On Tuesday 04 February 2003 01:55 pm, you wrote:

> Assuming high impedances are OK, there's a different way to address
> the problem with the common-mode input range of the comparator.
> Instead of connecting the RC filter to the comparator (-) input,
> connect it to the (+) input and put a resistor between the pot wiper
> and the (+) input, like this:
>
>    o +5V                       16F628
>
>    |                          +---------
>
>    \ 10K                      |
>    /     470K              RA2|
>    \<---/\/\/\---+------------| comp +
>    /             |  470K   RA3|
>    \             +--/\/\/\----| charge
>
>    V            =3D=3D=3D 0.02uF    |
>
>                  V            +---------
>
> Connect the (-) input to a fixed internal or external reference of
> 0.5*Vcc. Modify the firmware so that it inverts the state of the
> comparator output bit when it copies it to the charge pin. The
> circuit now operates so that the average voltage on the charge pin is
> the complement of the voltage on the pot wiper, and both inputs to
> the comparator are held at 0.5*Vcc.

That's pretty neat, I'll have to try that.  I'm guessing that the=20
advantage is reduced current consumption and the fact that the=20
comparator is being operated at it's most accurate point.  Does that=20
sound right?

michael

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