LM6172: Phase Reversal

Hi Sadanori,

if the datasheet doesn't explicitely say that no "phase reversal" or "inversion" can ever occur, then I would always consider it possible to happen, especially if the OPAmp is an older one and isn't specified for input rail-to-rail operation like the LM6172.

The datasheet need not to specify what happens, if you leave the recommended common mode input voltage range, as you should ensure by your circuitry that this will never happen. Having said this, it's rather unlikely that you damage the LM6172 when only exceeding the common mode input voltage range. But never allow the input signals to violate the "Absolute Maximum Ratings", as this will damage the chip!

Kai

Hi Kai-san,

Thank you for your prompt reply.

Have you ever seen a phenomenon where the output phase is reversed on the V+ side even if the common-mode voltage is within the recommended operating range ?
Is it supposed that the LM6172 is damaged, so what do you think is the possible cause ?

Best regards,
Kato

Hello Kato,

Can you clarify what common-voltages you inputting that you are seeing this phase reversal? Can you include some scope shots and a schematic?

Best,

Hasan Babiker

Hi Hasan-san,

I will continue to discuss via a private message if you  can accept my friendship request.

The output phase is reversed on the V+ side even if the common-mode voltage is within the recommended operating range. So, it is supposed that the LM6172 is damaged.

Best regards,
Kato

Hello Kato,

I've accepted your request. Feel free to message me the details.

Best,

Hasan Babiker

Hi Hasan-san,

Thank you for the approval.

I just sent the detailed information via a private message, so please check it.

Best regards,
Kato

Hi Hasan-san,

Thank you for your support.

I understand.

Best regards,
Kato

Hi Sadanori,

the occurence of phase reversal in the case when the common mode input voltage isn't exceeded is very unusual. I never have observed this with an OPAmp. Having said this, you should take into consideration that the common mode input voltage range depends on many factors like supply voltage, junction temperature, etc. And please note that the common mode input voltage range given in the datasheet of LM6172 is only a typical and is only valid for a junction temperature of 25°C!

If you want to know which is the guaranteed common mode input voltage range you should take a look at the specification of "common mode rejection ratio" in the datasheet.

Let's have an example: For the LM6172 the typical common mode input voltage range at 25°C junction temperature and +/-15V supply voltage is given in the datasheet as +/-13.5V. But the common mode rejection ratio, on the other hand, is only guaranteed to be better than 65dB for a common mode input voltage of Vcm=+/-10V. So, it's wise to keep the common mode input voltage always below +/-10V, even if the LM6172 might work with a common mode input voltage of up to +/-13.5V under certain circumstances.

These specifications make perfect sense when you consider that older OPAmps (like the LM6172) were designed to be able to handle +/-10V voltage signals (which is the typical voltage range of industrial signals) when running at a supply voltage of +/-15V. These older OPAmps were no input-rail-to-rail OPAmps.

Kai

Hi Kai-san,

Thank you for the information.

It would be very helpful for me.

Best regards,
Kato