18F4520 A/D working weird

[Warrier]

Oh, I forgot to mention another point. LF411 or LM741 CANNOT be used with single rail supply unless you bias the input up half way and ac couple the input....Then you are going to lose dynamic range (video signal is referenced to zero so you need balck level clamping).

If you are using the 741/411 with 0-5volt supply then your output will be "stuck" high or low depending on input and no signal will be seen at the output of the buffer!

- warrier

[TurboLS]

First of all about the frequency, this CCD is being run very slow... approximately 3.6kHz and as such, the frequency of the vout signal is also 3.6khz.

I will definitely take a look at the adcon 2 register to see if maybe that will fix the problem.

We were using a simple buffer with a negative feedback loop and powered with positive 18V and negative 18V.

I also read in the manual that the maximum input impedance for an analog signal is 2.5kohms. Does anyone know what the minimum or recommended impedance is>?

Thanks for everyone's help. Hopefully I can figure out a way to get the PIC to work with this setup.

[TurboLS]

Well, I slope coupled the input with a 1200pF cap and a 10k resistor and if go directly from that into a non-inverting x10 opamp circuit and then to the PIC, I can actually see a signal. As of yet, all I can get is noise on the A/D tho, so it may just be that I have to change the point at which I do the A/D operation in the software. I will be trying a combination of circuits with the slope coupler, amplifier, buffer, and sampling opamp circuitry.

Also, since I am running the chip so slow, I don't think the slew rate nor the ADCON2 register would be a problem.

[TurboLS]

also, do i expressly need the following line in my code?

ADCON0.0 = 1 ' enable a/d

Cuz I can do A/D without it, but I was just wondering if it might have been a config error. thanks.

[TurboLS]

I am pretty sure I just measure the impedance in parallel, but how do I do that (on the PORTA.0 A/D channel) with the PIC? thanks.