What is behaviour of the instrumentation amplifier if input common mode voltage (Vcm) exceeds specified range and what phase reversal concept? Does it happen every time in instrumentation amplifiers when ever Vcm exceeds range.
Instrumentation amplifiers reach the maximum output voltage in the correct polarity when they exceed the common mode voltage. The limit can be found by looking at the Input Common-Mode Range vs. Output Voltage plots provided in the amplifier's data sheet. You can see an example on page 10 on AD8221's data sheet:
Phase reversal refers to the inversion of the output voltage polarity when the common-mode voltage range is exceeded. This is sometimes a problem for op amps when they are configured as voltage followers (unity gain). As an example, if you decrease the voltage on the non inverting terminal, the output should decrease its voltage until the negative limit is reached and it saturates. From that point on, if you keep decreasing the input voltage, the output remains pegged to the maximum negative output voltage that is capable of generating. When phase reversal occurs, the output "jumps" to the maximum positive voltage (or viceversa, however it is more commonly observed on the negative polarity). Some amplifiers guarantee "no phase reversal" by implementing specific circuit design techniques. Please refer to the following document for a more detailed explanation:
Care has been taken to prevent phase reversal in instrumentation amplifiers when the common-mode range is exceeded.
To add to Gustavo's reply:
There are three cases we can consider for an instrumentation amplifier.
1) The voltage on both inputs are within the in amp's input operating range (the one specified in the table), but the common mode voltage is outside the plot shown in the "Input Common Mode Range vs. Output Voltage" plot.
In this case, there will be no phase reversal. It will appear as if the instrumentation amplifier has less gain than it should. Bandwidth may also decrease, and there will be large amounts of distortion.
2) The voltage on one of the inputs is within the in amp's input operating range and one of the inputs is not.
For the majority of newer instrumentation amplifiers, there will be no phase reversal, since our designers typically add circuitry to prevent this from happening. In some cases, adding this extra circuitry may not be possible (for example it may impact the noise too much.) To be safe, if this criteria is important, it's best to double check it yourself, especially if you need this behaviour when the input voltage goes beyond the supply rail (for example when using input protection resistors.)
3) The voltage of both of the inputs is outside the in amp's input operating range.
In this case the output voltage is undefined. Often this will result in an output voltage near the reference voltage - since both input amplifiers are railed, the difference will be near zero, so the difference amplifier will likely output something near the reference voltage.
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