Flash PIC with 12 bit ADC

[Steve_88]

I've read back in the day that the reason there aren't any 12 bit pic's is because it is difficult to keep the noise from the micro away from the ADC.

Now they have new flash pic's with 12 bit ADC. Are these chips really capable of a solid 12 bit result?

If so, can you suggest something similiar to a vanilla 16F876 that would be good to start with?

In addition, I would like to use an amplifier configured as a voltage follower/buffer to allow for higher input impedences. I'm just measuring DC signals and would appreciate some suggestions for a low power multi channel amplifier.

Thanks,
Steve

[BrianT]

12 bit ADC operation should be easy with even relatively sloppy layout boards. I recommend you use a copper pour over the entire board tied to ground.

I have found an external 16 bit ADC is a challenge and needs very careful layout, separate analog and digital grounds, a copper pour tied to analog ground around the ADC and tied to digital ground in relevant areas plus software tricks like multiple averaged samples to get expected results.

One neat trick that has saved the day for me several times is to have a low voltage mains input and to time the ADC read to a zero crossing, or at least the same point in the mains cycle every sample.

HTH
Brian

[Charles Linquis]

The (new) 18F8723 has 12 bit A/D's

[Steve_88]

Thanks for the tips. Any suggestions on using an amplifier as a low pass filter and to provide a low impedence output to the AtoD input?

I've been looking on the web etc, but is quite daunting.

Thanks,
Steve

[Charles Linquis]

Op amps are really quite simple. You should probably pick one with rail-to-rail input and output and low offset voltage. If unity gain is what you want, just hook the output to the (-) input and feed your signal into the (+) input. Most of the data sheets will have examples

[SteveB]

TI put out "Op Amps For Everyone.pdf (http://focus.ti.com/lit/an/slod006b/slod006b.pdf)" and "Handbook of Operational Amplifier Applications (http://focus.ti.com/lit/an/sboa092a/sboa092a.pdf)," both of which may help as well.

SteveB

[Steve_88]

Thanks for the links, sure is alot of information there. I found a microchip appnote AN699 which is helpful. I downloaded the filterlab program from microchip.

I'm shooting for single pole low pass filter which will beef up the output current to the AtoD of the pic.

Looks like I'm on my way, thanks very much