Instrumentation Amplifier

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ETRO_SSN583
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Using MATLAB -

   

 

   

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Anonymous
Not applicable
                
      

Hi Dana-san, "Konnitiwa"

      

Interesting report. I'm trying to do actually.

      

Result of CMRR

      

-41.6dB (x1)

      

-42.0dB (x16)

      

That was measured at output pin.

      

Calculated CMRR on input pin.

      

-42.0 - 24 = -66dB (x16)

      

 

      

Anyway, INS-Amp made by PSoC PGA is functionally working.

      

All depend on user's requirement.

      

 

      

schematic

      

insamp1.JPG

      

 

     
    
   
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ETRO_SSN583
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That approach suffers from same issues, accuracy of PGA to PGA

   

gain tolerances within a single chip. So yes, you can measure

   

reasonable CMR in a specific device, but based on datasheet specs,

   

since there is no spec PGAx/PGAy worst case G error, one is stuck

   

with datasheet for guarenteed performance, and as shown is poor.

   

And this discussion so far excludes AC CMR.

   

 

   

I will model the 2 OpAmp case to see what it looks like.

   

 

   

Regards, Dana.

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ETRO_SSN583
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Of course the other issue you have is the direct injection of

   

Vdd/2 noise into first PGA, not a preferred topology. If G needed

   

<= 8 using A/D differential front end as IA replacement would be a

   

better alternative.

   

 

   

Regards, Dana.

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Anonymous
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        Three OP Amp case! I am lookin forward.   
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ETRO_SSN583
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The first issue with PGA being used in 2 OpAmp IA implementation -

   

 

   

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ETRO_SSN583
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Forum software from the dark ages .....

   

 

   

What I was trying to post, attached.

   

 

   

Regards, Dana.

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ETRO_SSN583
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Found error in analysis conclusion, result still the same.

   

 

   

Regards, Dana.

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ETRO_SSN583
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Here is the Matlab analysis, PGA G = 2, 2% error used (even though datasheet

   

only has a value for G = 1 ), again only PGA G = 2 will work for IA solution :

   

 

   

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