AD8274 -- how is common-mode voltage defined?

Question

Looking at the specs, in particular figures 8 and 10 on page 6, the common-mode voltage seems to be limited to 25V for +/-15V supplies. 
 My question is: where is that 25V measured? At the pins of the IC, or at the inverting and non-inverting input pins of the op-amp? If the former (which seems reasonable, since the specs are given for the IC), then I cannot understand why if I use it in a gain=1/2 configuration, with +/-15V supplies, the circuit would fail with two identical input voltages of 30V (the voltage at the input pins of the op-amp would be 10V, which is within the power supplies range). 
 Can you confirm? (or clarify if there is something I'm misunderstanding?) 
 Thanks, 
 Carlos 
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krisf · Accepted Answer

Hi Carlos, 
 Apologies for the delayed reply. 
 The internal resistor network changes depending on the gain configuration that is used. See below: 
 
 As you will see, the actual input voltage at the amplifier will be a result of the input voltage divided down by the resistor network. Since the resistor network will be different for the two gains, the diamond plot will also be different. 
 For example, for the gain of 1/2, the actual input at the amplifier will be +IN*(6k/18k). The actual input must always be kept within 1.5V of the supply. For a 13.5V input at the amplifier (given a 15V supply), you would be able to apply 40.5V at either IN+ or IN-. However, you have the absolute maximum rating to consider, and in this case, it will be the limiting factor to the diamond plot. 
 For a gain of 2, on the other hand, the actual input will be +IN*(12k/18k). For a 13.5V input of the amplifier, the input can only get to 20.25V at IN+ or IN-. In this case, this voltage is what limits the diamond plot. (By the way, the diamond plot says 20.85V, but as this test checks for the output swing, it will not always be so precise so this difference is acceptable.) 
 You should be able to use it with a supply of 30V since you have a small differential voltage, and you plan to use a gain of 1/2 (is that correct?). May I inquire what input you will be using for the AD8274? What is your application? 
 Thanks, 
 Kris)

krisf · Answer

Hi Carlos, 
 You are correct that the 25V in the datasheet will be taken at the input of the pins of the device. The reason it is 25V is because that the absolute maximum ratings at each pin need to be observed. See table below: 
 
 If you have your supplies at +/-15V, then the maximum and minimum voltage at any input pin would be 25V (-15+40), and -25V (15-40), respectively. This is the reason why the input range is limited to +/-25V for a +/-15V supply. You can check page 12 of the datasheet (Input Voltage Range section) for more information. 
 I hope this helps! 
 Kris

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AD8274 -- how is common-mode voltage defined?