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INA138: input filter of INA138

Part Number: INA138

Hi team,

is the input filter necessary?

or should I remove it?

  • Hey Fred,

    It depends. 

    In regards to the differential input filter (C3116 and R2311 R2312):

    • How noisey is the current load?
    • Does noise from the load create significant noise on the output?
    • Can this output noise not be averaged out in post-processing or filtered with output filter?

    In regards to the common-mode input filter (C2303 and C2304):

    • How noisey is the VBUS?
    • Are there large VBUS transients that cause Vout disturbances due to AC CMRR?
    • Is there broadband noise in Vbus that is causing broadband noise at Vout?
    • Can Vout be filtered on the output?
    • Is VBUS switching periodically?
      • If so, what is the settling time needed at Vout after each time VBUS changes?
    • How much load capacitance can VBUS tolerate?

    These are the questions to pose when considering needing an input filter. The common-mode capacitors (C2203 and C2304) are usually not required, unless VBUS is especially noisey. The best thing to do it to leave pads for these passive here on the board and measure the noise of your system in a realistic condition to determine if filtering could be necessary. If both differential and common-mode is needed, keep the differential capacitor (C3116 > 10*C2203). This is to help keep sense voltage stable in the event of a large VBUS change in which common-mode capacitors will have to rebalance their charges which can cause a dynamic offset that will need to settle, thus increasing overall signal settling times.



  • Hi Peter,

    let me conclude your comment,

    so I'll tell customer to DY C2303 and C2304, keep the rest but increase the  C3116 to 1uF, right?

    how about  PR2311 PR2312, should I DY them also?

  • Hi Peter,

    I think the issue here is that the datasheet doesn't tell anything about an input filter.


  • Hey Fred,

    It all depends on how much customer want to filter the signal before it amplified and whether it is too cumbersome to filter signal on the output. Making the input differential capacitor 1uF may not be the optimal because capacitances these large are not great for signal chains. Usually NP0/COG capacitors are better. Nevertheless, the best way to answer this question is to first ask what is the minimum signal BW required and/or what is the fastest settling time required to a current step response. For example, if the system need accurate current data 30us after a large current change, then the input RC time constant needs to be 30us/(5*2) = 3us. The 5 is for 5 time constants it take for capacitor to reach full charge, the 2 is for the fact that this is a R-C-R filter and thus C takes longer to charge compared to just single-ended RC. Keep in mind that the cutoff frequency for this filter is 1/(2*2*PI*R*C).

    Hope this helps.