Hi,

in this application note:

http://sci-toys.com/scitoys/scitoys/computers/thermometer/how_the_LM34_works.pdf

page 5, section "Fahrenheit temperature sensors", it says:

"The sensor can be used with a single supply over the full −50 ̊F to +300 ̊F temperature range, as seen in Figure 10, simply by adding a resistor from the output pin to ground, connecting two diodes in series between the GND pin and the circuit ground, and taking a differential reading. This allows the LM34 to sink the necessary current required for negative temperatures."

What does "differential reading" mean in this context?

 

Hi,

in this application note:

http://sci-toys.com/scitoys/scitoys/computers/thermometer/how_the_LM34_works.pdf

page 5, section "Fahrenheit temperature sensors", it says:

"The sensor can be used with a single supply over the full −50 ̊F to +300 ̊F temperature range, as seen in Figure 10, simply by adding a resistor from the output pin to ground, connecting two diodes in series between the GND pin and the circuit ground, and taking a differential reading. This allows the LM34 to sink the necessary current required for negative temperatures."

What does "differential reading" mean in this context?

It means measure across Vout in Fig. 10, instead of using ground as the reference, as is done in Fig. 9.

 

It means you have to measure the voltage between (differential of) the two indicated terminals (Vout) not from one terminal to ground. For this you need a circuit that can measure differential voltages above ground such as an op amp configured as a differential amplifier, or an instrumentation amp IC.

 

So, in a uC world, I take a reading from the Vout + on one pin, and Vout - on another and subtract the - value from the + value?

ADC_pos = readADCpospin
ADC_neg = readADCnegpin

Temperature = ADC_pos - ADC_neg

 

So, in a uC world, I take a reading from the Vout + on one pin, and Vout - on another and subtract the - value from the + value?

ADC_pos = readADCpospin
ADC_neg = readADCnegpin

Temperature = ADC_pos - ADC_neg

That should work.

 

Great. Thanks guys.

 

So, in a uC world, I take a reading from the Vout + on one pin, and Vout - on another and subtract the - value from the + value?

ADC_pos = readADCpospin
ADC_neg = readADCnegpin

Temperature = ADC_pos - ADC_neg

That will work as long as the gain for each channel is the same (or close enough for the accuracy you want).

 

Differential reading is like then you use a voltmeter to measure the voltage drop over some resistor in a circuit. If the voltage changes are slow compared to the sample rate. You can use your method using two ACD readings. If not you can use an instrumentation amplifier(IA) setup.
http://en.wikipedia.org/wiki/Instrumentation_amplifier
Commercial or home build. For precession use a commercial one. As you need to use very close matched resistors to avoid offset errors.

 

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Commercial or home build. For precession use a commercial one. As you need to use very close matched resistors to avoid offset errors.

An IC instrumentation amplifier has the precision resistors on-chip so you don't need precision external resistors unless you need an accurate gain of more than 1.

 

An IC instrumentation amplifier has the precision resistors on-chip so you don't need precision external resistors unless you need an accurate gain of more than 1.

Indeed, I was refering to resistors(besides Rgain that is) used in the basic IA setup shown in this picture. Sorry if I was unclear