Common Mode Input Range of Opamp ignored?

[Flyback]

Why are we all so blessedly complacent about the Common Mode Input Range of an Opamp?

We all are cognisant of the evil that is the common mode input
range of an opamp.
Its so easy to accidentally make a circuit on the bench that works even though it violates the common mode input range of the opamp....(ie, it has input signals on either or both
inputs which are outside of the common mode input range of the opamp).....and so it then goes into production,
and then some unknown time later, a batch of opamps is used which goes into phase reversal
due to the common mode input range violation.....and the company goes bust!

 

[Tony Stewart]

Who do you know that went bust?

 

[ZipZapOuch]

Why are we all so blessedly complacent about the Common Mode Input Range of an Opamp?

We all are cognisant of the evil that is the common mode input
range of an opamp.
Its so easy to accidentally make a circuit on the bench that works even though it violates the common mode input range of the opamp....(ie, it has input signals on either or both
inputs which are outside of the common mode input range of the opamp).....and so it then goes into production,
and then some unknown time later, a batch of opamps is used which goes into phase reversal
due to the common mode input range violation.....and the company goes bust!

Lots of good companies go bust because of incompetent engineering or scientific inexpertise. Good companies have standardized processes, best practices, preferred parts (and suppliers), engage with suppliers to understand raw materials, maintain training records and engage in extensive reviews of designs and manufacturing processes. Those who don't have good management practices end up designing products that fail, cause liability and they go bust - eventually.

Another sign of a bad company: their "engineers" don't know the basics of their responsibilities or how to engage with vendors and, instead, they post questions on Internet forums to ask how to do their jobs.

 

[Nigel Goodwin]

Who do you know that went bust?

Most of the companies he's worked for?

 

[Diver300]

Unfortunately there are a lot of electronic designs from people who don't understand the components.

It seems that common-mode violation is something that a simulator could catch even if a physical test doesn't, but that relies on simulation model being quite sophisticated.

Common mode range might also be something that is different for different manufacturers of a part that is considered generic.

I did some repairs on some battery tools that used a 556 (dual 555). The design would take one of the input voltages higher than the supply voltage. That worked with one brand of 556, but not with another.

 

[Tony Stewart]

Using inverting gain limits the Vcm swing as the virtual null tracks the fixed Vin+ reference, so Vcm is always fixed = Vref.

Using the non-inverting input forces Vin- to track Vin+ which stresses the limits of Vcm for large input swings.

The Vcm of differential INstrument Amplifiers (INA) is worse.
It uses 3 amp's with the dual Vin+ used for the high Z inputs.

This reduces the Vcm and Vdm range even more such that you may recall INA's have a diamond shaped valid-bias input range. The addition of mismatched cables and high E-fields often stresses the accuracy of peak signals when the signals exceed the linear valid input range.

 

[danadak]

Unfortunately there are a lot of electronic designs from people who don't understand the components.

It seems that common-mode violation is something that a simulator could catch even if a physical test doesn't, but that relies on simulation model being quite sophisticated.

Common mode range might also be something that is different for different manufacturers of a part that is considered generic.

I did some repairs on some battery tools that used a 556 (dual 555). The design would take one of the input voltages higher than the supply voltage. That worked with one brand of 556, but not with another.

Totally agree on model completeness, Just look at the model when you place it
on sim workup, they generally are so simple one might as well be in Ohm and Kelvin
age, using Galvanometers, the ancient age of primal discovery.

Regards, Dana.