I want to measure the (negative) potential that develops on an electrode, but am not sure how because I believe that the signal passed to an analog reference pin must be positive. I have considered using a voltage divider between the negative and a positive voltage (for example from the 3.3V power pin) to achieve a positive value that can be fed into an analog input, but my concern is that if the negative electrode is floating then it will simply become charged with the supply voltage. Is my understanding here correct, and if so how might I solve this problem? Thanks. Here's a depiction of what I was considering but think won't work because of the described reason:
The voltage divider can work if the source impedance is low, relative to the voltage divider.
For example, A battery has low source impedance so the battery voltage won't drop with a 5 or 10K load so its voltage won't be affected 50/50 voltage divider. A 50/50 voltage divider made with 10K resistors biases the input at 1.65V. So a 1.5V battery will bring the voltage down to 0.15V.
For higher voltages you can put a resistor in series with the voltage source to make a "3-way" voltage divider. I this case, the original two resistors "seen" in parallel with each other (two 10K resistors make a 5K load when calculating the other part of the 3-way voltage divider).
Yes, but does that matter? What level of potential do you want to measure? -0.1v, -1v, -10v? What is generating it, is it high impedance or low?
What causes this potential to develop? Please describe the process in enough detail that forum members can make useful suggestions.
And just HOW do you know it is negative?
Reverse the leads ...
to measure "potential" without changing it you need a device with a high input resistance - like an arduino ADC input
however as soon as you start connecting resistors you will change the value you are trying to measure.
As others have said, to be much help we need to know the range of values you expect to measure, and ideally the source impedance.
Thanks all for the replies. I will try my best to provide here the inquired info that may help to understand the problem or my naivety about it:
Perhaps I misspoke (I’ve been away from electronics for some years) because after more investigation I’m thinking that I meant to say that it is a high impedance voltage source (I learned this is the case for an ion-selective electrode like in my situation here) as opposed to a floating signal source. The potential to measure is expected to range from approximately -0.35V to 0V as previously measured with a voltmeter.
This is for an ion-selective electrode so the potential is developed from a difference in dissolved ion concentration experienced at each (reference and working) electrode’s surface.
Most likely you will need a very high impedance amplifier/buffer (as with pH meter electrodes), in which case it is trivial to invert the sign of the output.
For more informed advice, post a link to the electrode (if a commercial offering).
So is this in an ionised fluid - perhaps aqueous? If so the impedance might be quite low but presumably you don't want much current to flow as it may polarise the electrodes. Since the test electrode is going negative w.r.t. ground (or could you simply swap the elctrodes?) you will probably need a negative rail as well as positive to run a suitable op amp.
You are dealing with (most likely) weak electrical voltages; in my mind, like a voltage reference cell or a pH meter electrode. Thus resistors for voltage divider is seriously flawed.
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There are numerous ways to approach this issue and each application has critical factors that must be considered:
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