AVR-Chat

This is excellent advice from Bob, using a fet to connect / disconnect
the power supply to the voltage divider also means you can use lower
value resistors for the divider, I have had some issues with resistors
> 1M OHM. Not sure what the input impedance of the A2D pin on the
micro is, but you want to pick voltage divider resistor values that
are much lower to prevent the A2D pin from loading the voltage divider.

What I have done in the past in a similar situation is actually used a
Output Port pin to drive the top of the voltage divider directly, this
eliminates using an external fet and the voltage diference between the
port pin output and supply rail is minimal.

Hope this helps.

Cheers
Hein B
Auckland, New Zealand.

--- In AVR-Chat@yahoogroups.com, BobGardner@... 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)
> 
> -----Original Message-----
> From: Richard Cooke <rcooke@...>
> 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
> 
> 
> 
>  
> Yahoo! Groups Links
> 
> 
> 
> 
> 
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