[SOLVED] How to detect micro volt changes in a voltage?

[JohnJohn20]

How to use a PIC ADC to detect micro volt changes in a voltage?

Using a PIC16F88 chip I can get a resolution of 5V/1024 = 5mV. Even using a 16 bit ADC will only give me say .08mV resolution.

So. How to get detect a relatively fast 10 uV change in the input voltage using the 16F88 chip?

2 ideas I had. What are their pros and cons?

1. Measure the input voltage V_in using the 10 bit ADC, then create two reference voltages V_in +- 1mV and apply them to the V_ref+ and V_ref- pins and then monitor the ADC value of V_in between these two reference values. That would give me 2uV resolution.

Will the PIC ADC work OK if V_ref+ - V_ref- = 2mV?

I thought that PWM would be too noisy to generate the reference voltages, but considered charging a low loss capacitor (controlled by the PIC chip) until its voltage reached V_in + 1mV, using that for the V_ref+ input (via a unity gain op amp buffer) and an op amp voltage subtractor to get the V_in - 1mV for the V_ref- pin.

The idea being that the V_in change would happen much quicker than the V_cap voltage decay which could be monitored and topped it up as required.

2. The other method was to charge a low loss capacitor until V_cap is close to V_in and then use a high gain op amp amplifier to monitor the voltage difference between V_cap and V_in for fast changes in V_in compared to the relatively slow changes in V_cap.

Again, as V_cap and V_in slowly drift apart, monitoring could pause and the capacitor could be charged or discharged accordingly.

Any better ways to detect micro volt changes in a voltage?

Thanks.

 

[lw1ecp]

I feel solution 1 is not practical, the ADC won't behave with such a low Vref.
A different approach to solution 2 is just AC-couple the signal to a high gain amplifier, and possibly add another AC coupling at its output. Choose the time constant longer than the expected duration of the voltage change.

 

[Win_Hill]

Some additional analog processing before going digital is appropriate. Why stand on your head digitally-speaking, when a simple analog solution is at hand?

One nice way to AC couple is to use a R + C filter on the signal, and present its output along with the signal to a differential amplifier. The latter could be a follower with a difference amplifier, or it could be an instrumentation amplifier. Such an approach insures that you'll not go beyond the common-mode range of an amplifier with large step inputs, as can easily happen with the standard simple C + R ac-coupling approach.

 

[paulfjujo]

hello,


:-xPost #1 this soluce is impossible ! delta +VRef and -Vref must be > 1,5Volt
:smile:You can try MCP 3424 or 3421 ADC delta sigma converter up to 18 bits
i did a lot of test on it .. i get 2µV / point with 17 bits of resolution ( reference =2,048V)

see code C18 and results on **broken link removed**
You easily can adapt the code to your MCU

 

[JohnJohn20]

Thank you all for your responses. 18 bit resolution ADC is not an option with my programmer. Sigh.

What I am trying to do is monitor a voltage input (that may fluctuate slowly anywhere between 0.5V and 4.5V) for relatively abrupt changes of perhaps 10uV over a period of 0.2s.

Win_Hill, I can see what you are saying. If I have a 1 Hz high pass filter and the voltage goes from 2V to 2.00001 V in 0.2s, what will the output of the high pass filter look like? Would it be a pulse 0.4s long?

Likewise, if the voltage dropped from 2V to 1.99999 V would the output show a voltage pulse in the -ve direction?

I have only considered filter circuits to be useful for AC signals rather than a single "half" pulse (as in a full pulse would return to 2.0 V)