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plz help ...its urgent....

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ss53

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the lm35 with a negative voltage

hi
i am working on a microcontroller 89c51
i want to interface an LM35 to the microcontroller using an ADC
the temp range of the temp sensor is -55C to 150C(-550mV to 1.5V) with a scaling factor of 10mV/C.
the analog input to the ADC is 0-5V
my question is how are the negative voltage values converted on the ADC?
plz help ....its urgent
 

IanP

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The ADC will not work with negative voltages ..
What you will have to do is to "mix" the output voltage from the LM35 with +550mV and one way of doing this is to use "adder" based on an opamp ..

Refer to this post for an example:


Regards,
IanP
 

    ss53

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rosarioantony

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first shift the minimum voltage(-550mv? ) into zero volt by summing the sensor output with the +550mv(use a pot - potential devider) using a opamp summing amplifier (level shifting) .
now u get 0V to 2.05V(1500+550mv) range. so use an non inverting amlifier with the gain of > 5/2.05 to get 0-5V input to ur microcontroller
 

lambtron

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Instead of LM35, how about using LM335?

The LM335 transducer has 10mV/°K, so its output voltage will be positive even when measuring negative (°C) temperatures.
 

mmike

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First detect that it is a -ve volatge and rectify the same..
 

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