> Hi everyone,
>
>    I´m trying to read negative temperature using LM35, and one 16F877.
>    It gives me 10mv/ºC, so at -55º, I´ll have -550mv.
>    But I want to have 0 Volt at -10ºC, How do I change the zero position ??
>
>     It it for being able to read this temperature with the 16F877.
>
>          Thanks in adavance.-
>
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>A) Use a diferential amplifier ( I hope you are familiar with the
>configuration), Vout = vp - vn where vp is the potential on
>+ input and vn is the potential on - input. >On Vp put a reference of +550mV, then for -55C the output will be 0V.
>For positive temperature the output will be +550mV +temperature potential.
> >B) If you can go only to -45...-50 C then no external operational is need.
>Take a look to my site:
>http://www.geocities.com/vsurducan/electro/PIC/f877.html
>
>On Thu, 10 Jan 2002, Gabriel Caffese wrote:
>
>>    I´m trying to read negative temperature using LM35, and one 16F877.
>>    It gives me 10mv/ºC, so at -55º, I´ll have -550mv.
>>    But I want to have 0 Volt at -10ºC, How do I change the zero position ??
>> >>     It it for being able to read this temperature with the 16F877.
>>

> The circuit as you described it, with +550 mV on the (+) input of the
> diffamp and the LM35 output connected to the (-) input of the diffamp,
> would give 0V output at +55C, not -55C.  The diffamp output would go
> positive for temperatures below 0C, and negative for temperatures above
> 0C.  Also, the output of the LM35 can't go below ground (at 0C) without
> some help.
>
> The LM35 data sheet (http://www.national.com/ds/LM/LM35.pdf) recommends
> a couple of ways for dealing with this.  One way, shown in Figure 7 of
> the data sheet (copy attached as Image2.gif), allows temperature sensing
> below 0C by placing a couple of diodes in series with the ground lead of
> the LM35, and lightly pulling down the LM35 output with a resistor to
> ground to allow the LM35's output to go a bit negative with respect to
> its ground pin.
>
> This method requires that two of the PIC's analog inputs be used; one to
> read the LM35's output pin, and the other to read the potential on the
> LM35's ground pin.  After taking the two A/D readings, a simple
> subtraction yields the temperature result.  The only components required
> are two 1N4148 (or equivalent) diodes and an 18K resistor, which should
> be somewhat less expensive than a differential amplifier.
>
> Dave
>
> Vasile Surducan wrote...
>
> >A) Use a diferential amplifier ( I hope you are familiar with the
> >configuration), Vout = vp - vn where vp is the potential on
> >+ input and vn is the potential on - input.
> >On Vp put a reference of +550mV, then for -55C the output will be 0V.
> >For positive temperature the output will be +550mV +temperature potential.
> >
> >B) If you can go only to -45...-50 C then no external operational is need.
> >Take a look to my site:
> >www.geocities.com/vsurducan/electro/PIC/f877.html
> >
> >On Thu, 10 Jan 2002, Gabriel Caffese wrote:
> >
> >>    I´m trying to read negative temperature using LM35, and one 16F877.
> >>    It gives me 10mv/ºC, so at -55º, I´ll have -550mv.
> >>    But I want to have 0 Volt at -10ºC, How do I change the zero position ??
> >>
> >>     It it for being able to read this temperature with the 16F877.
> >>
>
>

>A) Use a diferential amplifier ( I hope you are familiar with the
>configuration), Vout = vp - vn where vp is the potential on
>+ input and vn is the potential on - input.
>On Vp put a reference of +550mV, then for -55C the output will be 0V.
>For positive temperature the output will be +550mV +temperature potential.
>
>B) If you can go only to -45...-50 C then no external operational is need.
>Take a look to my site:
>http://www.geocities.com/vsurducan/electro/PIC/f877.html
>
>On Thu, 10 Jan 2002, Gabriel Caffese wrote:
>
>>    I´m trying to read negative temperature using LM35, and one 16F877.
>>    It gives me 10mv/ºC, so at -55º, I´ll have -550mv.
>>    But I want to have 0 Volt at -10ºC, How do I change the zero position

> >
> >    It it for being able to read this temperature with the 16F877.
> >
> >         Thanks in adavance.-
> >
>
> Hi Gabriel,
>
> You'll need to scale it with an op-amp to get zero at -55. Just subtract
> 550mV from the LM35 voltage.
>
> We've used them at very cold temps. here and they work great. We use a

> up table instead of an op-amp to scale the temps. Much easier in our
> situration.
>
> Good luck
>
> chuck
>
>
> wmwmwmwmwmwmwmwmwmwmwmwmwmwmwmwmwmwmwmwmwmwmwmwmwmwmwmwmwmwmwmwmwm
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>

> Martin Peach wrote...
>
> >Of course you could also use the LM335, which gives 10mV/K instead of
> >10mV/C, so your output is always positive. 0C is at 2730mV, -10C at 2630mV.
> >Then do the offset in software. No opamps required.
>
> Damn!  Why didn't I think of that?
> <KICK><KICK><KICK>
>
> Nice part, not too expensive either: US$1.19 at 100 ea. in Digi-Key.
>
> Dave
>
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>

> Vasile Surducan wrote...
>
> >But also you'll have the same problem regarding full scale resolution...
> >Without external op-amp you'll never have it ! And the software offset
> >will not solve the problem...
>
> Well, I'm not sure this is a "problem", as the OP didn't specify what
> resolution he needed over what temperature range; he simply asked for a
> way to read temperatures below 0C.  With a 10mV/K sensor (LM335) and a
> 10-bit PIC A/D operating from a 5.0V Vref, he gets a resolution of
> roughly 0.5C which may or may not suit his needs.  If 0.5C resolution is
> sufficient, it doesn't particularly matter that the entire A/D input
> range isn't utilized.
>
> Obviously, if he wants to get the maximum available resolution over a
> specific temperature range, the LM335 output would have to be offset and
> scaled appropriately; but he didn't say anything about that.
>
> If that's the case, a simple circuit consisting of a RRIO opamp (e.g.,
> 1/2 of a National Semiconductor LMC6482) and three resistors will do the
> trick, as follows:
>
> 1) Connect the opamp (+) input to the output of the LM335;
> 2) Connect the opamp output to the PIC A/D input pin;
> 3) Connect one of the resistors (R1) from Vcc to the opamp (-) input;
> 4) Connect resistor R2 from the opamp (-) input to ground; and
> 5) Connect resistor R3 from the opamp (-) input to the opamp output.
>
> Let Tlow be the temperature at which you want the opamp output to be
> zero volts (i.e., the lowest temperature you want to measure); at this
> temperature the LM335 output is Vlow = Tlow/100, where Tlow is expressed
> in Deg. K.  Let Thi be the temperature at which you want the opamp
> output to be 5V (your upper temperature limit); at this temperature the
> LM335 puts out Vhi = Thi/100, where Thi is expressed in Deg. K.
>
> 6) Choose an arbitrary, convenient value for R2, say 10K ohms.
>
> Since the opamp will act to keep its (-) input equal to its (+) input,
> we can write a pair of nodal equations for the (-) input--one at Tlow
> and one at Thi--that will allow us to solve for the other two resistors,
> R1 and R3:
>
> 7)    (5 - Vlow)*(1/R1) - Vlow*(1/R2) - Vlow*(1/R3) = 0; and
> 8)    (5 - Vhi)*(1/R1) - Vhi*(1/R2) + (5 - Vhi)*(1/R3) = 0.
>
> Plug Vlow, Vhi, and the chosen value of R2 into 7) and 8) above and
> solve them simulaneously to give the required values of R1 and R3.
>
> Voila.
>
> Dave Dilatush
>
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