What does "Differential reading" mean...?

Discussion in 'General Electronics Chat' started by nerdegutta, Apr 19, 2013.

  1. nerdegutta

    Thread Starter Moderator

    Dec 15, 2009
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    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?
     
  2. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    It means measure across Vout in Fig. 10, instead of using ground as the reference, as is done in Fig. 9.
     
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  3. crutschow

    Expert

    Mar 14, 2008
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    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.
     
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  4. nerdegutta

    Thread Starter Moderator

    Dec 15, 2009
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    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?

    Code ( (Unknown Language)):
    1.  
    2. ADC_pos = readADCpospin
    3. ADC_neg = readADCnegpin
    4.  
    5. Temperature = ADC_pos - ADC_neg
    6.  
     
  5. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    That should work.
     
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  6. nerdegutta

    Thread Starter Moderator

    Dec 15, 2009
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    Great. Thanks guys.:)
     
  7. crutschow

    Expert

    Mar 14, 2008
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    That will work as long as the gain for each channel is the same (or close enough for the accuracy you want).
     
  8. t06afre

    AAC Fanatic!

    May 11, 2009
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    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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  9. crutschow

    Expert

    Mar 14, 2008
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    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.
     
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  10. t06afre

    AAC Fanatic!

    May 11, 2009
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    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
    [​IMG]
     
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