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My question is somewhat similar to this one asked here before. I have the following circuit:

schematic

simulate this circuit – Schematic created using CircuitLab

The input signal is being fed to two (possibly three) inverting operational amplifier circuits. The positive input of each opamp is used to adjust the input offset to a desirable value. Resistor R11 is added because the input is a voltage drop from the 50Ω resistor. Another thing I should mention is that the input signal scale is approximately from 0 V to 10 mV.

The problem is that these circuits affect each other. This behaviour is explained in the answers to mentioned question, so I'm trying to design a different circuit. I have read this tutorial by AD and a similar by TI. So, there are several methods of adjusting the input offset. I'm using the one for the inverting opamp circuit and the impedance of my voltage source is not low enough so these circuits don't work correctly together. I thought of the following solutions:

  1. Use a non-inverting circuit. Offset adjustment can be done like this:

schematic

simulate this circuit

However, I'm worried about connecting multiple opamps together. In the above circuit, R2 changes the input signal; will multiple opamps with inputs connected together affect each other in this case?

  1. Simulate the whole circuit and optimize resistor values so all the opamps will have a correct gain. This method would produce a lot of optimization goals which would be difficult to achieve. How stable a circuit built like this would be?
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    \$\begingroup\$ If V1 is an ideal voltage source, which a simulator source will be, there is no interaction between the circuits. So I don't see exactly what you're asking. \$\endgroup\$ – WhatRoughBeast Nov 21 '15 at 20:12
  • \$\begingroup\$ It's not a simulator source — it's a voltage drop generated on a low-value resistor (50 Ω). Also, simulation results (specifically OA1 gain) seem to change when the second circuit is disconnected. \$\endgroup\$ – John Doe Nov 21 '15 at 20:16
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    \$\begingroup\$ OK, then edit your schematic to add a 50-ohm resistor in series with the voltage. If you're going to ask a question based on an erroneous description, you're going to confuse people. \$\endgroup\$ – WhatRoughBeast Nov 21 '15 at 20:19
  • \$\begingroup\$ I have edited the schematic, sorry for the confusion. \$\endgroup\$ – John Doe Nov 21 '15 at 20:25
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The simplest way to eliminate your problem is to reduce the effective impedance of the source, and a voltage follower will do nicely

schematic

simulate this circuit – Schematic created using CircuitLab

The choice of buffer op amp is left to you, depending on the frequency response you need.

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