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LMP7721: or alternative for pH and ORP probe reading

Jesse Slone
Prodigy 50 points
Part Number: LMP7721
Other Parts Discussed in Thread: ADS1115

I would like to be able to reasonably accurately measure electrochemical probes with an ADS1115.  The LMP7721 looks good, but is there a less expensive option that would still allow me to amplify the current in the +/- 1V range?  Since the ADS1115 only measures 0-5 +V is there anything special I will have to do to measure the +/- 1V from the probes?

Edit:

Prior discussion at ADS1115: AMP for pH and ORP Probe Interface

Also, the ADS1115 measures differential measurements to get bipolar readings, so all voltages have to be positive with reference to ground.  It has four inputs, so I would hope to be able to connect at least two probes per ADS, but if I tied all of the reference signals to the same pin, perhaps I could measure three probes?

  • 0
    TI__Guru* 93105 points
    Hi Jesse,

    Regarding your LMP7721 questions:

    The LMP7721 looks good, but is there a less expensive option that would still allow me to amplify the current in the +/- 1V range? - Not from TI.

    Since the ADS1115 only measures 0-5 +V is there anything special I will have to do to measure the +/- 1V from the probes? - The LMP7721 should be powered by dual supplies such as +/-2.5 V such as shown in datasheet Figure 50, to accommodate the +/-1 V input. Then, the output would need to be level shifted to a positive voltage level to be compatible with the 0 to 5 V input range of the ADS. That could be accommodated by another operational amplifier.

    For the specifics about applying the ADS1115, please post to the Delta-Sigma E2E forum.

    Regards, Thomas
    Precision Amplifiers Applications Engineering