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'[EE] Adjusting voltage output of a temperature sen'
2000\05\23@002310 by Dean Biddle

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Hi,

Are there alternative(s) to an operational amplifier (supplied by +ve and
-ve) to adjust the voltage output of an LM335 temperature sensor from
approx. 2.5 to 3.5 volts to span the 0 to 5 volt range for a P16F87x A/D
converter?  My application needs to consume minimal current and preferably
use only one battery.  Can another option for the configuration of the A/D
system be used other than limiting Vref+ to approx. 3.5 volts?

I assume this is common, so please simply refer me to a relevant past
PICLIST thread if I failed to match the thread in my "pre-post" search.

2000\05\23@002919 by Damon Hopkins

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Dean Biddle wrote:
>
> Hi,
>
> Are there alternative(s) to an operational amplifier (supplied by +ve and
> -ve) to adjust the voltage output of an LM335 temperature sensor from
> approx. 2.5 to 3.5 volts to span the 0 to 5 volt range for a P16F87x A/D
> converter?  My application needs to consume minimal current and preferably
> use only one battery.  Can another option for the configuration of the A/D
> system be used other than limiting Vref+ to approx. 3.5 volts?
>
> I assume this is common, so please simply refer me to a relevant past
> PICLIST thread if I failed to match the thread in my "pre-post" search.

Excellent question.. I await the answer because right now I'm using an
OP AMP to do it... I hope there is..

               Damon Hopkins

2000\05\23@095515 by Scott Dattalo

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On Tue, 23 May 2000, Damon Hopkins wrote:

> Dean Biddle wrote:
> >
> > Hi,
> >
> > Are there alternative(s) to an operational amplifier (supplied by +ve and
> > -ve) to adjust the voltage output of an LM335 temperature sensor from
> > approx. 2.5 to 3.5 volts to span the 0 to 5 volt range for a P16F87x A/D
> > converter?  My application needs to consume minimal current and preferably
> > use only one battery.  Can another option for the configuration of the A/D
> > system be used other than limiting Vref+ to approx. 3.5 volts?
> >
> > I assume this is common, so please simply refer me to a relevant past
> > PICLIST thread if I failed to match the thread in my "pre-post" search.
>
> Excellent question.. I await the answer because right now I'm using an
> OP AMP to do it... I hope there is..

Is it the op amp that's the problem or having to supply -ve? There are several
low-power, single-supply, rail-to-rail opamps in tiny SOT-23 sized packages that
would work nicely here.

All though I've never explored this, you could recover the A/D converter's
dynamic range by tying the LM335 to the 87x's vref pin and then measure a known
DC voltage source on one of the analog inputs. Perhaps two or three DC voltages
could be used as references. At first glance this would appear to prevent the
other analog inputs from being used. However, you could still switch the A/D to
use the power supply rails as the voltage reference.


Scott

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