Pic A2D Input Method.

[richard.c]

Hi,

I'm using a 18F2520 pic with an amplified sensor signal of 0 - 1v, which I would like to convert at a maximum resolution.

Having read Nigels tutorial on the use of Vref I can appreciate the handiness of using this method.
Equally, I was wondering if I modified the amplifier to a 4 or 5 volt output, it would seem to be a simpler way, plus saving the use of the Vref Port.

However what I would like to know is does the voltage shunt Vref method actually give a significant improvement on the stability of the resultant conversions over the basic VDD 5v method ?

Also wondered if there is any merit in fitting a 5.1v zener to the RA0 pin incase something caused the amplifier to exceed 5v.

thanks

Richard

 

[eng1]

I would use both an amplifier and a voltage reference. The amplifier provides some gain (note that there is a minimum value for Vref and your signal is too low if you want high accuracy) and a low output impedance. The precision reference provides a stable voltage with low drift; its tolerance is usually little if compared to a voltare regulator IC. For example the output of a 7805 can be anywhere beetwen 4.75 and 5.25 V.

 

[Nigel Goodwin]

Yes, using the Vref with a precision reference makes it more accurate and repeatable - and overcomes the limitations of the opamp feeding the input. There's no need for a zener, as the input is clamped anyway, which is why I have a series resistor feeding.