Instrumental amplifier gain and filtering

Thread Starter

moravas

Joined Sep 11, 2019
3
Hi Folks,

I am working on a medical instrument, like an ECG, and searching trough the net, there are some unclear point about the instrumental amplifier usage:
  • As you can see in the image, there is a far less resistor to set the gain, and parallel to it, two more in series with each other. What is good for this circuit instead of the 2 series resistor:
    https://i.stack.imgur.com/6hUym.png
  • Common practice is to feed the instrumental amplifier's ref input with its output signal trough a miller integrator. The final effect should be same as using high pass filter in series with it. What are the pros or cons of this solution?
 

danadak

Joined Mar 10, 2018
3,577

Thread Starter

moravas

Joined Sep 11, 2019
3
Hi Dana,

I know about the CM feedback (two 24.9k in series), but my question would about this third resistor (4.12k) attached directly to the gain inputs of the AD9220 (Page 6-22)
 

ericgibbs

Joined Jan 29, 2010
8,733
hi m.
The 4.12K in parallel with the 24k9 in series, sets the Gain of the IA.
Check out the Gain equation
E
 
Last edited:

AnalogKid

Joined Aug 1, 2013
8,103
Because the 4K resistor is less than 10% of the sum of the two 25 K resistors in parallel with it, its value is the major component of the gain-setting network. Loading (including asymmetrical loading) of the CM node has a relatively small effect on the circuit gain and DC shift.

ak
 

Thread Starter

moravas

Joined Sep 11, 2019
3
Hi Folks,

yes, what you wrote is right. What I don't understand is the purpose of using all of the three resistor:
  • setting the gain of an in amp without CM is enough the 4K resistor
  • if I need the CM, I can split up the gain setting resistor (4K) into two 2K resistors
but in this application, the gain setting has been done by 1 resistor and the CM voltage point is created by two more additional. That I what I would like to understand.

Thank you
 
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