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This amplifier gives incorrect V_OUT for small VIN difference


Hello there,

I've tried to simulate this with all sorts of precision amplifiers, and now I need some help, because I've failed.

I have a small voltage difference between non inverting and inverting input of 1mV. But still I get 40mV from the output. The amplifiers I've used, typically have some uV input offset, and low input bias currents.

Attached is my circuit.


In other words, check your Vref (pin 5 of the LT1991) to make sure it's at the zero point (0V if using a dual supply, 1/2 Vcc if using a single supply). The example shown on page 1 of the datasheet shows it at 2.5V on a single 5V supply. Your schematic shows the end of the 450K @ pin 5 is grounded, but the negative rail is also at that point, so... Vee=Gnd?


It works if I add a dual supply, but at ref = 1/2VCC, I have no luck. Why not?


At 1/2 VCC your output will also be at 1/2 VCC when your inputs are equal.
It's not right there because your 1K resistor is too small, change that to 10K and you will see.

Thanks. That worked fine :) M


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