LMP7721 as transimpedance in three electrode potentiostat

Hi Nirav,

You need to add a current source in the working electrode to create the sensor "current".

Right now you are creating a inverting amplifier with a gain of 1000, with an input of 300mV, which is railing the output of the "TIA" against ground.

Regards,

Hello Paul Grohe,

This is a equivalent model of three electrode potentiostat. I have put impedance, in term of resistance and capacitance, of working electrode (WE), Reference Electrode (RE) and Counter Electrode (CE). 

Actually, the sink or source current may be generated due to the concentration of electrolyte and bias voltage in potentiostat.

Due to 300mV bias voltage, the AM1 shows the -324.53 nA and provide 75.51 mV output at VF1.

I have simulate the same circuit with external current source to TIA with same feedback and I have found that from +300 nA I am getting 300 mV and so on, as it is true because the gain of TIA is 10^6 (as 1 M resistor used in feedback).

But due to negative current and less than +300mA, I am getting just 75.51mV.

I am not getting the logic behind it.

HI Nirav,

Remember there is a negative sign in the trans impedance formula:

-Vout = I * RF

There is an inversion. Negative current inputs will move the output positive. Positive currents will move the output negative.

The output cannot swing negative because you do not have a negative supply. It cannot output -300mV.

You can either bias the TIA above ground (by setting the positive input to a reference voltage), or use a lopsided or split supply (±2.5V) so that the output can swing negative.

If you do use a single supply, then bias the TIA (U1) positive input at, say, 500mV, and set the set the servo amp (U2) positive input to 200mV, to maintain the 300mV bias across the sensor.

At zero current, the output of the TIA will be +500mV, and will go ± from that proportional to the input current.

+300nA = 500mV - 300mV = +200mV
-300nA = 500mV +300mV = +800mV

Remember the potential at the TIA input (inverting input) will be equal to the non-inverting input. The bias across the sensor will be equal to the difference between the servo amp and TIA reference voltages. Pick a reference voltage that gives you the best dynamic range for the signal range you wish to measure.

Regards,

Hi Nirav,

I think you are having difficulty understanding what a current source is.

A current source is a very high impedance output device that will adjust it's output to maintain a set current.

Currently (pun intended) your model circuit is forcing 300mV into the input of an inverting amplifier through 2k - and the amp is doing exactly what it should - railing negative (or 75mV, which is the lowest the output can go). There is no "give" for the 300mV (the servo amp is at 300mV and the TIA input is at 0V - so a resistive path will always create current).

By adding the current source, you add a very high impedance that will adjust itself so that the desired current will flow. The current source will "break" the 300mV path and "leak" just enough to create the desired current into the TIA input.

Put a voltage source in series with the current source and adjust the voltage source. You will see that the voltage source has no effect - the current will always be the same (in an ideal environment like SPICE, the "real world" is more difficult).

So put a current source in series with the input - between R6 and AM1, with the positive end pointing towards R6. Now you should be able to properly adjust the current.

Regards,

Hello Paul,

I got your point.

Hello Nirav,

You still do not have a current source in the simulation. The "AM1" device is a ammeter.

The trans-impedance formula is only valid if the source potential and the reference voltage of the TIA are equal. A current source would separate the voltages and only pass a current.

Also, are you getting "wire not connected" errors? Looks like there may be broken connections at the negative input terminal (next to R3).

Please post your Tina simulation.

Regards,

Hi Paul,

As I am working on three electrode potentiostat, as shown in attached file, in which working electrode may sink or source the current and TIA is use as I2V convertor. 

I have post the tina simulation which is equivalent circuit of three electrode potentiostat.

Vbias is the voltage what we need to provide on reference electrode and its range is -500 mv to 500 mv. 

According to bias voltage, due to concentration of electrolyte, few uA or even less current may sink or source in working electrode.

It needs to convert in equivalent mV by TIA, which further measured by ADC and LCD.

Potentiostat Equivalent Circuit for positive bias voltage - autosave 16-05-24 17_40.TSC