say about their model: -

This model is intended to be an initial design tool that works well in the op amp’s linear region of operation over the temperature range. See the model file for information on its capabilities.

Bench testing is a very important part of any design and cannot be replaced with simulations. Also, simulation results using this macro model need to be validated by comparing them to the data sheet specifications and characteristic curves.

What this tells me is that if you sail to close to the input common mode specifications (for instance) the model may not work like the data sheet implies.

For a 3V supply the inputs can have a common mode voltage that is 0.9 volts below 3V i.e. 2.1 volts - you are pushing it quite close and this could be the actual problem. Try running it from 4 volts to see if things clear up.

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  • \$\begingroup\$ Increasing to 5V gives same output of 2.2V :S \$\endgroup\$
    – Jolle
    Feb 26, 2015 at 19:27
  • \$\begingroup\$ Sorry, @Andy aka. R9 and R19 set the + input at 1 volt. The output will try to keep the - input at the same level, so input range is well within spec. \$\endgroup\$ Feb 26, 2015 at 19:46
  • \$\begingroup\$ @WhatRoughBeast upvoted your answer as a form of thankyou! \$\endgroup\$
    – Andy aka
    Feb 26, 2015 at 21:43
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It looks like your ground line is at 2V. I'm not sure if your ground symbol is giving you any more than a connection to other ground symbols. That would mean your + terminal is also at 2V which would give you an output similar to 2.2V which is what you're seeing.

Can you probe the "ground" voltage and see if it's 2 or 0?


EDIT: R19 doesn't appear connected at all then according to your voltages. Alternately, your ground symbol isn't working as a net connector. If that's the case then the gnd off of R19 is 2V while the other is at 0V. Please probe the net between R19 and GND.

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  • \$\begingroup\$ Check edit :) .. \$\endgroup\$
    – Jolle
    Feb 26, 2015 at 20:05

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