32 votes
Accepted

Active low-pass filters --- good to what frequencies?

I believe your analysis to be good. I've made sallen-key 4th order filters that cut-off around 3 MHz with absolutely no worry about performance. I don't see that 10 MHz is unachievable. It's all ...
  • 396k
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 ...
  • 23k
15 votes
Accepted

Adding an anti-aliasing filter to op-amp before ADC

It is always good practices to use anti-aliasing filter before digitizing a signal. Although your target signal does not contain frequency components above the Nyquist rate, there might be other ...
  • 601
15 votes
Accepted

What is this circuit called, and what is it used for?

This is a capacitance multiplier: It behaves like a capacitor to ground with a capacitance of C*(1+R2/R1). This means you can use it to "simulate" very large capacitors without actually ...
  • 7,238
14 votes
Accepted

Unknown amplifier circuit Analysis

Theory What are the effective tools people use nowadays? The first, most effective, and primary tool to use is your brain. I first see is a high-pass passive RC filter leading into an active filter....
  • 73k
13 votes
Accepted

Cathode to cathode diodes in schematics

In my opinion this is not a well designed circuit, especially I have my doubts about the buck converter part. Also it isn't very well drawn either, for example around the LEDs D6, D8 and D9: there is ...
  • 79.1k
12 votes

Active filter with series inductor and resistor - do these exist?

Inductors are generally more expensive, more bulky and less ideal than capacitors so you'll usually see a strong preference for capacitors over inductors. Only very tiny values can be put into an IC, ...
11 votes

Adding an anti-aliasing filter to op-amp before ADC

Do I need to worry about anti-aliasing filters before the ADC Unless your ADC has a built-in anti-aliasing filter, then yes, you should take care about it even if you're only interested in ...
10 votes

How is the slope of the frequency response of an analog active filter defined?

If you're talking about "typical" transfer functions (those that express the behavior of a linear ordinary differential equation), then they take the form $$H(s) = \frac{s^m + b_{m-1}s^{m-1} ...
  • 38.9k
9 votes
Accepted

Why doesn't aliasing of broad band noise 'pile up' in the sample band?

You are correct: after sampling, the aliased noise components do pile up in the frequency band below the Nyquist frequency. The question is just what exactly it is that piles up, and what is its ...
  • 3,613
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:...
  • 23k
9 votes
Accepted

Second order all-pass filter using a single op-amp

One option for a second order all-pass filter with one OpAmp is the Delyiannis structure, as shown here (p.2): The transfer function of this filter is given by $$H(s)=\frac{R_4}{R_3+R_4}\cdot \frac{...
  • 3,613
9 votes

Completely different circuit response with 1000kΩ than with 1MΩ resistance

In PSpice m and M are thousandths. You need to use Meg.
  • 13.6k
8 votes

How was the design of active filter topologies arrived at, invented or discovered?

A filter topology, whether it's a passive one like a pi-section or an active one like the Sallen-Key circuit, is just a way to produce some poles and zeros. Generally, you can tune the circuit values (...
  • 122k
8 votes

Active low-pass filters --- good to what frequencies?

My first question: really? A mere 100kHz is already too high for active filters to be practical? No, 100kHz is nothing, but it all depends on the opamp. At some point the Gain Bandwidth Product is ...
  • 65.5k
8 votes
Accepted

Why am I measuring such high THD+N on my active high-pass filter?

The number one suspects are the ceramic capacitors. Here is a SE link that shows the severe voltage dependence that ceramics have. The X5Rs are some of the worst, especially in the small SMD sizes. ...
  • 3,694
7 votes

How can I improve the response time of a low pass filter?

You cannot greatly change response time of a low-pass filter by changing its' order. What you have to do is change its cutoff frequency - the higher the cutoff frequency, the faster the response. ...
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

How to make perfboard circuits more compact

The ultimate answer is to use SMD components on a PCB - the parts are smaller, and using traces instead of wires will reduce stray capacitance; this can have a large effect in sensitive circuits. I ...
  • 920
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

Unexpected plots for active low pass filter response in LTspice

In the first circuit the active Low Pass filter is a 2nd order filter while the passive Low Pass filter is a 1st order filter so I totally expect the curves to be different and they are what they ...
  • 79.1k
7 votes
Accepted

How is the slope of the frequency response of an analog active filter defined?

Well, generally we have two things that we look at: dB/decade: $$\lim_{\omega\to\infty}\left(20\log_{10}\left|\underline{\mathscr{H}}\left(10\omega\text{j}\right)\right|-20\log_{10}\left|\underline{\...
  • 5,485
6 votes
Accepted

Gain of filter is higher than calculated - why?

The transfer function is $$H(s)=\frac{-sR_1C_2}{1+s(R_1C_1+R_2C_2)+s^2R_1R_2C_1C_2}$$ and the maximum gain is $$A_{\text{max}}=\frac{R_1C_2}{R_1C_1+R_2C_2}=2.04$$
  • 23k
6 votes

How to design a band-pass filter with 2.4GHz center frequency?

Have someone stand there and request each electron's paperwork which should include a inter-circuit migration paperwork, quantum state, energy level details ect. On page 3 of the Feynman documents, ...
  • 4,509
6 votes
Accepted

Does Q-Factor matter for low pass and high pass filters?

Here's a picture (I drag out now and then) that explains the effect of Q on a 2nd order low pass filter: - The top three pictures show you the effect of varying the Q-factor. Q-factor can also be ...
  • 396k
6 votes
Accepted

Why does my electret microphone amplifying and filtering circuit output so much noise?

There are a number of problems with this circuit: The input to the reference is not filtered. R5 and R4 make half the supply voltage, but also transfer half of whatever noise is on the supply. ...
6 votes

Active filter with series inductor and resistor - do these exist?

To add to Spehro Pefhany's reasons, the circuit as shown has a gain at DC, which may cause issues with the following circuitry because of any DC offset voltage in the amplifier. Also, if the power ...
  • 1,090
6 votes
Accepted

ADS1115 ADC input filter - active or passive?

Summary: A band pass of 300Hz to 1MHz won't do you any good - the ADS1115 can only sample at maximum 860 samples per second. You cannot capture a signal above 400 Hz with the ADS1115. TI recommends a ...
  • 61k
6 votes
Accepted

Derive the transfer function

I use a concept of outflowing and inflowing currents, which is how Spice programs develop their matrices as well, when developing my KCL equations. I place the first on the left side and the second on ...
  • 73k

Only top scored, non community-wiki answers of a minimum length are eligible