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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 :-)

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  • \$\begingroup\$ Your theory is correct, just some non-idealities of real opamps to watch out for, your circuit will work as expected if you add a -9 supply to your opamp. \$\endgroup\$ – sstobbe Sep 23 '17 at 14:39
  • \$\begingroup\$ @sstobbe, thank you! indeed. The TL082 starts working as soon as I apply -9V to the negative supply rail, it then outputs 49,99mV. I will invest some time in split power supply to avoid any "understanding barriers" :-) After some tests with some newer opamps, capable of single rail supply, I see that they work but the output calculations seem to be more complicated for single rail. Make an answer, so I can accept it please. \$\endgroup\$ – Artur Cichosz Sep 23 '17 at 19:27
  • \$\begingroup\$ Glad it works! You can make the TL082 work on a single supply as differential amplifier. But since you allude to wanting to build an instrumentation amp the front-end buffers will definitely need a negative supply to operate near ground. \$\endgroup\$ – sstobbe Sep 23 '17 at 19:48
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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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  • \$\begingroup\$ thank you! indeed. The TL082 starts working as soon as I apply -9V to the negative supply rail, it then outputs 49,99mV \$\endgroup\$ – Artur Cichosz Sep 23 '17 at 20:10
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TL082 is not rail to rail output by 2V offset so NG (no good)

LM7321 is not true R2R output by 50 ~200mV offset due loading effects and design choice .

There are thousands of other R2R Op Amps to choose from, one needs to specify this offset to rail which is often a tradeoff with current drive, GBW and tons of other specs and cost.

Your key parameter to search and sort from any OEM website is;

Output Saturation Voltage vs Load Current

e.g. LMV601 VO Output swing RL = 10 kΩ to 2.5 V Swing high 7 30 mV (typ max)

< 50mV is a difficult task and your specs matter.

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  • \$\begingroup\$ Thank you Tony, my specs for the current project are fairly tollerant. I need to amplify a gauge pressure sensor output ranging from 0 to 40mV to detect some ballpark pressure limit. TL082 seems to handle 10-40mV just fine giving me 0.5-2V at a gain of 50. At least in simulation :-) The actual problem with it is, that it does not seem to like single rail supply, which sstobbe pointed out in his comment. \$\endgroup\$ – Artur Cichosz Sep 23 '17 at 19:46

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