Use a 2 resistor divider, no op amp needed.  Try a 22K from the battery
to the A/D, and a 47K from the A/D to GND.

Sherpa Doug

> -----Original Message-----
> From: Kosmerchock, Steve [mailto:Steve.Kosmerchock@RFSWORLD.COM]
> Sent: Wednesday, March 27, 2002 3:20 PM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: measuring battery supply via A/D
>
>
> Glen,
>
> Two diodes would give you a drop of 1.4VDC.
> So REF V would be 5.8V, still too high. You could use
> an op-amp and resistor network.
>
> Steve
>
>
>
> -----Original Message-----
> From: Glen Wiley [mailto:gwiley@GWILEY.COM]
> Sent: Wednesday, March 27, 2002 1:12 PM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: measuring battery supply via A/D
>
>
> From a software guy to the EE's of the world...
>
> I would like to measure the voltage level of my battery
> power supply (6 NiMH AAA cells) which should start at around
> 7.2V.  Using the A/D (AN0) pin on a PIC16F872, what is the
> best way to do this.  My first guess was to insert 2 diodes
> in series to take the max input voltage down to 4.8V.
>
> The goal here is to log the battery supply voltage (along
> with some other info which is stored in serial EEPROM) for
> later analysis. This does not have be absolutely accurate,
> only relatively accurate - this portion of the data will be
> used to generate data for trending battery life under normal
> operating conditions for the device.
>
> thanks in advance for your time
> --
> Glen Wiley
>
> Everything should be made as simple as
> possible, but not simpler. -- Albert Einstein
>
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