# Common voltage of 2 op amps instrumentation amplifier

I am trying to build an amplifier for a strain gauge sensor using the 2 op amps instrumentation amplifier design provided in the Texas Instruments note: https://www.ti.com/lit/an/sboa247/sboa247.pdf?ts=1605266073942&ref_url=https%253A%252F%252Fwww.google.com%252F.

I was able to derivate the expression for voltage gain of the amplifier, but I have troubles understanding the expression for the common voltage gain and also why the total resistance of the bridge is 4 X 120.

Can someone provide an explanation for my dilemmas regarding the common mode voltage of this amplifier design? I appreciate your help!

• I don't understand your dilemma and why you are confused. It's certainly confusing the 4 x 120 ohm bit but it's not clear whether you think it should be 4 x 120 or someone or something is telling you it's that and, a bridge doesn't really have a total resistance parameter because there are four terminals so, that's a little confusing too. Nov 13 '20 at 15:32
• I don't think that the resistance should be 4 X 120 ohm, this is stated in the link that I provided. The expression for the common voltage is similar to that of a voltage divider, but I cannot work out the resistors that form the divider because of the strange term R_bridge. Nov 13 '20 at 15:43