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How to compare PET and PP film capacitors for active low pass filter stages? If I implement an active filter which film capacitor is more temperature stable? I guess temperature might vary the capacitor capacitances hence the filter characteristics. So Im wondering which type is safer to use in this respect. Is there a convention to use a specific type?

More specifically here is a KEMET 47n PP type cap: https://eu.mouser.com/datasheet/2/212/F3301_R79-1103144.pdf

And here a KEMET 47n PET type: https://content.kemet.com/datasheets/KEM_F3101_R82.pdf

Im stuck which type would be suitable for such application. From the data-sheets where should we look at? Does anybody have experience?

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  • \$\begingroup\$ what frequency range? how deep must any nulls be? can you compute how DF dissipation-factor-versus-frequency will affect the poles and zeros and the nulls and the passband-ripple? \$\endgroup\$ – analogsystemsrf May 30 '19 at 2:25
  • \$\begingroup\$ Which capacitor more stable with temperature? Theres no filter yet. Its a general question. \$\endgroup\$ – pnatk May 30 '19 at 13:07
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They are both temperature stable dielectrics but PP (0.5∆C/°C%) is more stable than PET (1.5∆C/°C%). The data sheets you are referencing do not show this curve.

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  • \$\begingroup\$ Are those values 1.5 and 0.5 for the parts in question? Or in general? \$\endgroup\$ – pnatk May 31 '19 at 16:37
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from specs:
PP Temperature coefficient -200 (ppm/°C)
PET (±200)ppm/°C at 1kHz

Tolerances are cost sensitive 5,10,20%

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  • \$\begingroup\$ Can we than say they are almost same? Have you ever designed a LP active filter? What cap types do you use in general for that? Thanks \$\endgroup\$ – pnatk May 31 '19 at 16:38
  • \$\begingroup\$ It depends on specs. Maybe I would use NPO ceramic and large R with low input bias current unless it was an integrator then longest time constant plastic. But yes almost same. a fixed -200ppm is better when using other parts with +200ppm for balance. but absolute tolerance errors may be more significant. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 May 31 '19 at 17:32

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