How do I calculate the Output of this potentiostat

[newbie_hs]

I am using this gas sensor in my circuit.The below circuit is obtained from vendors application note.

1)How do I calculate the voltage at the output of IC2
2)May I know the role of Rload

 

[kd502]

You can't calculate it, potentiostat compensates electrode polarisation voltage inside the sensor. Usually several dozen millivolts when sensor current is zero.

 

[newbie_hs]

Thank you.But how I will design this circuit.How to select the components and opamp

 

[FvM]

There isn't much to design. Application note suggests OP90/296 micropower bipolar OP, but any low offset, low noise FET OP can do as well.

Application note reports +600..+800 counter electrode potential for Chlorine sensor. Can be considered as first sensor health check, calibration with test gas is however required for a reliable function check.

Rload creates negative feedback for reference control loop and increases output stability.

 

[kd502]

From AN4348 by ST
The recommended load resistor of the sensor, specified in its datasheet, gives the value Rl .
Rl is a tradeoff between response time and noise. Indeed, any noise on the WE node is
amplified by the op-amp with a gain of 1 + (R T /Rl ).
Due to its capacitive behavior, the output signal converges with a time constant C s (R s +Rl ),
assuming that the sensor is a capacitor, C s , in series with a resistor, R s .
Also, Rl should not be too high or it may unbias the sensor (because of an Rl ×I sense voltage
voltage drop)

 

[newbie_hs]

There isn't much to design. Application note suggests OP90/296 micropower bipolar OP, but any low offset, low noise FET OP can do as well.

Application note reports +600..+800 counter electrode potential for Chlorine sensor. Can be considered as first sensor health check, calibration with test gas is however required for a reliable function check.

Rload creates negative feedback for reference control loop and increases output stability.

I am confused about the circuit analysis of IC2 side.Suppose the voltage at reference electrode(left side of R1) is some X.How much will I get to counter electrode.

 

[kd502]

I am confused about the circuit analysis of IC2 side.Suppose the voltage at reference electrode(left side of R1) is some X.How much will I get to counter electrode.

This is an integrator. If there is a constant voltage at the input, output rises with constant speed

This circuit changes output voltage until there is zero at the input

 

[newbie_hs]

Thank you.May I know ,

1) why two RC sections.
2)what is the reference voltage in this case

 

[FvM]

1. Double RC is unusual, but doesn't change principal operation and DC levels. Don't know if has significant effect, may be reduces noise.

Don't remember to have ever used additional RC in potentiostat, not even with Alphasense.

2. Still zero.

 

[kd502]

1) why two RC sections.

Second section have ten times lower time constant. This attenuates frequencies far beyond crossover frequency. Looks like noise reduction.

Ic1 gives noisy signal because huge gain of IC1, I don't what part of this noise comes from integrator.

 

[newbie_hs]

2. Still zero.

If the reference electrode is zero volt ,then why it is connected to counter electrode using integrator.
This section is not clear for me.
For an integrator When the input is zero the output also will be zero(assuming zero initial conditions).Please correct me if I am wrong

 

[FvM]

Integrator is varying counter electrode voltage until reference electrode voltage is zero.

 

[newbie_hs]

May I know can I use this AFE for the above mentioned electrochemical sensor. I checked with Alphasense team what they told is that they never worked with these AFE so they cant comment.
Please share your thoughts.