# Voltage drop at input of opamp simulate this circuit – Schematic created using CircuitLab

At VM1 I theoretically expect about 1mV. When I connect with the opamp I get a voltage drop of 0.5mV at VM1, I get this values in simulation(theoretically). What is the cause of this (something to do with the 10MOhm) and what can I do to prevent this in the practical circuit.

• Welcome to EE.SE A schematic would be useful if you want to get good answers - there is a schematic editor widget available in the edit toolbar. The question also really needs to be edited for readability. Apr 10, 2016 at 16:27

Edited question for non-inverting case, removed circuit + analysis of negative op-amp amplifier.

Yes the problem is most likely the 10MOhm resistor. This happens because while the op-amp assumption is that $$I_+ = I_- = 0$$ this is not really the case, and there is a small input current.

You should decrease the size of both voltage divider resistors while keeping their ratio the same. This will give the required attenuation while also allowing the op-amp terminals to sink some current.

You can look at the datasheet to see how much current the op-amp inverting/non-inverting terminals actually draw. You want this current to make only a small difference compared to the voltage you are amplifying.

I will say that trying to amplify a 500uV signal like this is going to create a lot of noise, and the voltage divider will draw a lot of power. A better way is probably to control the gain with the feedback resistor ratio rather than pre-attenuate it like this, especially for a 1V signal.

• Sadly, this well-written answer is now more or less irrelevant thanks to the original question being updated with a schematic showing that an inverting configuration is not being used.
– pipe
Apr 10, 2016 at 22:42
• The goal of this circuit is to measure currents in de nA range, the source voltage will be around 3V instead of 1V. Is there another option than lowering the resistance values (e.g. another opamp, something with feedback resistors)? Apr 11, 2016 at 10:56
• I found a solution for my problem, by using a JFET opamp the problem does not exist anymore. The reason is that the input impedance is very high (around TOhm) and lower bias current. Apr 11, 2016 at 12:47
• Yes, the '741 op-amp is ancient and no longer used in new designs. Apr 11, 2016 at 19:15