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.

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

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.

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.