AVR-Chat

Inaccuracy in the reference should have very little
effect on measurments near ground. 

But, yes, the source impedance may be too high. Try this
experiment:

1. Make a measurment with the top of the divider (the end
normally connected to 10V) grounded. This will make the
divider resistance as seen by the ADC input 5K. Note the
reading.

2. Make a measurement with the top of the divider not
connected to anything. Make sure that there are no nearby
signals (processor clocks, PWM, etc) that can be coupled in
to upset things. This will make the divider resistance seen
by the ADC input 10K. Note the reading.

3. If the second reading is significantly greater than the
first, its an impedance (resistance) issue. If this is the
case and you really need the higher resistance, try adding
a cap across the bottom resistor. This may help because the
current flows only during the sample time. I would try a
0.01uf cap.

Jim


On Tue, 22 Jun 2004 14:34:45 -0500
 David VanHorn <dvanhorn@cedar.net> wrote:
> 
> 
> 
> 
> At 06:57 PM 6/22/2004 +0000, p_anglum wrote:
> 
> >I'm using a Tiny 15 to convert a 0-10 volt input to
> PWM output.
> >I use a voltage divider (10K - 10K) to scale the 0-10
> down to 0-5 
> >before applying it to PB3(ADC3).
> >
> >Everything seems to work except at the lowest range
> of input voltage 
> >the A/D seems to always read some small
> voltage.  In fact, with my 0-
> >10V input removed and only the voltage divider on the
> input to to 
> >ADC3 I read about .9 volts on ADC3.  It appears
> the source of the 
> >voltage is the ADC3 pin itself.  Any ideas?
> >(All voltage measurements were made in refernce to
> pin4 - GND on the 
> >Tiny15)
> 
> You may have too high an input impedance. 
> Also, what's your reference voltage? On the Tiny-26's,
> the 2.5V "precision" reference comes out at
> 2.7V. 
> 
> 
> 
> 
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