Electronic – Effect of capacitor type in an active filter circuit

active-filtercapacitor

Below is a unity-gain low-pass filter implemented with a Sallen–Key topology:

enter image description here

If C1 and C2 is large, are electrolytic capacitors recommended?
Similarly, if C1 and C2 is small, are any type of ceramic capacitors recommended?

What types of capacitors can be used if C1 and C2 are in uF range and nF range for a good filter application?

I cannot find information on this.

EDIT:

Lets say the desired LP active filter cut-off is 50Hz.

I go to this amazing Sallen-Key Low-pass Filter tool and set R1=R2=3.3k and C1=C2=1uf.

And I obtain 48Hz cut-off.

These tools do not show the types of the caps.

And depending on the opamp I get TOTALLY different results in SPICE simulations.

So basically some tools do not suggest an opamp and some tools do not suggest capacitor types. And some tools offer weird resistor values like 3.1235869804k

My specs are as follows

  • Bandwidth -3dB = ___Hz
  • Frequency tolerance = ___%
  • Gain at DC = ____ with tolerance ____%
  • Bandstop rejection >=____ dB at frequency ____ Hz
  • Band pass ripple <1dB Butterworth?
  • Maximum R value should be___ ? due to Iio error=___uA

I have tried http://www.ti.com/lsds/ti/analog/webench/webench-filters.page

and my result is ……

Best Answer

Last Edition

When considering large C , dont' consider more than ~0.33uF until you consider larger R's 1st . If OA has high input bias current that may cause Vio offset voltage, then Change 0 ohm feedback to Vin(-) to matched R load on Vin(+) then the offset is reduced to the spec for Iio input offset current.(Iio x Req + Vio= Vio_total) x gain > = Voutput offset error


Any GOOD designer knows how to define specs before attempting to choose parts. LEARN THIS. by defining as much as possible the following;

  • Bandpass f
  • Bandpass gain
  • Bandpass ripple ( possible with higher than 2nd order)
  • Stopband f (important breakpoint)
  • Stopband min attenuation.
  • rise time , latency or group delay
  • step response % overshoot and/or resonant frequency & Qu
  • single or split supply
  • input Z (optional)
  • max DC offset error and gain error,
  • Vin CMRR requirements ( and differential impedance balance)
  • supply voltage , supply noise and power consumption.
  • group delay distortion and Q are other ways to specify some of the above characteristics.

http://www.ti.com/lsds/ti/analog/webench/webench-filters.page

e.g. Here a gain of 10 for a single supply with Vcc/2 = Vref

enter image description here R2C2 product of the filter for example cannot approach the leakage Rp*C of the capacitor, otherwise unity gain no longer exists. Rp is equiv parallel leakage.

What rise time? t= 0.35/ f-3dB YOu largest R possible such that 2% * Iin*R is acceptable offset with matched R's on each OA input with 1% tolerance.

  • Tantalum Polymer = 0.1CV is the DC leakage current thus 10uF @10V = 10uA =V/R (ref AVX)
    • so 0.1CV= V/R thus RC = 10 seconds for all values and voltages for Tant. polymer
  • Plastic , metal film PET Polyester film (polyethylene terephthalate)
    T >= 1000 seconds for C> 1.0uFand > 5000 seconds (0.33~1uF) and 15000 seconds (<0.33uF)

So PET caps are your best bet. with values < 0.33uF and large R values. up to 15ks/0.1uF *1%error= 1.5GOhm for 150 seconds with 0.1uF as minimum but much higher is possible with DC gain error managed.

REF KEmet: R82 Series, Radial, 5 mm Lead Spacing,

XR7 are poor C values with sensitivity to large -dC/dV unless highV rated relative to operating range. In which case NPO/COG are much more stable and accurate and low leakage but lower k so smaller C range in same size.

enter image description here