16F886 and Voltage reference

[retepsnikrep]

I have a project using all 11 ADC Channels on the 16F886.

I note the external voltage reference channels/pins are shared with the ADC channels AN2 & AN3 :?

Anyway round this? I can't disconnect the device the adc is measuring from the adc pins AN2,AN3.

I'm fairly happy with the VDD/VSS reference method but was just thinking about using something with more precision than the power supply.

[HenrikOlsson]

Hi,
No way around it that I can think of.
How much current does the system draw? Perhaps you can power the PIC from a (possibly buffered) voltage reference chip thus making Vdd more stable/accurate.
Or use an external analog swich to connect either a voltage reference or your normal analog signal to one of the inputs, sample the voltage reference and use its value to apply correction in software.

/Henrik.

[Acetronics2]

Hi, Peter

may we understand your board design is already done and can't be altered ?

If so ... see Hendrik's answer.

another solution could be, if your board current draw is quite constant, to add a little calibration routine in your program : @ first power up, you use a known reference voltage on one ADC pin ... then the routine calculates a correction coeff to use in your ADC measurements ...

say, IF the EEPROM cal value is $FF or $ FFFF ( generally is with newly programmed components ) , THEN you enter the cal. routine @ power-up ...

Alain

[retepsnikrep]

The system has quite a high precision 5v regulator so I think I will stick with the VSS/VDD references for now but thanks for the ideas.

Peter