Electronics > Microcontrollers

ADC user definable + and - Vreffs

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Simon:
So I have a project where I will need to sense the current in a high side line. Naturally I want the voltage drop to be quite low so I'll be sensing in the low mV range, I could of course set my +Vreff and -Vreff to suitable values so as to avoid other circuitry. but will that work ? how small a range can I give to a pic's ADC ?say i wanted it to be 10 mV, can I ?

As always there is no information at all readily available from micro chip, their datasheet only talks registers and the so called separate pic manual is just the datasheet stripped of the part numbers it is seriously worrying that they even bothered as it takes the piss

Psi:
i think you will run into noise issues with a difference that low, but just how bad i'm not sure.
Only way to find out is to try it first.

I know some avrs have a programmable differential adc gain of up to 200x so it probably is possible with an equivlent pic.

If you cant get it to work there are some quite nice current sense chips designed for monitoring the voltage across a low R resistor and generating an amplified analog output

 

Simon:
Yes so far my design has incorporated a current sensing chip, I thought that if I can make the pic do what i what i may be able to do away with it, I mean if you specify the two limits surely the chip know no better. Or does it reference to another voltage as well ?

ChrisKiwi:

--- Quote from: Psi on October 13, 2011, 12:10:31 pm ---i think you will run into noise issues with a difference that low, but just how bad i'm not sure.
Only way to find out is to try it first.

I know some avrs have a programmable differential adc gain of up to 200x so it probably is possible with an equivlent pic.

--- End quote ---
Some of the PIC's do allow that, some also allow you to apply a DC offset to the differential amp as well.  I have a feeling they might be 16 bit parts but I can't remember any part numbers of the top of my head, sorry.  I have gone down to 512mV but not 10mV (on 18F13K50).  I would have thought a differential amp with some gain would have been the best bet (apart form a dedicated IC)

Simon:
yes looks like I'd best use a dedicated chip

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