PIC AD + Vref- using external IC

[skimask]

See if I got this straight:
You have now:
Sensor minimum 1.8v - maximum 5v
(PIC A/D minimum 0v - maximum 5v)

You want to shift the input down by 1.8v, so a zero input is really a ZERO input, then you want to multiply that input by 1.5625 so you can regain the full range of 0-5v at the A/D input?

You could do all the above...but, yes, you can reference the Vref- to 1.8v giving you a new ZERO (0) at the A/D input, and use 5v for Vref+ giving you MAX (1023 for 10bit A/D) at the A/D input of the PIC. You could use something as simple as 3 silicon diodes in series or even a red led (hand picked parts of course) as this would give you the 1.8v needed for Vref-. Of course the Vf would vary a bit, temperature, etc. But using the red led would also give you a power led.

And what is this sensor? How much accuracy do you REALLY need?

[mister_e]

lm4128, LM4132 are nice. more you pay, more they're accurate.

[jorge]

lm4128, LM4132 are nice. more you pay, more they're accurate.

Thanks , i will try them !

[jorge]

See if I got this straight:
You have now:
Sensor minimum 1.8v - maximum 5v
(PIC A/D minimum 0v - maximum 5v)

You want to shift the input down by 1.8v, so a zero input is really a ZERO input, then you want to multiply that input by 1.5625 so you can regain the full range of 0-5v at the A/D input?

You could do all the above...but, yes, you can reference the Vref- to 1.8v giving you a new ZERO (0) at the A/D input, and use 5v for Vref+ giving you MAX (1023 for 10bit A/D) at the A/D input of the PIC. You could use something as simple as 3 silicon diodes in series or even a red led (hand picked parts of course) as this would give you the 1.8v needed for Vref-. Of course the Vf would vary a bit, temperature, etc. But using the red led would also give you a power led.

And what is this sensor? How much accuracy do you REALLY need?

Hi, first of all thanks for you reply.
Regarding the sensor it ´s a pressure sensor. And regarding the reason , it is simple: since i am discarding by software the first 1.8v of the 5v AD readings i am loosing almost ~36% of the pressure range i could obtain -> 1.8v to 5v only !!!...3.2v only to be converted into valid/useful steps of the AD!
Thanks again for the LED hint , but i really need a Temp free device...