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AMC1300: Floating voltage

ACGUY
Guru 11170 points

Part Number: AMC1300

Hello E2E,

When INN-pin connect to GND and INP-pin to open, INP-pin has about 4-mV floating voltage. Is it normally spec?

Regards,
ACGUY

  • 0
    TI__Guru 57662 points

    Hi ACGUY,

    Max spec for input offset is 2mV which may explain some of the 4mV.

    The rest depends on your schematic. There is a bias current that exits both INN and INP of approximately 30uA. This bias current will flow across filter resistors and the shunt resistor, potentially creating an offset depending on the connections. Can you share your schematic so I can help confirm? 

  • 0 in reply to Alexander Smith
    Guru 11170 points

    Hello Alex,

    Thank you for your response.
    We are evaluating the AMC1300 on our engineering board. VDD1 and VDD2 are 5V, insulated from each.
    INN-pin is connected to GND1, INP-pin is floated.
    OUTP-pin and OUTN-pin are tied to GND2 by 10-kΩ resisters.

    >> There is a bias current that exits both INN and INP of approximately 30uA.

    If is there not source for input bias current for INx, is the input offset voltage of the AMC1300 larger than the spec value stated in the data sheet?

    For your information, even if the Toshiba device had the same circuit, the offset voltage was on-spec.

    Regards,
    ACGUY

  • 0 in reply to ACGUY
    TI__Guru 57662 points

    Hi ACGUY,

    If INP is floating, then I do not understand how you are taking a measurement?

    Can you please share a schematic?

    The source of the input bias current for INx is the AMC1300. The AMC1300 offset spec in the datasheet refers only to the AMC1300 offset performance, not he performance of the passive components interacting with the bias current. Depending on how the device is connected schematically, additional offset error can be seen. However, this can easily be accounted for in post processing.

    I cannot comment on the Toshiba devices performance since I do not know what the devices input bias current is.

  • 0 in reply to Alexander Smith
    Guru 11170 points

    Hello,

    What is this 4mV on INP?

    Regards,

    ACGUY

  • 0 in reply to ACGUY
    TI__Guru 57662 points

    Hi ACGUY,

    I see. Thank you for providing a sample schematic. 

    More than likely it is an accumulation of charge on the positive input pin due to the bias current. More than likely, the bias current of the Toshiba device is lower, hence them not having an issue. 

    Try lifting VINN as well, then measuring the difference between VINP and VINN, this would be more representative of an offset measurement.  

    However, this schematic has no real functionality and we do not specify our devices functionality in a configuration like this. What is the purpose of this test?