# Problems with a bootstrapped instrumentation amplifier

Thread Starter

#### Johannes

Joined Jun 8, 2008
1
Hello everyone!

I´ve biult an instrumentation amplifier with 3 opamps, but the cmrr has been to low. Ive increased it with bootstrapping the resistor of the input highpass.
Unfortunatelly I cant calculate the function of the amp analytically, there are mistakes all the time. So I dont know how to choose the values for high cmrr.

Please can anyone explain to me how I got the values of the resistors and capacitors? Maybe the function of voltage or Impedanz of 1 op-amp of the input-stage would be enough.

So the first schematic of the pdf is the instrumentation amplifier. I use C3 to bootstrapp R12 and C4 to b R13. With the bootstrapping I want to compensate the mismatch between the two input-high-passes!

Again, maybe its enough to explain the 2. schematic of the pdf.

best regards from germany (I´m sorry for the bad englisch)

Johannes

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#### hgmjr

Joined Jan 28, 2005
9,029
My initial concern is that R11 is an unusually low value. I take it you are trying for a high gain. That high gain may be the root cause of your problem. You may be seeing some of the offsets from the opamps with the high gain applied.

Have you built the circuit yet or have you only simulated it thus far?

hgmjr

#### beenthere

Joined Apr 20, 2004
15,819
CMRR in an IA comes mostly from the gain match between the input buffers and very good resistive matching around the difference amp. Even 1% resistors may not give good results.

Your gain appears to be 200 overall. And the resistive values are very low around the input buffers. Is there some reason for the low resistances?

#### Distort10n

Joined Dec 25, 2006
429
Beenthere brings up a crucial point. What are the tolerances of your resistors? This is one major benefit of a monolithic in-amp: matched resistors. 1% matching equates to 40dB!!!

#### SgtWookie

Joined Jul 17, 2007
22,221