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Filtering noise for high impdance voltage source

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prateek_k_chd

prateek_k_chd

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Hello folks
I have a very high impedance voltage source (a pH sensor) that I'm buffering via a BiMOS opamp (CA3140)
The problem is ,the opamp being wired as voltage follwer has a rather high i/p impedance and that's causing a relatively large noise signal.

I've tried RC filters without much success. The only thing that seems to work is a 0.1uF cap at the input. But the sensor being high impedance,the cap slows down the response to pathetic levels.

So can anyone please tell me a way to filter that noise?
Thanks a ton !!
 

I presume, that you are talking about noise, not 50 Hz hum caused by a inappropriate measurement setup.

the opamp being wired as voltage follwer has a rather high i/p impedance and that's causing a relatively large noise signal.
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The noise is generated by the sensor itself rather than the buffer impedance. The latter adds current and voltage noise to some extent, but most likely far below the sensor noise itself. A bad pH electrode (dried reference diapragma) may cause excessive noise. In this case, you have to replace the sensor.

The buffered pH signal should be filtered with a suitable low-pass. In case of digital signal processing, averaging can help.
 
Thanks fvm !
The circuit is powered by an 7805+0.1uF cap connected to an smps. So I guess hum should not be a problem
However,the circuit is right now on breadboard. Does that increase errors ?
The sensor is fine-I checked it just now.

and I'll try a RC filter at the output as well.
 

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