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
>
>

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