>-----Original Message-----
>From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] 
>Sent: 02 October 2006 23:39
>To: Microcontroller discussion list - Public.
>Subject: Lower bounds for ADC Vref?
>
>
>Hi,
>
>I'm wanting to measure a current with a 12F675.  I want to 
>have a low-side sense resistor and measure it directly with 
>the ADC.  Of course, when building a current sensor, it saves 
>power to use a low-value sense resistor, which makes a small 
>full-scale voltage.  I plan to compensate for this by setting 
>Vref to a low value.  But I'm not seeing anything in the 
>datasheet to tell me how low I can go, or what problems I will 
>see if I go too low.  Noise?  Fixed offset?  Do others have 
>experiences with this?  Am I looking in the wrong place?


Look in "Electrical Specifications" for A/D Converter Characteristics (Page 98 of the PDF datasheet I have).  Parameter A05 gives the minimum full scale range as 2.2volts.  Since the low end voltage reference is tied to Vss within the PIC, this means the Vref must be a minimum of 2.2 volts.

Regards

Mike

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