What happens if the common-mode voltage of the op amp exceeds the reference common-mode voltage specified in the specification?

乱世煮酒论天下

What happens if the common-mode voltage of the op amp exceeds the reference common-mode voltage specified in the specification? [Copy link]

Now it is an op amp with a common-mode voltage rating of 4V and a single power supply of 3.3V. It is used to detect the current collected by the low-value resistor of the inverter lower bridge. The problem is that it is very easy to trigger overcurrent when the inverter output changes suddenly.

Later, I found that the problem was most likely caused by the common-mode voltage exceeding the rated parameters of the op amp. When the external output suddenly changed, the ground potential floated. Measuring the voltage of the two input pins of the op amp found that the common-mode voltage exceeded 30V.

maychang

[What happens if the common-mode voltage of the op amp exceeds the reference common-mode voltage in the manual? ]

In the worst case, the op amp will burn out.

乱世煮酒论天下

maychang posted on 2024-8-9 10:11 [What will happen if the common-mode voltage of the op amp exceeds the reference common-mode voltage in the manual? ] At the least, the op amp output will be wrong, and at the worst, the op amp will burn out.

However, the op amp works normally during normal use. This large common-mode voltage will only appear when the output changes greatly. Moreover, the op amp did not burn out after many tests.

maychang

Luan Shi Zhu Jiu Lun Tian Xia published on 2024-8-9 11:47 However, the op amp works normally in normal use. This large common mode voltage will only appear when the output changes greatly, and the op amp will be used for multiple times...

[And the op amp did not burn out after multiple tests. ]

That is, the common-mode voltage is not high enough to burn out the op amp.

Gen_X

The reason why it can withstand a large common mode is because the input circuit of the op amp is a symmetrical high-impedance input circuit. Generally, the op amp can withstand a common mode of 10-35V or even higher. The measured 35v AC common mode has almost no effect on the calculation accuracy of UA741. Only when it is above 50vAC will it cause large errors or dysfunction of the op amp, and the accuracy will drop significantly or saturate on one side.

However, for the op amp input itself, the maximum common-mode resistance is only 1-2V. When the common mode is measured at 0.5V, the operation error reaches 50-100%. It can be seen that the common-mode resistance of the op amp is brought by the designed circuit, not entirely possessed by the op amp itself.

However, the op amp does have the ability to adapt to the power supply, that is, a certain power supply error or fluctuation will not affect the results or accuracy of the calculation.

乱世煮酒论天下

Gen_X posted on 2024-8-16 14:03 The reason why it can withstand a large common mode is because the input circuit of the op amp is a symmetrical high-impedance input circuit. Generally, the op amp can withstand 10-35V or even higher...

I think the most accurate op amp circuit I can come across is this differential probe. It has several common parameters, including common-mode rejection ratio, differential input voltage, common-mode input voltage, bandwidth, error, input impedance; and output-related parameters, including output voltage swing, output offset, noise, and impedance.

Gen_X