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WEBENCH® Tools/OPA1664: Low pass filter ; capacitor size too big generated from Webench

Part Number: OPA1664

Tool/software: WEBENCH® Design Tools

Hi,

I am using webench to create a low pass filter (image pasted, report attached). The BOM generated by the filter seems odd to me. The feedback capacitor size (C2) in third stage and fourth stage is 1812 (4532 Metric). The rated voltage level of the capacitor is 50V. Input voltage is only +/- 2.5V. Can we select capacitor of the size 0603 or 0805 separately. 1812 (4532 Metric) size is too big and will occupy lot of estate of the PCB.

Regards, 

400_webench_design.pdf

  • Hi Tapas Pandey39,

    It should be fine to replace those with a smaller package, so long as you use a C0G/NP0 type dielectric. High-K ceramic capacitors (X5R/X7R, etc) are not suitable for filter applications due to their voltage coefficient of capacitance. More information can be found here: www.edn.com/.../Signal-distortion-from-high-K-ceramic-capacitors.

    A quick search on Digikey turned up a 120nF C0G ceramic capacitor in a 1206 package, but 1206 appears to be the smallest available.
  • Hi Alex,

    Thanks for pointing out. The article mentioned in the above thread is very useful. I am hoping manufactures should start make C0G/NP0 type capacitor in 0805/0603 packages.

    Regards,

  • Hi Alex,

    Some of the capacitor generated by the webench tool are X7R type (webench file attached in the first thread), but the article mentioned to use C0G/NP0 type capacitor only. Do we have to manually choose C0G/NP0 type capacitor or some X7R/X5R selected by webench application, can be used without degradation in the audio quality.

    Regards,
  • Hi Tapas Pandey39,

    The issue with C0G/NP0 ceramics in small packages is that the C0G dielectric has a low-k, or relative permittivity. This unfortunately means that they can have less capacitance in a given volume than a Class-2 ceramic material (like an X7R). I believe the largest capacitance I've seen in a 1210 C0G was around 100nF, but higher values may be available.

    For ceramic capacitors, I'd recommend sticking with C0G/NP0. You might also consider a film capacitor instead. You will definitely see a measurable degradation in THD using a class-2 ceramic capacitor.