# Tag Info

27

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 with much better selectivity (higher pole Qp) and various possible approximations (Butterworth, Chebyshev, Thomson-Bessel,...). However, there is one big ...

15

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 having to use a physically large capacitor.

14

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 sources of noise which do. First of all you need to decide which bandwidth you want to cover. If your ADC samples at 75kHz, then there should not be any ...

13

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 no reason why the wires should cross over one another. If the LEDs were placed from left to right: D9, D6, D8 then no wires need to cross. Another one: the ...

12

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, whereas useful values of capacitors can be integrated. However, series inductance is frequently used where conductors enter a shielded enclosure or signals ...

11

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 frequencies below the nyqist limit. The reason is, that frequencies higher than the nyquist limit fold (mirror) back into your frequency range of interest. For example ...

10

If you're talking about "typical" transfer functions (those that express the behavior of a linear ordinary differential equation), then they take the form $$H(s) = \frac{s^m + b_{m-1}s^{m-1} + \cdots + b_0}{s^n + a_{n-1}s^{n-1} + \cdots + a_0}$$ where I'm using $s = j\omega$, to save on typing and on trying to keep track of minus signs. Note ...

9

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 consequence. In the following I assume that we deal with random noise modeled as a wide-sense stationary (WSS) random process, i.e. a random process for which we ...

9

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: For rising frequencies the "classical" output signal from the opamp decreases (as desired) - however, at the same time there is a signal arriving at the ...

9

In PSpice m and M are thousandths. You need to use Meg.

8

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 (resistances, capacitances, inductances) to move those poles and zeros around in the s-plane. A filter design, like Butterworth or Chebychev, is a choice of ...

8

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 $$H(s)=\frac{R_4}{R_3+R_4}\cdot \frac{s^2-s\frac{2}{R_2C}+\frac{1}{R_1R_2C^2}}{s^2+s\frac{2}{R_2C}+\frac{1}{R_1R_2C^2}}\tag{1}$$ where $$\frac{R_2R_3}{R_1R_4}=4\tag{2}$$ must be satisfied. For a ...

8

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 going to cause problems. If you had an op amp with a 1MHz or 10MHz GBWP (which may have been typical at the time of the first edition of AofE, maybe they didn't ...

7

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. Look at it this way. A low-pass filter removes high frequencies, right? And if you want the filter output to change more quickly it must contain more high-...

7

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, changing the opamp type you use. Use a faster opamp, a slower one, and a perfect one. I don't know LTSpice specifically, but most simulators have a generic opamp ...

7

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 understand your reluctance to use a PCB because it is more work and more expensive initially - however, in the long run it sounds like the only way you will get ...

7

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. Some people sneer at it because of limitations like this - but it is still a fine opamp if you design to its limitations. Its Class B output stage is specifically ...

7

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 should be. In the second circuit you're expecting the impossible from that poor LM324 opamp. You give it a single (positive only) supply yet the circuit expects it ...

7

Well, generally we have two things that we look at: dB/decade: $$\lim_{\omega\to\infty}\left(20\log_{10}\left|\underline{\mathscr{H}}\left(10\omega\text{j}\right)\right|-20\log_{10}\left|\underline{\mathscr{H}}\left(\omega\text{j}\right)\right|\right)\tag1$$ dB/octave: $$\lim_{\omega\to\infty}\left(20\log_{10}\left|\underline{\mathscr{H}}\left(2\omega\text{... 6 The transfer function is$$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

6

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, section e− should indicate the modulation and central frequency information. If the number under "Central Frequency" is not exactly 2.400000GHz call the layout ...

6

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. There should be cap to ground across R4. I'd start with around 2 µF. There is no bias supply for the microphone. You said this was a electret, so there ...

6

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 supply is disconnected at the instant while current is flowing through the inductor, the induced (possibly large) voltage spike may damage other components. Adding ...

6

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 the right side. These, of course, must equal each other. I present this as an alternative for you to consider. Traditional teaching tells you to make decisions ...

6

What is the purpose of C3? (corrected) Pardon my poor engineeringlish. I meant Negative Inductance not -L. L is always a positive interger, with positive reactance, while C is a negative reactance, unless used in an impedance converter with feedback. (so always think in terms of impedance) C4 is an active inductor or negative capacitor as the negative ...

5

Quantum231 - I do not fully understand what you really want, because: The four types you have listed are NOT different filter topologies (functions) - instead, they are 4 different approximations to an IDEAL lowpass response (and they can be transferred to corresponding highpass and bandpass approximations); All these filter functions can be realized ...

5

If you keep all the poles and zeros the same, and use ideal components, the transfer function will not be changed. If you use real components instead of ideal components, then the parasitics will likely change the transfer function somewhat. This could happen, though, even if you just changed from one active realization to another. If the components are ...

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