TLC4502: Unable to record sensor response

What is connected to the input and to the output? What are the voltages at the inputs and output? (If they change, show an oscilloscope trace.) How do the differ from what you expect?

Hi Gopi,

Gopi Krishna Saladi said:

Can someone help us to understand what where might are we missing?

Please provide us the Vcc and the pH sensor design requirements.

My guess is that the pH sensor's redox potentials are outside of input common mode voltage ranges of the op amp, which it will not be operating linearly. Also,  I need to know the type of pH's sensor, what is its output impedance? 

TLC4502 has a low nominal bias current from 1-60pA at 25C.  If this is general purpose pH electrode, it may be ok. If the pH electrode is high impedance type, the op amp may load the electrode down, and it may not work properly.  

Gopi Krishna Saladi said:

point out mandatory items that are to be taken care of during the PCB design in such cases?

Enclosed are LM7721 IC, application notes and design guideline for pH electrode.  

https://media.digikey.com/pdf/Data%20Sheets/National%20Semiconductor%20PDFs/LMP7721%20EvalBoard.pdf

https://www.ti.com/lit/ds/symlink/lmp7721.pdf?ts=1678720350847

https://www.ti.com/lit/an/snoa514c/snoa514c.pdf

https://www.ti.com/lit/ug/snou004/snou004.pdf?ts=1658761475325&ref_url=https%253A%252F%252Fwww.google.com%252F

If you have additional questions, please let us know. 

Best,

Raymond

Hi Gopi,

these circuits all look simple and easy but they can make issues during power-up and power-down or connecting and disconnecting the PH sensor. Then unwanted leakage currents can fow through the PH sensor which have the potential of destroying the PH sensor. Think about how discharging currents are flowing out of C1 and C2 during a sharp power down. Keep in mind that even the smallest current through a PH sensor can destroy it.

Because of this I would always connect the PH sensor between the input of very high input impedance OPAmp and signal ground, even if this makes the circuit more complex.

Kai