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Please help on odd reference voltages


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Why not use a Pot? You could set a reference voltage quite easily, and you should already have a rather stable power supply for your PIC


POT's.JPG

Wind the POT's to the required resistance and check them with a multimeter and your set.

If for some reason your voltage regulator decides to conduct harder/less than the voltage Vref (the reference for your ADC conversions) on your PIC is also going to be affected proportionally.

Eg; Vout from reg rises too 5.2 Volts, therefore the output of the potentiometers will rise by 0.2V but so too will what ever Pin (probably Vcc) you have as a reference for the ADC conversions, so your PIC will still read 2.26 or 3.673 volts (well the analogue equivalent), as its a digital representation of Vref

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With the resolution you seek, standard values won't get you there in one step.  I recommend choosing one common value, and for the other leg combining resistors in parallel to trim the value down.

For example:  with a TL431 I would use a 10k 1% on the bottom leg which always sees a nominal 2V495, which I would measure with ref shorted to cathode.  For the upper leg I would start with 750R and measure the actual resistance.  After measuring the shunt voltage, expecting  2V655 to 2V709, I will know how far the '431 is off the nominal voltage and about how much reference current the chip needs.  Then I would select a resistor to parallel with 750R to get the target value of 516R to 729R, which could be from 1k6 to 26k.

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