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LM2901-Q1: Common mode input_Simulation_Pspice

Sunney Kumar
Prodigy 190 points
Part Number: LM2901-Q1
Other Parts Discussed in Thread: LM2901, LM339, LM393B
Hi,
During simulation of LM2901QPWRQ1 using LM2901.5_1 Pspice model in OrCad, following was observed.

Even if both the inputs at inverting and non-inverting terminals are out of common mode range, LM2901QPWRQ1 comparator is
working fine in simulation.
Vcc = 3.3V, Vcm =   3.3-1.5 = 1.8V
VRef = 2.5 at non inverting terminal via voltage divider from 3.3V.
Vin at inverting terminal, triangular pulse, 0 to 5V.
 
At output, I am getting nice pulsed output as if common mode range violation  doesn't have any adverse effect on ouput.
From Datasheet:
"When IN– and IN+ are both higher than common-mode, then the output is low and the output transistor is
sinking current."
Is it expected behavior of LM2901 in simulation?
Thanks in advance,
Sunney
  • +1
    TI__Guru 51316 points

    Hello Sunney,

    The "5_1" models are very simple. They do not model exceeding the input range.

    You should really use Vcc-2V as the limit over temperature, which would be 1.3V max, or you may have issues at cold.

    5V at the input is violating the input range, though it will not damage it and should still "work".

    Please see the LM339 family application note, particularly sections 2.2 and 2.5:

     Application Design Guidelines for LM339, LM393, TL331 Family Comparators (Rev. A)

    You can try using the newer LM393B model (same family), which does model input range:

     https://www.ti.com/lit/mo/slcm004c/slcm004c.zip

    The model output will go to half-supply, which should not happen in real-life, to signify an error condition (exceeding input range or incorrect supply voltage). Keep in mind that these models only model typical specs at room temp and do not show temperature effects.

    Yes, the "real" device will do what the datasheet says...

  • 0 in reply to Paul Grohe
    Prodigy 190 points

    Hi Paul,

    Thank you for the information and sorry for delayed response.

    With this I can assume that in order to simulate the input offset voltage I need to add a voltage source in series at non inverting terminal right?

    Thanks and regards,

    Sunney

  • 0 in reply to Sunney Kumar
    TI__Guru 51316 points

    Hello Sunney,

    Yes. The offset is modeled as a voltage source in series with the non-inverting input - the same as an op-amp. A positive offset voltage would add to the input voltage, causing the threshold to shift negative (since the offset is "boosting" the actual input voltage, causing it to reach the threshold sooner).

  • +1 in reply to Paul Grohe
    Prodigy 190 points

    Hi Paul,

    One last question for this thread, 

    In LM2901 datasheet, the input offset voltage is mentioned as 15mV (over temp range).

    Does it mean +/-15mV or just 15mV?

    It will help me to understand more about my threshold settings

    Thanks in advance!

    Sunney

  • +1 in reply to Sunney Kumar
    TI__Guru 51316 points

    Hello Sunney,

    +15mV to -15mV.

    The offset voltage, unless there is a specific sign (such as +8mV or -7mV), then the offset should be interpreted as plus & minus that value.

    So a "7mV" offset spec will range from -7mV to +7mV. This also applies to Op Amp offsets, too.