>>    Tad is the convertion time, right? I think FOSC/32 @ 20MHz (3.2us)
would
>Fosc/32 @ 20Mhz = 1.6uS (at least according to the tables in the data
books).

    Oops, sorry. I mean Fosc/32 @ *10*MHz.

> Why are you using a float anyway?
> A 32-bit integer will fit AND be more accurate AND faster!!

    CCS-PCM doesn't have 32-bit integers.

>   Assuming the software it's ok, you have 80% a measuring error or
>noise problem. First, this looks like an offset error. Have you use
>some sort of breadboard without nice ground lay-out? Make a
>measurement with a good voltmeter (4 1/2 digits ) between 877
>ground and adc input pin, you will get something... change the
>ground point let say near the power supply, you will get something

    YES!!! I have 60mV of voltages betweem some points on the breadboard.
I've corrected it putting some wires between various ground points. Now it's
2mV max.

>else...
>  Have you filtered the analogic input signal ?

    Do you mean putting a capacitor between the analog input and ground?
What value? I was thinking that my *software* filter was enough. I'm
averaging 2048 readings. It's a lot, I know. I hope to use no more than 256
on the final product.

>  Finally I've manage with my 10bites adc readings ( it was a
>compiler problem, of course at bank switching ) and I haven't this
>problem... all measured points are in +- 2LSB  with a 10K pot and a
>good 5.00V supply.

    I'm not making tests with a 25-turn 5K precision trimpot. I'm using
FOSC/32 and 4MHz clock. Here is my last results.

    Multimeter    PIC ADC
        0.004V     0.000V
        1.000V     0.992V
        2.000V     1.988V
        2.500V     2.474V
        3.000V     2.989V
        4.000V     4.000V
        5.010V     5.000V

    This values are usable on my application. Hope in the PCB it will work
better.

    Now, I will have some headaches with the precision AC/DC converter.
Ouch. Someone have a T**3 (tested, tried and true) precision AC/DC converter
circuit that works over the entire PIC ADC voltage range and 1:1 gain? I
mean 0-5VAC input, 0-5VDC output.

    Thanks to everyone that helped me with this.

    Regards,

    Brusque
-----------------------------------
Edson Brusque
Research and Development
C.I.Tronics Lighting Designers Ltda
(47) 323-2685  /  (47) 9993-6453
Blumenau  -  SC  -  Brazil
-----------------------------------