LM35: reading negative temperatures with confugiuration from figure 18

Magno,

Can you please provide the full orderable part number you are using?  There are multiple versions of this device and different owning product groups.  This will help us move to correct forum for support.

Regards,

Wade

Hi Magno,

From figure 18 we see that

Vout ("+" node relative to supply GND) = 2*Vd+10mV/^oC,

where Vd = forward voltage of the 1N914 diode, which depends on the diode forward current (should be close to the quiescent current of LM35)

The diodes are used to create a 2*Vd reference for the LM35 so that there will be headroom for LM35 to measure negative temperatures.

Note that the differential output (See the "+" and "-" nodes in figure 18) is still Vout = 10mV/^oC.

Example calculations:

Let's assume Vd=0.6 V for figure 18 and the operating temperature is -55^oC to 150^oC.

Vout ("+" node relative to supply GND) ranges from 0.65V to 2.7V.

@-55: -55^oC.*10mV/^oC+2*0.6V = 0.65V

@150: 150^oC.*10mV/^oC+2*0.6V = 2.7V

ADC considerations:

Single-ended ADC

As you can see, for single-ended measurement the output accuracy depends on the reference voltage provided by the diode stack.  The user will need to monitor both the LM35 and diode reference voltage in order to have reasonable accuracy over temperature.

Differential ADC

The Vout ("+" and "-" nodes) is independent of Vd.  However, you still need a stable common mode reference 

The bottom line is your system accuracy can't be higher than your reference voltage.

-Kelvin