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Common Mode Input Range on LMP8640

Donald50451
Prodigy 40 points

Other Parts Discussed in Thread: LMP8640, LMP8480

I am a little unsure about the usable input common mode range on LMP8640.

The data sheet repeatedly suggests that it operates with a common mode range down to -2V.  It even has a CMRR ratio specified from -2V to +2V or 60dB.

But if I look at the block diagram on the front page the circuit shows that all current that goes into Rg has to come from +In.  If +in is negative then the voltage on Rg (and hence output) can not be positive - so it would not be operational.

Looking at the detailed data operation (other than CMRR) is only specified at Vcm=2.1V.

Is this part suitable for low side sensing (ie at 0V) or not?

  • 0
    TI__Guru 68686 points

    Hello Donald,

    Like I mentioned previously  - there is internal circuitry that allows operation below +2V (including under ground). The circuitry injects current into the nodes to offset the currents - and is why the bias currents change under +2V. That is part of the "IP" of the device...

    Remember that the device internally is measuring CURRENT DIFFERENCE - not absolute voltage. The resistors are converting the input voltages into currents. The internal circuitry offsets the currents so that the differences can still be measured. Common mode rejection will be slightly worse -2 to +2V due to the extra circuitry kicking in - but it is still fully operational. 2.1V is just what we use for production testing.

    Yes. It is suitable for low-side sensing - down to -2V.

    Regards,

  • 0 in reply to Paul Grohe
    Prodigy 40 points

    Paul,

    Thanks for the reply.  Actually I have not heard form you before.  The previous TI contact allocated to this case by the TI email support system told me that the part would NOT work with negative common mode voltages. That is when I posted this question on e2e.  Since then, the TI email support contact has changed their view and said that the part WILL work with negative common mode voltages and you have confirmed this also.

    Maybe I will come straight to e2e next time :-)

    Thanks.

  • 0 in reply to Donald50451
    TI__Guru 68686 points

    Hi Donald,

    Welp...I am in the group that designed it and released it under National...so I am familiar with it and the whole LMP8xxx current sense family.

    They may have confused it with the LMP8480/81, which cannot go negative.

    The forum is probably your safest/fastest bet for answers...many eyes looking over shoulders so there is continuous error checking and redundancy...

    Regards,