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AMC1305M25: the diffrence between FSR and Vcm

Hirotaka Matsumoto
Guru 27585 points
Part Number: AMC1305M25

Hi all

Would you mind if we ask AMC1305M25?

The datasheet shows follows;
FSR : Specified linear full-scale range (AINP-AINN), -250mV(Min) 250mV(Max)  
Vcm : Operating common-mode input range, -160mV(Min),  VDD-2V(Max) 

<Question1>
What is tthe diffrence between FSR and Vcm?

<Question2>
Vcm Min value is the small value(-0.16V) compared with Max value(VDD-2V).
So, is it possible to detetct with full scale(±250mV) at shunt resister?

We need your help.(Our customer needs this rely as soon as possible,,,,)

Kind regards,

Hirotaka Matsumoto

  • 0
    TI__Guru**** 180931 points
    Hi Matsumoto-san,

    For the AMC1305M25, the common mode voltage is a voltage offset that is "common" to both the inverting and noninverting (i.e. "+" and "-") inputs of the amplifier. The FSR, or full scale range, is the +/-250mV you noted above. So, with Vcm min of -0.16V, the inputs can swing from -410mV to +9mV (+/-250mV around -0.16V). At the maximum Vcm and assuming AVdd is 5V, the inputs could swing +/- 250mV around 3V or from +2.75 to +3.25.
  • 0 in reply to Tom Hendrick
    Guru 27585 points

    Tom san

    Thank you so much for your prompt reply!

    Just in case, it is possible to input  less than -0.16V(AINP-AINN -250mV upto +250mV), right??

    Vcm means VCM=2.0V which describes on the datasheet p28 Figure56. And then, VCM depends on IIB(bias current).

    Is our recognition correct?

    Kind regards,

    Hirotaka Matsumoto

  • 0 in reply to Hirotaka Matsumoto
    TI__Guru**** 180931 points

    Hi Matsumoto-san,

    Take a look at the ABS MAX values in section 7.1 of the AMC1305M25 datasheet, the input can go as low as -6V.  There is an internal 2V Vcm generator on the output stage of the AMC1305M25 front end (Figure 56, page 28) which impacts the IIB depending on the input common mode voltage (see Figure 6, page 14).   The X scale of Figure 6 is the input common mode voltage.  When the input common mode is 2V, it balances out the 2V internal Vcm and IIB goes to 0A.

  • 0 in reply to Tom Hendrick
    Guru 27585 points

    Tom san

    Thank you very much for your reply!

    Just in case, we would like to confirm one point.

    -VCM means offset voltage of AINP and AINN
    -In generally, "input commmon voltage range" means VCM=((IN+) + (IN-))/2.
    ->Otherwise, does "AMC1305's VCM" mean offset voltage or bias voltage, right?

    Kind regards,

    Hirotaka Matsumoto

  • 0 in reply to Hirotaka Matsumoto
    TI__Guru**** 180931 points
    Hi Matsumoto-san,

    The Vcm on the AMC1305 can be looked at like an offset or bias signal that is applied to both inputs.