Hey Forum
Just had a look at the discussion on Impedance matching, however
it seemed to deal more with audio stuff, anyway my question involves a
pH sensor working at DC levels. I know the priciples are the same but recently i got a description from the company selling the pH sensor saying that it had an initial impedance of 2-3Gigaohms, his argument was that our signal converter (-5mV -> +.5mV converted to 4mA -> 20mA) may not be able to read this. My question: So what if its impedance is high, if it outputs 4mV then why wouldn't the converter amplify it and convert it to 4-20mA???

Cheers
Michael

 

Hi,

Your pH probe acts like a very weenie battery, and produces voltages depending on the acidity or alkanility of the solution its in. It can only source or sink currents in the range of nanoamps, so a device that is connected to it must have an input impedance of 10^12 ohms or better.

You will need to buffer the probe's output with an op amp with a suitable input impedance. The pH probe will output +/- 414.4 millivolts full scale.

 

The concept of impedance is applicable from DC to beyond blue light. It isn't limited to audio frequencies but applies to any AC source and circuit.

 

Thanks for the reply!