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Question 1:

Recently, i'm working on a design which needs to design an INA front end. The most used circuit like this (please omit the component values):

enter image description here

You see, the two 10M resistors which provide the path for biasing current are at the most front of the circuit. But occasionally, i saw designs like this:

enter image description here

The 'biasing resistors' come to the back, just before the INA. Which one is right, or which one is better?

Question 2:

In ADI's application note: Designing Amplifier Circuits: How to Avoid Common Problems, they give a similar circuit:

enter image description here

And they suggest:

In these circuits, there will be a small offset-voltage error due to mismatches between the resistors and/or the input bias currents. To minimize such errors, a third resistor, about 1/10 th their value (but still large compared to the differential source resistance), can be connected between the two in-amp inputs (thus bridging both resistors).

They suggest to parallel a smaller resistor to bridge the unbalanced input. But why not reduce the value of the 'biasing' resistors directly. Are there any differences?

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The second circuit divides the input voltage by the resistor ratio \${Rg}\over {Rin+Rg}\$, of course, so it's fine if you want to divide the input voltage down.

The third circuit is AC coupled. The resistors must be on the In-amp side of the capacitors otherwise they can't conduct the bias currents.

If you analyze the circuits with a differential voltage vs. a common-mode voltage applied you can see that it is indeed different whether you use two low value resistors vs. one low value and two high value resistors. Consider 10M + 10M to ground and 1K across the inputs. The inputs are practically floating wrt ground (5M\$\Omega\$), but a differential voltage sees 1K. If you use two 500\$\Omega\$ resistors to common, then the differential voltage sees 1K, but common mode voltage sees 250\$\Omega\$ to common. Very different.

What's better depends on the application.

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  • \$\begingroup\$ About Question 2, if i don't care the common mode resistance seen by the source, decreasing the resistor to ground should have the same effect to parallel a smaller resistor, right? \$\endgroup\$ – diverger Sep 1 '14 at 14:28
  • \$\begingroup\$ Yes. If you really don't care about the common mode resistance (and high CM rejection) you might want to consider using a single op-amp differential amplifier rather than an instrumentation amplifier. \$\endgroup\$ – Spehro Pefhany Sep 1 '14 at 14:48

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