How to read datasheets
The datasheet of an op-amp used on a single 5 V supply gives the common-mode input range as "0 V to (V+ − 1.5 V)." Which is correct?
- A0–5 V is within the absolute maximum ratings, so even with a 4 V input, a voltage follower outputs correctly
- BIt means the output voltage can swing only over 0–3.5 V; there is no limit on the input range
- CCorrect operation is not guaranteed for inputs above 3.5 V, so a voltage follower cannot handle a 4 V signal
Show answer and explanation
Answer: C. Correct operation is not guaranteed for inputs above 3.5 V, so a voltage follower cannot handle a 4 V signal
The common-mode input range is the range within which the device amplifies as specified, provided the voltages on both input pins are within it. Here the upper limit is 5 − 1.5 = 3.5 V, so applying 4 V to a voltage follower, where the input voltage is directly the common-mode voltage, causes the output to distort or stick. Outside the range but within the absolute maximum ratings, the device will not be damaged, but operation is not guaranteed. The range over which the output can swing is specified separately as "output voltage swing," and devices usable close to both supply rails are called rail-to-rail.