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ACF2101: total output nose vs C1 and C2

Akihisa Tamazaki
Genius 11675 points
Part Number: ACF2101

Tool/software:

Hello team,

What do C1 and C2 mean in the chart "total output nose vs C1 and C2" at Page6 in datasheet?
If C1 is Sensor capacity Cin and C2 is Externally installed Cintegration,
Total Noise value in "total output noise vs C1 and C2" does not match the value calculated in the calculation formula described in “noise” in Page10?
How should we consider C1 and C2?

Your comment is appreciated.

Best Regards,
Akihisa Tamazaki

  • 0
    TI__Intellectual 1520 points

    Hi Akihisa,

    Firstly, thank you for your patience. 

    Based on the figure referenced in the Noise section (highlighted below), it seems that the title of the figure should be "Total Output Noise vs Cin and Cintegration" rather than "Total Output Noise vs C1 and C2". So to answer your question, C1 and C2 refer to Cin and Cintegration, respectively, where Cintegration = Cinternal + Cexternal. 

    So in the figure, C1 is the sensor capacitance, and C2 is the combination of the internal 100pF integration capacitor and external integration capacitor.

    I calculated the total output noise based on the equation in the noise section for the same capacitor values shown in the figure, which I attached below. The plot in the datasheet is based on measured data, and should be a more accurate representation of the characteristics of the part compared to the approximation of the noise equation.

    Please let me know if you have any other questions.

    Kind Regards,

    Alex Curtis

  • 0 in reply to Alex Curtis
    TI__Genius 11675 points

    Alex san

    Thank you for your comment.

    There seems to be a gap between the graphs in the datasheet based on actual measurements and the formula for the noise calculation. Can you tell what is causing this gap?
    The measured graph shows a smaller increase in noise with respect to Cin than the formula.
    I would like to know what factors are suppressing the increase in noise.
    If you can answer this question, please let me know.

    Best regards

    Akihisa Tamazaki

  • 0 in reply to Akihisa Tamazaki
    TI__Intellectual 1520 points

    Hi Akihisa-san,

    I'll need some additional time to work on your question. You can expect a response within 24 hours.

    Best Regards,

    Alex Curtis

  • +1 in reply to Alex Curtis
    TI__Intellectual 1520 points

    Hello Akihisa-san,

    I appreciate your patience. 

    I consulted with another engineer about this, and we don't have a definitive explanation for the difference between the datasheet plot of the total noise and the calculated total noise. The relationship of total output noise vs Cin provided by the equation looks to be a linear approximation of what is shown in the datasheet, and although the plots don't line up exactly, using the equations will give a more conservative estimate of noise error.

    The most accurate way to determine noise for a given combination of Cin and Cintegration would be to use the datasheet graph.

    Best Regards,

    Alex Curtis

  • 0 in reply to Alex Curtis
    TI__Intellectual 1520 points

    Hi Akihisa-san,

    We are going to close this thread, since we have not heard from you. 

    If you have any other questions, please let us know Slight smile

    Best Regards,

    Alex Curtis

  • 0 in reply to Alex Curtis
    TI__Genius 11675 points

    Alex-san,

    Thank you for your support and sorry for my delay in responding.
    The customer can understand our answer.

    Best Regards,
    Akihisa Tamazaki