LT1999 with Vout levels

Hi!!

I'm studying how to use LT1999-20 for a project and I have simulated a circuit to learn about it.

I want to amplify an AC current signal, so I simulate next circuit. I adjust VRef to 1.41 V in both circuits and I add a current source but it is ZERO. The difference between circuits is the resistance. In the left cirucit, it is 1 ohm and in the right circuit is 100.

When I simulated the circuits, I get the following results: Vout of the left circuit is in red, and it is 1.42 V and Vout of the right circuit is 2.32 V.

In theory, Vout shall be 1.41 V because of the current is zero and both circuits have 1.41V VRef.

I don't know why this difference exists if I change the resisteance and I have searched for information in the datasheet, but I don't find where the trouble is.

Thanks.

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Parents
• Hi glomumu,

I think you're seeing the LT1999-20 input bias current flow in the 100ohm sense resistor showing up as an output voltage shift.

I measure about 500uA current modeled in LTspice for the conditions you've shown which amounts to 1V (=500uA * 100ohm * 20) output voltage shift. Regards,

Hooman

• Hi glomumu,

I think you're seeing the LT1999-20 input bias current flow in the 100ohm sense resistor showing up as an output voltage shift.

I measure about 500uA current modeled in LTspice for the conditions you've shown which amounts to 1V (=500uA * 100ohm * 20) output voltage shift. Regards,

Hooman

Children
• Hi Hooman!

Your answer has helped me a lot. I'm so grateful to you.

I had not considered the input bias current. I need to work with smalls AC currents so I need to consider this characteristic and the common mode voltage range, because I have negative values.

Regards • Hi glomumu,

You're welcome!

Using a single 5V supply, the LT1999 can work with negative input CM voltages as negative as -5V, and positive CM voltages up to +80V.

Let me know if you have any other questions.

Regards,
Hooman