On Fri, 16 May 2003 12:35:59 +0000, you wrote:

>Hi everyone, I am using a microcontroller (M16C) to do simple analog to
>digital conversion. Due to the rather limited documentation and support =
for
>this chip (by mitsubishi), I have some queries on certain aspect of this=
 A-D
>stuff.
>
>The manual mention that the A-D functionality uses 3 other pins beside =
the
>analog input pin(AN0-AN4...). They are:
>AVcc =3D Power source pin of the analog section
>AVss =3D GND pin of the analog section
>Vref =3D Input pin of reference voltage
>(These pins can be tied to internal source or externally)
>
>My question is:
>1) If my external analog signal(-5V to 5V) is using a different power
>supply, how shall I connect these 3 pins?
If your input can go negative, you will need to level-shift it to bring =
it inside the range of the
ADC. An inverting op-amp configuration with a gain of 0.5 will convert it=
 to 0..5V. You may also
need to buffer the input if the source impedance is significant or =
non-constant.=20

e.g. 10K from input to opamp -ve input, 5K from opamp -ve to output, =
connect opamp+ve to a reference
which is 0.5 Vref (e.g. 10K/10K divider, with 100n decoupling from the =
0.5Vref to ground.).

Avss,Avdd should normally be connected to digital Vss,Vdd, occasionally =
via some noise-filtering
components. you can NOT use different AVdd/ss to level-shift your input.

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