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AVR ADC question
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AVR ADC question

by foobillious on Mon Feb 22, 2010 9:44 pm

Hi All,

I'm working on a circuit based around a at90s4433. I realise this is an ancient chip, but i've got a bunch of them lying around and I wanted to use them up.

I'm having an issue trying to use the chip's ADC to measure the voltage the high (5-35v) voltage output from a boost converter thats PWM control is handled by the same chip. In order to measure this I was connecting the ADC ping through a voltage divider comprised of a 10K and 1k resistor, with the junction connected to the chips ADC0 pin. The problem is when the pin is connected to the junction the voltage reads like the 1k resistor is a 500 Ohm (measured with both the ADC and my multimeter). When the pin is not connected the voltage reads as expected (1/10th of the input voltage) Is this normal behaviour or have I screwed something up?

When measuring 5Volt and lower voltages directly (no voltage divider) with the ADC the voltage reads almost the same as my multimeter (certainly within the error range that i'm happy with).
foobillious
 
Posts: 15
Joined: Fri Jul 03, 2009 12:36 am

Re: AVR ADC question

by zener on Tue Feb 23, 2010 4:21 am

Is there an option for an internal pulldown? Maybe try assuming that a 1K pulldown is there (as it seems to act like) so just put a 9K series resistor to it and see if it acts like a 10:1 voltage divider.

zener
 
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Joined: Sat Feb 21, 2009 2:38 am

Re: AVR ADC question

by foobillious on Tue Feb 23, 2010 6:51 pm

There is an option for an internal pull-up. hmm.. maybe I just stuffed my maths.

I'll check again and try the 9k resistor idea if the maths was right. Thank you for your help Zener
foobillious
 
Posts: 15
Joined: Fri Jul 03, 2009 12:36 am

Please be positive and constructive with your questions and comments.