LMC6062: pH measurement interfacing problem

Hi Onkar,

what means "significant offset"?

And you sure that your ph-sensor is still ok? Have you checked the circuit with a different ph-sensor? Have you checked the circuit with a fresh LMC6062?

Have you connected the guard ring to any potential? A floating guard ring would not necessarily do the trick. Are you using a triax cable as discussed in the datasheet of LMP7721?

Kai

Hi Kai,

"Significant offset" I stated because my circuit is measuring 8 pH instead of 4 when I connect the actual sensor which is immersed in 4pH buffer solution, but measures 4 pH correctly when I use calibrator.

I used total 4 different types of pH sensors, so this is the observation.
Guard ring is not connected to any potential, its floating. The sensor doesn't comes with triax cable.

Thanks,
Onkar

Hi Onkar,

just noticed that you bias the ph-sensor with 4.096V / 2 = 2.048V? This is quite a lot and you can easily violate the common mode input voltage range of LMC6062 by doing this, if the supply voltage of LMC6062 isn't high enough. Remember that according to the datasheet of LMC6062 the common mode input voltage must not rise above Vcc - 2.5V.

What is the supply voltage of LMC6062?

Kai

Supply voltage of LMC6062 is 5V.

Hi Onkar,

the datasheet doesn't give any additional information on the input bias current versus common mode input voltage. But it could be that at your common mode input voltage the input bias current is rising and by this loading the ph-sensor too much. Could you decrease the bias voltage of ph-sensor for a test?

Also, have you experimented with connecting the guard to a suited potential?

Kai

Hi Kai,

I will try reducing the bias voltage by changing voltage divider resistor value, but in the final circuit I will need 2.048 Vbias.
Is it advisable to connect guard ring to system ground?

-Onkar

Hi Onkar,

you can try to connect the guard to pin 1 of LMC6062. But if the ph-sensor cable isn't included in the guard system, this wouldn't help much.

Guarding isn't a simple task because of risking stability issues. You might want to read this:

http://www.ti.com/lit/ug/snou004/snou004.pdf

And: Carefully clean the printed circuit board!!

Kai

Hi Kai,

Yes, pH sensor signal is inside guard ring. 
I will connect ring to pin 1 and see the results.

I read the cleaning process as mentioned article, I will try that way but difficult to get all the material. 

Do you have any idea of using ultrasonic cleaning method? I am planning to use it,  Because it’s easily available to me. 

-Onkar

Hi Onkar,

ultrasonic cleaning is a nice to have, but not urgently required, me thinks. With the brush it will merely take a bit longer to get the printed circuit board clean. Important is the rinsing phase with destilled water, though.

The LMP7721 with it's special pin out can also help to keep the unwanted leakage current low.

Kai

Hi Kai,

Simple shorting guard ring to pin 1 of LMC6062 didnt worked.
Now I am switching to LMP91200, this will cost less than two LMP7721.

Any advice on PCB layout before starting redesign process with LMP91200?

-Onkar

Hi Onkar,

have you tried to reduce the bias voltage?

By the way, as the output of a pH-sensor is typically 59.16 mV per pH unit at 25°C and the output range is 414 mV to −414mV when the pH changes from 0 to 14 at 25°C, I see absolutely no reason to work with such a high bias voltage. You only risk the damage and destruction of ph-sensor during power-up or power-down, if the potentials rise and fall unevenly.

You could even work with a bipolar supply voltage, as shown in the datasheet of LMP7721 (and you wouldn't need two costly LMP7721 either). Then you wouldn't need a bias voltage at all, which was the safest method of running your ph-sensor. A negative supply voltage can very easily be generated from the positive supply, especially when only a small supply current has to be sourced. TI has tons of chips doing exactly this.

Kai

Hi Kai,

The reason I am using this high bias voltage is, the same circuit will be used for ORP measurement, as required by the client.
In case of ORP its voltage is between +2V to -2V so I am using 2.048V bias.

I was just trying to avoid negative voltages all they way in my circuit. Looks like this is the way forward.

Thanks
-Onkar

Ok, I understand. But I wouldn't do that, when the result is merely a poor performance of ph-sensor amplification.

Think about how expensive a good ph-sensor is and what you risk when applying a dangerous bias voltage. No, I wouldn't do that. I would only work with a zero bias voltage

Kai