ADC common mode input range

Discussion in 'General Electronics Chat' started by Anestis88, Dec 7, 2014.

  1. Anestis88

    Thread Starter New Member

    Aug 9, 2011
    19
    2
    Hi
    I am trying to choose an ADC for differential voltage measurement. However, I get confused with terms in datasheets such as "Common mode input range" , "Input differential Voltage range". Can somebody help me to understand the difference of these terms ?
    Thanks.
     
    NTSMTK likes this.
  2. Papabravo

    Expert

    Feb 24, 2006
    10,135
    1,786
    Input differential voltage is the maximum voltage difference between the two inputs.
    Common mode range tells you what range of voltages can be applied to both inputs at the same time for a differential reading of 0.0 volts.

    For example if the common mode range is ±10.0 VDC and the differential input voltage was ±2.0 VDC, and I put +8 volts on one input -2.0 volts on the other input which spec is violated?
     
    NTSMTK likes this.
  3. Anestis88

    Thread Starter New Member

    Aug 9, 2011
    19
    2
    Thanks for the response. I supposed to the example you give that the differential input range is violated since V+ - V- = 10V.
    Usually, I see the CM as a positive value as shown below. So with Vref=+5V the Vcm = +2.5V
    . Does that prevent me to measure Vdiff >2.5V ?

    Capture.PNG
     
    NTSMTK likes this.
  4. Papabravo

    Expert

    Feb 24, 2006
    10,135
    1,786
    I can't tell without looking at the rest of the datasheet. Can you provide a link?
     
    NTSMTK likes this.
  5. Anestis88

    Thread Starter New Member

    Aug 9, 2011
    19
    2
  6. Papabravo

    Expert

    Feb 24, 2006
    10,135
    1,786
    OK. So there are three things you must ensure:
    1. Vref must be equal to +5.0 V
    2. -5.0V < (Vin+) - (Vin-) < +5.0V
    3. -0.1 V < Vin+ < 5.01V ; AND -0.1 < Vin- < 5.01 V
    Do you understand the implications of all three requirements?
     
  7. Anestis88

    Thread Starter New Member

    Aug 9, 2011
    19
    2
    Yes, I fully understand these requirements. But these three implications they do not come from the common mode input range, are they ?
    That was the point I got confused while reading the datasheet.
     
  8. Papabravo

    Expert

    Feb 24, 2006
    10,135
    1,786
    Sure they do. You can consider each input to have a common mode voltage of 2.5V = Vref/2 = Vref * 0.5
    Now whereever the signal goes in the range 0.0 V to 5.0 volts it has a common mode part of 2.5 V and a differential part of (V - Vcm)
     
    Anestis88 likes this.
  9. cmartinez

    AAC Fanatic!

    Jan 17, 2007
    3,552
    2,487
    Now I'm a little confused here. @Papabravo, please correct me if I'm wrong, but are the signs in statement 3 correct? Shouldn't it read:
    -0.1 V < Vin+ < 5.01V ; AND -5.01 V < Vin- < 0.1V
     
  10. Papabravo

    Expert

    Feb 24, 2006
    10,135
    1,786
    No, I don't think so. The ranges in statement 3 say that the inputs cannot go outside of the absolute range bounded by the supply voltages.
    In differential terms if IN+ = 5V and IN- = 0V the differential is (5 - 0) = 5Volts
    On the other hand if if IN+ = 0V and IN-=5V then (0 - 5) = -5V is the differential voltage
    This ADC will not tolerate negative voltages. Negative differential voltage is another matter.

    Reread the datasheet very very carefully.
     
    cmartinez likes this.
  11. cmartinez

    AAC Fanatic!

    Jan 17, 2007
    3,552
    2,487
    I will, thanks for the clarification.
     
  12. cmartinez

    AAC Fanatic!

    Jan 17, 2007
    3,552
    2,487
    Yeap, you were right. I finally got it when I saw figure 23 on the datasheet.

    Capture.JPG
     
    Papabravo likes this.
Loading...