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# Comparator LM2901AV: Common-mode input-voltage range: How to calculate / define

Other Parts Discussed in Thread: LM2901AV, LM2901, TLV3011

We are using the LM2901AV and the datasheets requires that the Common-mode input-voltage range stays below Vcc-2V.

My questions is: How do I calculate the Common-mode voltage?:

1. Ucm = (Uplus + Uminus) / 2; E.g. Ucm=(2V+5V)/2=3.5V

2. Or is it the lower voltage of the maximal voltages on one of the two inputs. E.g. Uplus.max=2V; Uminus.max=5V; Ucm=2V

If the device is suppiled with 5V, case 1 would violate the VICR stated in the datasheet, where case 2 would be fine.

Regards Lars Hummel

• Lars,

Each input is considered separately. To be truly in the common mode range, both inputs must be less than VCC-2V.
However, most comparators (including LM2901) only require that one input remains in common mode range for proper output.
The LM2901 also allows for input voltage greater than VCC, if the other input is in the common mode range.

Therefore only one input needs to be in the common mode range at a time. Effectively case 2.

Regards,
Ron Michallick

• Ron,

Does the "most comparators (including LM2901) only require that one input remains in common mode range for proper output" comment apply to the TLV3011?

Gary Whelan

• Gary,

The TLV3011 is a rail to rail input comparator.
The data sheet allows the inputs to be up to 200mV beyond the supply rails.
There are diodes from the inputs to both supply rails so the inputs should not exceed the rails by 200mV at any time.

• Ron,

I guess I did not ask the right question.  If one of the TLV3011 inputs (the inverting for example) is driven to a voltage high or low enough such that the diode to the associated supply rail conducts, will the output still be as expected?  Or can the output phase reverse?  Let's say there is a 10kOhm resistor to limit the current.

Gary

• Gary,

Start a new thread with TLV3011 in the subject.
I do not have this information, but some else will.