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LMP2011: Adjusting the offset

hiroshi.yamada
Intellectual 535 points

Part Number: LMP2011
Other Parts Discussed in Thread: MSPM0G1105, LMP2021, TINA-TI

Tool/software:

I'm using an LMP2011.

Even when I connect the input to ground, it still outputs about 10mV.

Please tell me how to correct the offset.

Thank you.

Hiroshi Yamada

  • 0
    Guru 320610 points

    The load might be too high. Please show the schematic.

  • 0 in reply to Clemens Ladisch
    Intellectual 535 points

    Hi Clemens,

    Thank you for your quick reply.

    I forgot to attach the circuit diagram.

    The input OUT- is connected to an external load, but the problem this time is when no load is connected.

    Pull-down is provided by the 100Ω shunt resistor R42.

    The output(AN1) is connected to the ADC MSPM0G1105.

    The gain is set to 15 by adjusting the variable resistor and combining it with R44 to give a combined resistance of 140KΩ.

    In the simulation the output is just over 10mV, but even when TP26 and AG (analog ground) are actually shorted with this circuit,

    Note that AN0, the output of U8, is not affected by this problem this time.

    it is just over 20mV. Thank you in advance.

    Best regards,

    Hiroshi Yamada

  • 0 in reply to hiroshi.yamada
    TI__Genius 12885 points

    Hi Yamada-san,

    Could you please clarify where you are observing this offset? Is it only in simulation, or is this circuit physically built out on a PCB and you're measuring this offset? If you are measuring it, which nodes of the circuit are you probing to measure this offset?

    The increased offset you are observing could be due to the increase in input current when the LMP2011 operates at the low VCM range which by flowing through the input imbalanced impedances at the IN+ and IN- cause an increase in observed offset:

  • 0 in reply to HO SIU
    Intellectual 535 points

    Hi HO-san,

    Thank you for your comment.

    I checked the circuit diagram attached in my previous post, which I made on a prototype PCB,

    and even though the OUT- input of the LMP2011 in U10 was not connected, the output was about 20mV,

    so when I simulated it with Tina9-TI, the result was about 10mV.

    Thank you in advance.

    Best regards,

    Hiroshi Yamada

  • +1 in reply to hiroshi.yamada
    TI__Guru 70391 points

    Yamada-san,

    First, I assume you use a single 3.3V supply and if so what would you expect ideally to see at the output with LMP2021 max/min offset of +/-25uV?

    As you may calculate, the output would be between +/-375uV (+/-25uV*15) - see below.

    However, since the LMP2021 minimum output swing is limited to around 30mV or more above negative rail, this means that with negative rail at the ground the output could NOT get below 30mV and thus would always show ~30mV irrespective of the actual offset of the LMP2021 (see below)

    This means that the measurement will be valid ONLY for R42 shunt current of 20uA or more when output is driven to 30mV or higher  - see below.

    This would also mean that the LMP2021 circuit would be only able to measure shunt current between 20uA and 2mA - see below.

    For your convenience I have attached Tina-TI circuit schematic - see below.

    Hiroshi LMP2011.TSC

  • 0 in reply to Marek Lis
    Intellectual 535 points

    Hi Marek

    Thank you for the specific details.

    I understand it now.

    Perhaps I didn't understand or look at the LMP2011 standard well.

    I will reconsider this, including the circuit and the method of sensing the MSPM0G1105 ADC.

    Thank you very much.

    Hiroshi Yamada