pH measurement with TI's INA116

Discussion in 'The Projects Forum' started by topgun303, Jun 10, 2014.

  1. topgun303

    Thread Starter New Member

    May 31, 2014
    I am trying to implement a pH measuring device with INA116 chip. One of the applications of the chip is pH measurement.

    Constrants: pH ranges from +- 430 mV. Max input/output 2 Hz frequency. Gain of 11.5 V/V

    PH probe have very high impediance ranging from 100 - 1000 MOhms. So INA116 chip is a good choice.

    I have attached a my current ckt and INA116 data sheet for reference.

    Problems I am having right now:

    1. Noissy signal. On the output signal, I am getting frequency in thousand to mega Hz.

    2. The offset voltage is fluctuating from -800 mV to 300 mV or sometimes stable at -400 mV. Now if I place a .10 uF capacitor between inputs the output goes down to -70 mV. Have a offset is a big problem for me as I need 0 mV offset to start with other wise my pH reading screws up.

    NOTE: with gain set to 11.53 v/v

    3. I don't know what type of (LPF @ 2Hz) filter should I implement to at the input and output to block any noise [ without blocking dc input mV signal] from entering my signal and high frequencies. AC-DC power supply do sometimes introduce 60k Hz frequencies too.
    Last edited: Jun 10, 2014