Noise Calculation in MS-2022


how to get the below value from AD8641? from datasheet, In should be 0.5fA, right?

another question is about 

in calculation, BW is equal to 10Kz. but in application MT-049, BW = 1.57*FCL,  FCL = GBW / NoiseGain, this is my understanding,

so please help to clarify, thanks.

wang jian

  • 0
    •  Analog Employees 
    on Oct 19, 2020 5:25 AM 1 month ago

    Hi Wang Jian,

    Could you elaborate more about the figures in table 1? I would like to know more about the context and where you got it from.

    For your next question, noise calculation typically involves getting the noise bandwidth. The noise bandwidth is calculated by dividing the gain-bandwidth product with the noise gain. Ideally, this would result in a low pass filter resembling a rectangular shape, wherein the roll-off rate is a straight line.

    The image above represents the different responses of a low pass filter. The ideal brick wall filter is just a rectangular shape. Now for the calculation in MT-049, the computation assumes a single pole filter response (corresponds to the 1st order graph on the image). And to properly approximate the noise bandwidth, a constant of 1.57 is multiplied to the noise bandwidth to account for the 20dB/decade roll-off rate of the single pole response. 

    I hope this clears up the computation in MT-049.