Adc Ref. V.

[Melanie]

The ADC section in the Datasheet for your PIC will tell you what you can and can't do with references. If you want to change the reference away from Vdd, then you will need to lose a pin for that task (see ADCON1 Register PCFG configuration 0001).

There is only one ADC on the PIC... it is however multiplexed to several pins. Providing you allow enough settling time for the sampling capacitor to charge before initiating a conversion (50uS is a good starting point), you will have no problem switching between multiple inputs.

[modifyit]

Melanie,

I guess this is a little off topic, but you mentioned 50us between reading in different a/d values on porta. Are you basically saying we should not attempt to do....

ADCIN 1, var1
ADCIN 2, var2
ADCIN 3, var3

Are you saying we need at least 50us between differnt ADCIN commands or wierd issues may result?

Thanks for clarifying

[Warrier]

Internally, the sampling switch and the channel multiplexers are CMOS switches and they present an ON resistance that is a linear function of supply voltage. (it can vary from about 5kohm to 10kohm for Vdd ranging from 5V to about 3V). When a signal is applied to any pin, the internal holding capacitor has to charge to the input voltage through this resistor. With a 10bit ADC, you need the holding capacitor to be about 0.1% of the applied voltage to get accurate results.

You also need to make sure that any external amplifier also settles down to this accuracy before you let the ADC perform the digitization. Hopefully you are not increasing this value outside the PIC using any resistors. The acquision time thus is close to about 20usec at 5V with a 10k signal(source) resistance (impedance). To allow for other switches to settle, Ms. Newman is suggesting 50usec.

No, you don't need to add a Pauseus 50 if you have already defined DEFINE ADC_SAMPLEUS 50 at the header. This keeps the program waiting for 50usec between setting the selected channel and starting the AD conversion.

When you set your external reference to 500mV keep in mind that your LSB is only 500microvolt. The ENOB of the PIC ADC is only 9.5bits even with severly bandlimited signal. Glitches when operating internal switches and amplifier settling time may all be comaprable to this value. Keeping signal path clean also becomes critical. Sloppy Vref can eat into this resolution. Bypass is a must.

-warrier

[GEEZER]

Thanks everybody, lot's of information. Just what I need.