AVR-Chat

Hi group:
The problem I initially had on using the A/D on the ATmega128 was that I am holding an A/D input at steady state and am changing all other 7 channels in small increments until they reached the max level then they would start from ground again. The problem I initially had on using the A/D on the ATmega128 was that the steady state input would start to drift higher (i am using hyperterminal and software to see the output) for a certain range of inputs on the other A/D inputs. This problem got traced back to a dipping AVcc. The power supply to AVcc is a shunt reference(LM4040AIM3-5.0) with a 590 ohm series resistor. I changed this resistor to 200 ohm and the problem went away. It should be noted that AVcc and AVref are tied together and was initially designed for an ATmega103. I know the current draw for AVref, but cannot find the Maximum current draw for AVcc. Does anyone know where or how to find this?
Thanks
Draper

Thanks for the quick response. Unfortunately this board cannot be redesigned except for the current limiting resistor. Future boards will follow the guidelines you suggested. AVcc and Vcc are both at 5V. 6.25V is feeding the regulator. All channels of the A/D are setup for inputs and are connected to LM324 outputs. So, I need to calculate max current draw for each pin, add them together and then add that to the Vref current draw since Vref and AVcc have a trace tying them together. Without doing that calculation I found that changing the current limiting resistor to allow 6.5mA to flow fixed the problem. This seems like a lot of current. I was just trying to minimize the amount of current flowing to keep things cooler.
Draper  

--- In AVR-Chat@yahoogroups.com, wagnerj@... wrote:

>
> > Hi group:
> > The problem I initially had on using the A/D on the ATmega128 was that I
> > am holding an A/D input at steady state and am changing all other 7
> > channels in small increments until they reached the max level then they
> > would start from ground again. The problem I initially had on using the
> > A/D on the ATmega128 was that the steady state input would start to drift
> > higher (i am using hyperterminal and software to see the output) for a
> > certain range of inputs on the other A/D inputs. This problem got traced
> > back to a dipping AVcc. The power supply to AVcc is a shunt
> > reference(LM4040AIM3-5.0) with a 590 ohm series resistor. I changed this
> > resistor to 200 ohm and the problem went away. It should be noted that
> > AVcc and AVref are tied together and was initially designed for an
> > ATmega103. I know the current draw for AVref, but cannot find the Maximum
> > current draw for AVcc. Does anyone know where or how to find this?
> > Thanks
> > Draper
> >
> >
> 
> You should NOT tie AVcc and Vref together. And AVcc MUST be within a half
> volt of Vcc. Big problems if you try otherwise.
> 
> Maximum current draw on AVcc depends on the loads connected to the analog
> pins in normal I/O configuration. All out-flowing current from those pines
> sources from AVcc.
> 
> Instead, power your reference from Vcc or AVcc. Connect your Vref to the
> reference output and set the control bits for external reference.
> 
> Jim Wagner
> Oregon Research Electronics
>

> Hi group:
> The problem I initially had on using the A/D on the ATmega128 was that I
> am holding an A/D input at steady state and am changing all other 7
> channels in small increments until they reached the max level then they
> would start from ground again. The problem I initially had on using the
> A/D on the ATmega128 was that the steady state input would start to drift
> higher (i am using hyperterminal and software to see the output) for a
> certain range of inputs on the other A/D inputs. This problem got traced
> back to a dipping AVcc. The power supply to AVcc is a shunt
> reference(LM4040AIM3-5.0) with a 590 ohm series resistor. I changed this
> resistor to 200 ohm and the problem went away. It should be noted that
> AVcc and AVref are tied together and was initially designed for an
> ATmega103. I know the current draw for AVref, but cannot find the Maximum
> current draw for AVcc. Does anyone know where or how to find this?
> Thanks
> Draper
>
>

You should NOT tie AVcc and Vref together. And AVcc MUST be within a half
volt of Vcc. Big problems if you try otherwise.

Maximum current draw on AVcc depends on the loads connected to the analog
pins in normal I/O configuration. All out-flowing current from those pines
sources from AVcc.

Instead, power your reference from Vcc or AVcc. Connect your Vref to the
reference output and set the control bits for external reference.

Jim Wagner
Oregon Research Electronics