30 votes
Accepted

How to identify filter behaviour without memorizing different topologies

Look at the extremes. DC and very high frequencies. For DC you can remove the capacitors and short the inductors. For high frequencies you can short the capacitors and remove the inductors. By looking ...
  • 8,045
27 votes
Accepted

What is the advantage of a Sallen-Key filter over a normal second order filter?

What you call "normal" is a simple two-stage RC filter with very bad selectivity (two real poles only). In contrast. the Sallen-Key topology is capable of producing a second-order lowpass response ...
  • 22.9k
23 votes
Accepted

Why don't decoupling/bypass capacitors need resistors to perform their function, like regular filters?

I wouldn't think of a decoupling capacitor as a filter in the way you describe. Like an RC filter like this, where the source of the noise is the power supply and your "decoupling" capacitors are ...
22 votes
Accepted

Decoupling Capacitors: What happens to the DC on the cap once noise is introduced and it shorts?

The capacitor does not "short out", it has charged up to a constant voltage by storing energy as electrical charge, and if something external tries to change the voltage over the capacitor, it means ...
  • 96.3k
15 votes
Accepted

How to do low pass filtering on a differential signal

how would I do a simple RC low pass filter on this differential signal? The simplest method: - Source. This circuit only filters differential signals; if there is an in-phase signal (or interference)...
  • 395k
13 votes

Significance of cut-off frequency in a low-pass filter

H(w) =1/ (square root of 2) That sounds a bit confusing, we usually refer to the cutoff point as the -3 dB point. That is the same though, -3 dB is half the power. Let me explain: take your \$H(\...
  • 79.1k
12 votes

Why is inverting op-amp behaving like a low pass filter?

can anyone explain why this circuit behaves like a low pass filter despite having no capacitor in the feedback loop? All op-amp amplifiers will behave like low pass filters due to parasitic ...
  • 395k
11 votes

Low pass filter with two capacitors?

It's simply an AF (audio frequency) coupling capacitor, removing any DC that is on the signal.
  • 38.5k
11 votes

Cut-off frequency, gain and quality factor calculation of complex 2nd order low pass filter

1st stage If you set \$\gamma=\frac{C_1}{C_2}\$ and \$\rho=\frac{R_3}{R_4}\$, then the left-side 2nd order filter (1st stage of two) has \$\omega_{_0}=\frac1{R_4\, C_2 \,\sqrt{{\vphantom{M}}\gamma\,\...
  • 72.5k
10 votes
Accepted

Low pass filter with two capacitors?

The first RC does indeed act as a low pass filter. The second capacitor (the one on the right), combined with the load impedance presented by whatever is connected to the audio out, acts as a high ...
  • 4,908
10 votes

Cut-off frequency, gain and quality factor calculation of complex 2nd order low pass filter

These are very good answers and let me add mine considering the rainy day in Occitanie : ) I am using the fast analytical circuits techniques or FACTs as described in my book. The principle is quite ...
  • 15.5k
10 votes
Accepted

Problem with the simulation of two different low-pass filters

I think that you'll find that this is "a" definite problem: -
  • 395k
9 votes
Accepted

Low pass filter issue - is the 10k resistor part of the circuit?

If you have an ideal OpAmp the 10k resistor would carry no current and could be replace by a resistor with an arbitrary value, including a piece of wire. Therefore the answer is no, the 10k resistor ...
9 votes

What is wrong with this butterworth filter, how can it be improved?

I am afraid, changing the opamp type will not help. The observed effect (less damping for rising frequencies) is the typical disadvantage of the lowpass Sallen-Key topology. The reason is as follows:...
  • 22.9k
9 votes
Accepted

Where to ground low pass filter before dual supply operated op-amp?

The correct place to ground it is to 0 V. It is the reference for all voltages in your circuit. simulate this circuit – Schematic created using CircuitLab Figure 1. For perfectly regulated ...
  • 163k
9 votes
Accepted

How to calculate the frequency of oscillation of this low-pass phase shift oscillator?

Your schematic uses a low pass filter approach. One way to mentally consider a solution is to look at the following schematic: simulate this circuit – Schematic created using CircuitLab Above, ...
  • 72.5k
9 votes

Digital control of electromechanical gauges

Your approach is ultra sensitive because you are driving the gauge open-loop. The opamp circuit has a fixed gain, but it is driving a FET running wide open. Its transconductance changes with ...
  • 13.3k
8 votes
Accepted

Why a low-pass filter applied to a square wave gives a weird wave?

There are many types of low pass filter. The one that gives your 'weird waveform' is the simplest 1st order RC filter. This passes the fundamental signal. However, it not only passes all the higher ...
  • 145k
8 votes

Bypass capacitor vs low-pass filter

The trace from the power source to the capacitor has inherent parasitic inductance. This provides the series element to make a low-pass filter. A more complete model of the circuit looks like this: ...
  • 122k
8 votes

Decoupling Capacitors: What happens to the DC on the cap once noise is introduced and it shorts?

The fundamental nature of a capacitor is that stored charge = capacitance x voltage: - $$Q = CV$$ We also know that current is the rate of change of charge with time hence, if the formula is ...
  • 395k
8 votes

Decoupling Capacitors: What happens to the DC on the cap once noise is introduced and it shorts?

The capacitor does not become an actual short circuit. When we say that a capacitor is a short circuit at high frequencies, we are talking about the impedance of a capacitor when a sine wave voltage ...
  • 7,750
8 votes
Accepted

Trying to implement a digital IIR filter why is this happening?

It looks you already found the first bug: https://arm-software.github.io/CMSIS_5/DSP/html/arm__biquad__cascade__df1__f32_8c.html the function needs float arrays not int arrays By fixing this you just ...
  • 1,920
7 votes

Why don't decoupling/bypass capacitors need resistors to perform their function, like regular filters?

You are basically correct. A decoupling capacitor is needed because the trace from the power source to the consuming chip behaves like an inductor the power source itself is not infinitely fast, it ...
7 votes

What is wrong with this butterworth filter, how can it be improved?

The standard Sallen-Key design assumes you use perfect opamps. An LM324 is pretty slow as opamps go, I'm surprised it shows the filter working as well as it does. Perform a few more simulations, ...
  • 145k
7 votes
Accepted

Strange Distortion from Second Order Active Low Pass Filter

The LM324, while a brilliant achievement with 1970s transistors, has one well known bug - actually documented in its datasheet. This answer is based on a guess that you are running into this bug. ...
  • 66.1k
7 votes
Accepted

Divider and lowpass combined

Not that I'm aware of, call it a filtered voltage divider :) \$fc=\frac{(R_1+R_2)\sqrt{(10^{3/10}-1)}}{2\pi*R_1*R_2*C_1}\approx \frac{(R_1+R_2)}{2\pi*R_1*R_2*C_1} \$ \$R_1//R_2//C_1 \$ There are ...
  • 728
7 votes
Accepted

Instrumentation amplifier noise amplified

LM358 has very low gain-bandwidth product (1MHz) therefore the Sallen-Key filter topology you used might not work well as a lowpass above a few tens of kHz. What happens is the HF jumps over the opamp ...
  • 63.2k
7 votes

Low Pass Filter Wrong Pole Location

It's just a conversion from radians to degrees. The expression for a capacitor's impedance uses units of radians/sec. for frequency. The simulator is reporting frequencies in Hertz. 10 rad/sec is 1....
  • 2,530
7 votes

Why is inverting op-amp behaving like a low pass filter?

basically, every physical system in this world has a finite bandwidth and hence exposes some low-pass behaviour, so in the larger scheme of things, this is to be expected You use an ancient (LM)741 ...
7 votes

Why are the theoretical and measured frequency responses off a low pass filter different?

What is the reasoning behind this other than measurement errors? tolerances of components wrong theoretical calculation incorrect measurement procedures (wrong impedance scope-probe) test equipment ...
  • 395k

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