AVR-Chat

Most microcontroller's ADC will not like a 10M load, the charge time will
be very high and not very accurate.  I have used a low power OpAmp to run
very high Z dividers into and the output of the OpAmp is nicely low Z for
that ADC to be happy with.  Its a few more pins and passive parts than the
FET but the readings tend to be more repeatable.

DLC

> On Mon, 18 Jun 2007 13:02:17 -0400 you wrote:
>
>>I think you need to disconnect the voltage divider using an fet. The
>> voltage divider will draw more juice than the avr in sleep mode! I assume
>> you read the datasheet about the half dozen or so best ways to cut down
>> on power consumption? (turn off a/d, turn off pullups, which sleep mode
>> to use, etc)
>>
>
> Forget the FET. 10M resistors are cheaper if the consumption is that
> critical.
> The only buffer needed is a 0.1uF cap.
>
>>-----Original Message-----
>>From: Richard Cooke <rcooke@digitalwavelength.com>
>>To: AVR-Chat@yahoogroups.com
>>Sent: Mon, 18 Jun 2007 12:30 pm
>>Subject: [AVR-Chat] Using the ADC to measure the battery supply?
>>
>>
>>
>>Hi Folks,
>>
>>I'm using the ATmega48V in a project and I need to monitor the battery
>>level (2 AAA cells) every so often and when low give an indication to
>>the user that it's time to change them.
>>
>>I'm trying to minimize current consumption and I'm using the internal
>>1.1V reference so I'm using a voltage divider 1M + 3M to bring the
>>signal down to below the 1.1V reference.  I don't care how long the
>>conversion takes as I'm only making the measurement every 10 minutes
>>or so.
>>
>>Do I need to use a buffer between the divider and the input to the
>>ADC?  If I do, what small, cheap and low power chip would you recommend?
>>
>>Thanks,
>>
>>Richard Cooke
>
> --
> Ralph Hilton
> http://www.ralphhilton.org
> C-Meter: http://www.cmeter.org
> FZAOINT http://www.fzaoint.net
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