Glucose Measurement Using PSoC 3

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Anonymous
Not applicable

Hi to all,

   

I am doing glucose measurement , in this i am Interfacing IDAC, VDAC, TIA(Transimpedance Amplifier).Glucose Test Strip gives current so i  am taking IDAC .

   

The Output Equation at TIA  Vout=Vref-I*R, Normally for I=0 case Vout= Vref , but here I=0 i.e i am not connecting any thing to terminal , but i am measuring Vout, what i am getting is 0.29v, why is this strange output or else i am wrong any where.i am uploading Image of TOPDESIGN.

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1 Solution
ETRO_SSN583
Level 9
Level 9
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What is Vref (VDAC8) set to ? With a scope is TIA output DC or noisy, 60 cycle, etc.....

   

 

   

Per your diagram Pin_6 = Pin_5 as measure din DC voltage.

   

 

   

The TIA in this circumstance should act as a follower, although you might

   

want to disconnect the pin from TIA inverting input to minimize pickup for the

   

offset test.

   

 

   

To get an offset of .29 V with 40K TIA G set R one would have to have a leakage

   

of ~ 7 uA, which would be a pretty high bias current for the OpAmp used in TIA.

   

I looked for Ibias / Ileakage specs on OpAmp, TIA, saw none. This is just an

   

observation.

   

 

   

Regards, Dana.

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3 Replies
Anonymous
Not applicable

one Mistake on top post, with I=0, ie not connecting IDAC, not getting any value on multimeter.

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ETRO_SSN583
Level 9
Level 9
250 likes received 100 sign-ins 5 likes given

What is Vref (VDAC8) set to ? With a scope is TIA output DC or noisy, 60 cycle, etc.....

   

 

   

Per your diagram Pin_6 = Pin_5 as measure din DC voltage.

   

 

   

The TIA in this circumstance should act as a follower, although you might

   

want to disconnect the pin from TIA inverting input to minimize pickup for the

   

offset test.

   

 

   

To get an offset of .29 V with 40K TIA G set R one would have to have a leakage

   

of ~ 7 uA, which would be a pretty high bias current for the OpAmp used in TIA.

   

I looked for Ibias / Ileakage specs on OpAmp, TIA, saw none. This is just an

   

observation.

   

 

   

Regards, Dana.

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ETRO_SSN583
Level 9
Level 9
250 likes received 100 sign-ins 5 likes given

I set this up, using 1M Ohm TIA G set R to "aggravate" Ibias and Ileak errors.

   

 

   

For VDAC = 0                      I got 29 mV of TIA offset

   

For VDAC = .512 V              I got ~ 3 mV of TIA offset

   

 

   

If you look at G error and zero scale error for DAC that pretty much would cover

   

the 29 mV I got. To be exact one should calc the spec errors to see what one can

   

expect.

   

 

   

Regards, Dana.