ORP Schematic

[bryan]

Hello:

My knowledge of electronics is somewhat limited, and would appreciate some help on this schematic. the first stage (gain ~5x)is borrowed from a schematic for a PH measurement circuit. I replaced the inverting second stage with a buffer which feeds a pic.

I am wondering if there should be any changes or improvements in the design. So far it seems to work, but would appreciate a second opinion. Not confident I have the right values for the resistors and capacitors, or the correct noise filtering included.



-=Bryan=-

 

[MikeMl]

IMHO, you can leave out the voltage follower, and the zener clipper. The first opamp stage can drive the ADC as well as the second. The clipper is not needed because with only 5V on the opamp supply, it can just barely pull the ADC input to 5V, and no higher.

 

[bryan]

Thanks Mike.Yes, forgot to remove the zener in the schemnatic before I uploaded. Not needed as you suggest, the voltage would never get above 5v. However on the voltage follower,would it not be required to match the impedance of the pic input. My understanding is the ORP probe has a extremely high impedence, or does the first stage take care of this?

 

[MikeMl]

The output impedance of an opamp with a reasonable closed-loop gain (<100) is like 1 Ohm or less (as long as the max current spec for the opamp is not exceeded). Thats why I said: The first opamp stage can drive the ADC as well as the second. The high impedance load requirement for the sensor is taken care of by the way the first opamp is configured (non-inverting).