switching external vref+ and vdd vref

[alejandro_halon]

Hi, on more little problem with my project. I´m using 3 ADC inputs, two needs internal vref (5.0v) and one a external vref+ (11.0v to 14.0v, using a resistive divisor (20k + 10k)). The readings becomes unstable when I decided to use the external vref (on RA3, with a PIC 16f873A). I´m switching the ADCON1 with 10000010 (internal vref) and 10000011 (external vref).

Exist some problem to do this? Need some "pause" before alternate the vref?

thanks in advance,
Alejandro.

[mister_e]

i`ve never try this kind of stuf but, what about if you disable the a/d before to switch to external Vref and re-enable it after?

[alejandro_halon]

I tested right now... without diferences...

tks.

Originally posted by mister_e
i`ve never try this kind of stuf but, what about if you disable the a/d before to switch to external Vref and re-enable it after?

[Warrier]

alejandro:

I think you need to supply the reference voltage from a low impedance source; 10k-20k may be too high which may be causing the instability.

Also, allow sufficient time for the reference chain to settle!

_warrier

[alejandro_halon]

If I use some like 2k/1k divisor the current becomes little high, like 4.6 ma... I need the 13.8v (car) reference... The vref needs to be together with the voltage variation of the car... what do you sugest?

About the time, I´ve tried 10ms, need more?

thanks.

Originally posted by Warrier
alejandro:

I think you need to supply the reference voltage from a low impedance source; 10k-20k may be too high which may be causing the instability.

Also, allow sufficient time for the reference chain to settle!

_warrier

[Warrier]

If the driving impedance is small then the 10msec settling time should be adequate.

You can use single rail CMOS operational amplifiers to buffer your reference...these amplifiers have very low quiescent current -less than 1mA but can provide low impedance output.

A simpler way is to use an emitter follower with the collector connected to the Vcc (same supply as your MCU); but remember that the output will be about 0.66volts less than your voltage divider...

A third method would be to use a zener in series to drop the battery voltage to below Vcc (9.1volt zener will give you 4.5volt max at 13.6volt) and take your reference from, say, 2.2k connected with the zener to ground. This will limit the current to ~2mA.

_warrier