Build an instrumentation amplifier ..#2

Yves_Michel

Joined Apr 20, 2022
4
Most of the instrumentation circuits I see have a reference to ground . What I like to have is using the INA111 and have the maximum input impedance.
I want use an electro-potential and connecting the input to a ground is out of question. Consider as a high impedance floating cell. Any suggestion?
regards

Yves

crutschow

Joined Mar 14, 2008
33,962
connecting the input to a ground is out of question. Consider as a high impedance floating cell.
The input to the amps need a reference to ground to provide a path for the small input bias currents and establish the common-mode voltage.

What is the impedance of this mysterious "electro-potential"?

Yves_Michel

Joined Apr 20, 2022
4
The input to the amps need a reference to ground to provide a path for the small input bias currents and establish the common-mode voltage.

What is the impedance of this mysterious "electro-potential"?
Nothing mysterious about electro potential if you are a chemist. Basically, you measure the oxido/reduction potential between a reference electrode that has a potential around 200mV against a platinum sensing electrode that may vary from +300 to -300mV from the reference electrode. You may consider as a floating cell. The desire impudence acceptable will be around 10mega Ohm.

LowQCab

Joined Nov 6, 2012
3,922
Building an I-A using FET-Input Op-Amps could theoretically have almost infinite Input-Impedance.

Why can't the "Reference-Electrode" be grounded ???
( it's not actually "Grounded" when Battery-Powered )

Are You trying to solve a "Noise" issue ?
.
.
.

Yves_Michel

Joined Apr 20, 2022
4
Building an I-A using FET-Input Op-Amps could theoretically have almost infinite Input-Impedance.

Why can't the "Reference-Electrode" be grounded ???
( it's not actually "Grounded" when Battery-Powered )

Are You trying to solve a "Noise" issue ?
.
.
.
I suppose I could have the reference electrode grounded to the 2.5V virtual ground of the INA111. so the potential couls swing either way?
Is anyone have the INA111 for the Proteus V8.13 version?
Regards,

Yves

MisterBill2

Joined Jan 23, 2018
17,720
If you already have a "Ph/Poh" meter, it will have suitable buffering circuits included, not much additional required. And if that meter has recorder outputs, even better. So there is a simple cheap work-around suggestion. As the TS already has the electrodes, the meter may also be on hand.
Of course, getting any information about the internal circuit may be a serious fight.

Yves_Michel

Joined Apr 20, 2022
4
I suppose I could have the reference electrode grounded to the 2.5V virtual ground of the INA111. so the potential couls swing either way?
Is anyone have the INA111 for the Proteus V8.13 version?
Regards,

Yves
I Have made a simulation with INA122 as my Proteus don't have INA111. I have place a 10MOhm resistor on the virtual ground, do you think it is a good thing or shall I connect directly. Note that I haven't place capacitors to reduce noise for the simulation. Please let me know if you approve the setup or there is a flaw in the circuit. Many thanks
Regards,

Yves