# 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.

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2. ### Papabravo Expert

Feb 24, 2006
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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?

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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 ?

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4. ### Papabravo Expert

Feb 24, 2006
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I can't tell without looking at the rest of the datasheet. Can you provide a link?

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Aug 9, 2011
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6. ### Papabravo Expert

Feb 24, 2006
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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
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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)

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9. ### cmartinez AAC Fanatic!

Jan 17, 2007
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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
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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.

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11. ### cmartinez AAC Fanatic!

Jan 17, 2007
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I will, thanks for the clarification.

12. ### cmartinez AAC Fanatic!

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

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