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XTR111: mV to mA Signal Conversion for pH probe

Benjamin Mears
Prodigy 10 points
Part Number: XTR111
Other Parts Discussed in Thread: LMP7721, , LMP7715, OPA172

I am trying to measure and control pH using a PLC. pH probes typically output on a -420 to +420 mV scale. The analog input card of the PLC I want to use accepts 4-20mA inputs. Will this converter be able to accompligh this conversion? If not, is there one which will allow me to do that?

Thank you

  • 0
    TI__Guru* 93105 points
    Hello Benjamin,

    You will need a couple of amplifier stages to convert the pH probe current output, to a voltage range compatible with the XTR111 input. Have a look at the LMP7721 datasheet, Figure 50. - LMP7721 applied as a pH Electrode Amplifier. An LMP7715 is used as a guard driver. This first stage will take care of the very low input bias current and input guard drive needed for this kind of very high impedance application.

    A second op amp stage would follow the LMP7721/LMP7715 input stage to dc level shift and amplify the -414 to +414 mV output range so that it aligns with the XTR111 input voltage range of 0 to 12 V. An OPA172 should serve well in that position. You should be able to power the OPA172 off the XTR111 24 V supply.

    You can find the LMP7721, LMP7715 and OPA172 datasheets here:

    www.ti.com/.../lmp7721.pdf
    www.ti.com/.../lmp7716.pdf
    www.ti.com/.../opa172.pdf

    Regards, Thomas
    Precision Amplifiers Applications Engineering