AVR-Chat

If the cap is across it then the ADC might not get the correct voltage 
either.  The ADC relies upon charging a cap inside the module.  That 
relies upon a certain target RC charge rate, that charge rate also must 
not exceed the current capacity of the pin.  You are playing a finicky 
game by using a cap to boost the inrush current to offset the RC time 
constant.  It might work under some circumstances but since the ADC 
modules specifically states that you should only be using slowly 
changing signals on the circuit with low-pass filters that would tend to 
contra indicate using a bypass cap I think.  Have you used such a 
circuit successfully before?

curious,
DLC



Ralph Hilton wrote:
> On Mon, 18 Jun 2007 15:44:07 -0600 (MDT) you wrote:
> 
> 
>>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
> 
> 
> It isn't 10M if a cap is across it! 
> 
> 
>>>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.
>>>
> 
> 
> --
> Ralph Hilton
> http://www.ralphhilton.org
> C-Meter: http://www.cmeter.org
> FZAOINT http://www.fzaoint.net

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Dennis Clark          TTT Enterprises
www.techtoystoday.com
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