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PIC Floating voltage supply, with accurate ADC

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Rigor69

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pic adc zener reference

Hi there.
I'm working with a Battery Management Project.
On each cell, 2,5 to 4,25 V, I will put a PIC, powered directly from the cell voltage.
This gives me a variable power supply to the circuit.
However, as I monitor the cell voltage with the same PIC built in ADC, I cannot figure out how to create my reference. Clearly it must be lower then the minimum voltage, say 1.8V.
But How can I know the supply voltage to the PIC using an ADC...???
I could DC-DC to 5V, but this is very expensive.

Anyone got any suggestions?

Best Regards, Jens
 

what is floating voltage

As you already find out, your idea can't work without a voltage reference. A possible solution with minimum part count is to use the battery voltage as ADC reference and measure an e. g. 1.2V reference at the ADC input instead. Actual battery voltage is calculated then by a division. To conserve power, the supply current to the reference can be enabled by a port output.
 

measuring supply voltage to a pic

A current source feeding a diode would give a voltage reference (unfortunately variable with the temperature)of some 0.6 volts(cheep solution)
a specialised voltage reference chip will give a really stable reference (at a price).
Or a Pic with a special V ref output..(there are a few)

Then config the Pic for the Vref on one pin(if not a special pic with Vref) (where the reference is connected)and the reading formatted to the reference.
I hope I am not too difficult to make sense off..

gebepic
 

floating referecen for adc

Add a voltage reference IC (bandgap or Zener) to you circuit with the voltage below the minimum battery voltage. Connect it to the analog input of the PIC, not necessarily the Vref input. Set up the ADC to use the supply voltage (which will vary) as a reference and measure the reference voltage IC (which will remain constant). You can calculate the supply voltage from this measurement.
 

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