I have breadboarded a pH sensing circuit using the circuit shown on page 9, figure 7 of the INA116 data sheet using an INA121, and it works somewhat as expected.
Notes: I have V+ at 24 v, V- at ground, and Ref tied to a resistor voltage divider to provide a voltage offset on 3.5v to shift the pH swing (6.5 - 8) into the input range of a PIC ADC. I also use this voltage as the "Solution Ground". I am using a gain of 50. The pH sensor provides a +/-0.059 volt change for every unit of pH, with a pH of 7.0 being 0 volts.
I have now had boards made and the circuit does not work. I have cleaned the boards to insure there is no stray resistance.
Any assistance would be GREATLY appreciated!!!!!
I played around with +/- 5 v supplies on my breadboard yesterday and that seemed to work better, although the output did not stabilize, it kept climbing.
Any ideas on why that would happen? Lead lengths on components acting as antennae?
What's the best way to shift the output to a 0-5v for the PIC?
Do you have a part number and/or datasheet for your pH sensor?
Precision Analog Applications
I'm using the Sensorex S200C.
Here's the link to the datasheet, but I don't see any electrical specs. I believe the impedance is around 100 Mohm.
The output shifts 0.059 mV/pH.
Thanks for the additional information. The problem here is going to be the input bias current. The INA121's Ib is specified at 50pA (max). If the sensor has an impedance of 100Mohms you could have as much as 5mV of error on your 59uV signal. This does not include leakage currents due to the PCB layout. The INA116, on the other hand, has only 25fA (max) of input bias current, which would represent only 2.5uV of possible error. The INA116 has a unique pinout to allow for PCB guard rings that can minimize PCB leakage.
I found an article and an app note (attached) that have some good information. Here is the link to the article:
So, in the end, you'll want to verify the impedance of the sensor and select a device with ultra-low input bias current.
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