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