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.
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.
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