Electrical – Understanding OpAmps in TINA simulation

operational-amplifier

I have a hard time to understand why two different opamps in a substractor configuration deliver highly different output in a TINA simulation.
At the end I try to implement a high impedance differential aplifier (instrumentation amplifier), which second stage is a subtractor, and I got very different result with different opamps in simulation.

So I first read some basics here: http://www.ti.com/lit/an/slyt213/slyt213.pdf.
They explain an instrumentation aplifier as well as simple substractor and in regard to the later there is a formula for Vout which contains only the values for input voltages and external resistors.

So I think then, the particular opamp model does not matter at all for this configuration and DC operation.

Now in TINA I made a DC operation point simulation, for an ideal opamp, for TL082 and for LM7321. The schematic shows my configuration.

schematic

simulate this circuit – Schematic created using CircuitLab

Now with an ideal opamp I get the correct 50mV at the output, with LM7321 and 9V supply I get something around 45mV which I suppose is stil OK, but with TL082 there is 1.52V at the output (but the circuitlab simulation results in exactly 50mV). What do I miss? Is it a buggy spice macro in TINA or is there something else in TINA or in the datasheets I should consider or am missing?

I would like to use TL082 because its cheap and I have already some in stock bought as "general purpose opamp".

Please educate me a little 🙂

Best Answer

Since one of your requirements is to work away at your TL082 stock pile, the easiest solution is to add a negative supply rail.

The TL082 can tolerate supply voltages of up to +- 15V.

The input commmon-mode range is narrower than the output swing, so your negative supply needs to be low enough so that 0V is within the allowable CMR of your amplifier. Minimum guarantee input CMR from the datasheet is 4V above Vee. So you must add a negative supply rail of at least -4V to meet CMR.

Perhaps choosing more a more common value like -5V or -9V is even easier, as you may have a 79xx laying around.

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