Another way of handling bipolar inputs is to just bias them up with a
couple resistors. I don't recall the output resistance of the LM35 chip,
but if it's relatively low, you could just put a couple 10k resistors on
th PIC ADC input pin. The other end of one goes to the reference voltage
(often +5V). The other end of the other resistor goes to the LM35. If the
LM35 were to output -5V, the ADC would see 0V. If the LM35 were to output
0V, the ADC would see +2.5V. If the LM35 were to output +5V, the ADC would
see +5V.

While the absolute values of the resistors is not critical, the matching
(and tempco) IS critical. You are often best off using a couple resistors
on a network, since they are pretty precisely matched in both terms of
initial resistance and tempco.

Harold


> On Mon, Oct 02, 2006 at 12:19:41AM +0800, genome wrote:
>> Hello..
>> What happens if a PIC pin configured as a 10bit a\d receives negative
>> voltage... I'm interfacing an lm35c temperature sensor to a pic and it
>> says
>> on the datasheet that lm35 will produce a negative output voltage when
>> measuring less than 0 degrees celsius...
>
> You can do a few things: don't use the full-range application shown in
> figure 2 of the datasheet or using a summing op-amp to add a constant
> voltage to the lm35 output. I'm sure there are other solutions.
>
> Take care.
>
> Matthew
>
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