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Original question:

OPT3101: Finding a suitable laser diode

OPT3101: Signal to Noise Ratio Issue

Andrew Marques
Prodigy 40 points
Part Number: OPT3101

Calculating the signal to noise ratio of this AFE is problematic. 

ISSUE #1: According to equation 4 on page 18:

It is unclear where the value of 94.8 pA comes from.  After some digging I was able to verify the origin of this value based on the example provided on page 105:

Based on this, it seems that the value calculated above of 12.6 pA was found using 2.25*sqrt(31.25), meaning that the value is found by multiplying the pA/sqrt(Hz) value (found on Figure 2 of page 8) by the square root of the 'samples per second' (SPS) calculated based on Equation 1 of page 14.  Using this same logic on the explanation offered on page 18 yields an SPS of 4000 given 1.5 pA/sqrt(Hz).  This does not make sense given that the register NUM_AVG_SUB_FRAMES = 31 meaning that NUM_SUB_FRAMES must be 31 or greater according to page 14 which is impossible for an SPS of 4000 according to Equation 1 also offered on page 14.  Something isnt quite right here, I would appreciate if someone could clarify how to correctly determine the SNR.

  • 0
    TI__Mastermind 25815 points

    Hi Andrew,

    I don't see the images, can you please re attach? You can use the insert/edit media button to do this.

    Best,

    Alex

  • 0 in reply to Alex Bhandari-Young
    Prodigy 40 points

    Thanks for the response Alex, I Have amended the original post to include the images:

    Calculating the signal to noise ratio of this AFE is problematic. 

    ISSUE #1: According to equation 4 on page 18:

    It is unclear where the value of 94.8 pA comes from.  After some digging I was able to verify the origin of this value based on the example provided on page 105:

    Based on this, it seems that the value calculated above of 12.6 pA was found using 2.25*sqrt(31.25), meaning that the value is found by multiplying the pA/sqrt(Hz) value (found on Figure 2 of page 8) by the square root of the 'samples per second' (SPS) calculated based on Equation 1 of page 14.  Using this same logic on the explanation offered on page 18 yields an SPS of 4000 given 1.5 pA/sqrt(Hz).  This does not make sense given that the register NUM_AVG_SUB_FRAMES = 31 meaning that NUM_SUB_FRAMES must be 31 or greater according to page 14 which is impossible for an SPS of 4000 according to Equation 1 also offered on page 14.  Something isnt quite right here, I would appreciate if someone could clarify how to correctly determine the SNR.