This amplifier gives incorrect V_OUT for small VIN difference

[king.oslo]

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.

Thanks.M

[6502nop]

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?

nop

[king.oslo]

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

Thanks.M

[PA0PBZ]

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.

[king.oslo]

Thanks. That worked fine M