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.
I will appreciate any information about this.
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.
I hope this answers your question.
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.
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.