# Questions tagged [low-pass]

A low-pass filter lets lower frequencies through and blocks or reduces higher frequencies.

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### 2nd order low pass filter sallen key variation

I've come across what looks to be a variation of a sallen key low pass filter but it has an extra resistor (R10) and extra capacitor (C7). Does anyone know what the variation is called? [EDIT] The ...
4k views

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

simulate this circuit – Schematic created using CircuitLab I've been using decoupling caps for a while now and I understand their purpose in keeping DC signals clean since capacitors allow high ...
12k views

### cutoff frequency of low pass filter with two resistors (differential signal)

How do I calculate the cutoff frequency for this design? simulate this circuit – Schematic created using CircuitLab Or simply,
157 views

### Is it possible to achieve a square root analog filter?

I am attempting to create a (3/2)th-order active low-pass filter circuit, with a zero[es] at ω=10000 and pole[s] at ω=100. The only way I can think to do this is to simply cascade three simple first-...
1 vote
28 views

### Kuroda's Identities for Impedance Scaling

I am working on my understanding of RF filters and am in the process of designing a low-pass filter with a 3dB point of 1800 MHz and 10 dB of insertion loss at 2200 MHz. Based on these made-up ...
156 views

### lowpass prototype to bandpass filter

Bandpass filter with a center frequency of 10 rad/sec and bandwidth of 20 rad/sec here, wo= is mentioned as geometric frequency that's why I am confused. The solution in which picture is correct?
166 views

### Can't derive transfer function of RC filter with load

I am trying to go along with an example on a simple RC filter with load in a book called Practical electronics for inventors. simulate this circuit – Schematic created using CircuitLab Although ...
1 vote
101 views

### Low pass filter with Sallen Key topology

I'm designing a low-pass filter with a passband up to 1 MHz and a stopband starting at 1.8 MHz. I've designed a 4-stage filter using Sallen-Key topology and built it using NE5532P op-amps, supplied ...
151 views

### How to calculate a differential + common mode LPF circuit?

I'm trying to validate my understanding of a differential + common mode LPF circuit using the AK4499EX datasheet (019001308-E-00 2022/07 version, p. 47). It presents the following circuit: and its ...
1 vote
2k views

### RC low pass filter for NTC

I want to measure temperature using an NTC resistor and ADC (of an MCU). A low pass input filter is required with a certain cutoff frequency. Here is the recommended schematic: So I replace ...
1 vote
80 views

### Unexpected behaviour: Single pole RC LPF on output of op-amp buffer

I'm working on a project to show students how to use a Raspberry Pi to take automated I-V measurements of diodes. We've already done this using an Arduino, but there is a strong desire to use the Pi ...
158 views

### Dielectric absorption in capacitor (audio crossover)

I want to understand the selection of a proper capacitor type for a simple audio crossover (passive lowpass filter before woofer, passive highpass filter before the tweeter). Various blogs more or ...
78 views

### Bandwidth measured is two orders of magntiude less than expected TIA circuit

I have the following photodiode schematic, all soldered onto a small PCB. The first stage is a TIA, and the second stage filters out high frequency signals. The calculated bandwidth of the circuit ...
53 views

### Second order low pass filter cut off frequency calculation

I found this circuit which filters an incoming pwm pulse train from a Delta-Sigma ADC to an analog value. The PWM pulse has an amplitude from 0 to 5 V and duty cycle varies from 10% to 88%. And the ...
158 views

### Loop Filter Bandpass Jitter Characteristic in PLL

I was watching some PLL video lectures by Professor Elad Alon, and he explains why intuitively the jitter transfer function of the loop filter to the PLL output has a bandpass characteristic. He says ...
45 views

### Switching voltage divider and low pass filter combined

I have an input signal which I want to switch from 12V-5V and a low pass filter directly after that, the issue is, if there wasn't a switch I can calculate the cut-off frequency easily. In my case, ...
8k views

### How to do low pass filtering on a differential signal?

If I have a differential signal on a pair of wires, then how would I do a simple RC low pass filter on this differential signal?
168 views

### LPF and AMP for ADC with op amp not giving sufficient gain

I have a project where I need to have audio input into atmega168PA. I've decided to implement it by using an electret microphone which will feed into op amplifier for low-pass filtering and ...
115 views

### Non-ideal active low pass filter for LV25P output

Hello everyone I need some help understanding the active low-pass filter below. I was told to use this circuit to filter the measurements of the output voltage of an inverter, but there is no ...
214 views

### About PLL input frequency

I am working on a PLL design. I am at the research stage now. I am confused about the PLL input frequency range. This frequency has to remain constant. If this input frequency stays constant, how can ...
70 views

### Should you include multiple low-pass capacitor circuits in integrated IC PCB design?

I have two circuits, one is a POE to 5V, another is an IC for LEDs. They both include capacitors between 5V and gnd. Should I include the first 5V circuit POE capacitors since it should already be ...
78 views

### Equalizer circuit using low pass filter

I'm working on my circuit theory project. My goal is to make a simple 3-channel equalizer using 1 low pass filter and 2 band pass filter. Here is my circuit. Problem is, the 8kHz passive low pass ...
74 views

### RC filter cutoff and impedance

I wanted to create RC lowpass filter with cutoff frequency 15 kHz and output impedance 50 ohm. What should be R and C values? I'm a little confused how to interpret 50 ohm output impedance to ...
72 views

### What happens if a low-pass filter and a high-pass filter are interchanged in a band-pass filter?

what happens in this circuit if I interchange the highpass filter and lowpass filter how does it affects the gain and phase shift
276 views

### Using the same ultrasonic transducer for Tx and Rx in distance measurement

Goal: Measure distance to surrounding objects using 5 automotive grade ultrasonic transducers mounted to different points on a small robot. Requirements: Multiplex the drive and receive pulses so ...
1 vote
225 views

### Amplifier and Low Pass filter not responding after few minutes of correct output?

I am trying to design an amplifier and low pass filter circuit for a piezo sensor. My frequency of interest is .3 Hz to 5 Hz, for first stage I have used charge amplifier and the subsequent 3 stages ...
87 views

### How to design a 2nd order low pass filter [closed]

I have a question about second order low pass filters. I have a DC/DC power supply at 3.3 V and I would like to filter this signal to address any disturbance from 50 Hz (ideally only keep the DC ...
2k views

### Why does my integrator slowly saturate to negative rail in the following configuration?

So I have the following basic integrator circuit: The input signal is a 100mVp-p square wave of 50k frequency. In this case I get a stable triangle wave output. However, when I make the amplitude ...
1 vote
6k views

### Second-Order Low Pass Filter Configuration

Can someone explain why this is a second order filter, and what both sets of resistors and capacitors do? Also, I read somewhere that both sets are low-pass filters. If that's true, why do we need ...
16k views

### Divider and lowpass combined

I am using the following circuit for two purposes: As a voltage divider, to reduce DC sensor output to a range suitable for my ADC. As a lowpass filter, to remove high frequency noise from the signal....
139 views

### How can I select a common mode choke for DC/DC power conversion stages

I want to attenuate 15 MHz noise signal using a common mode choke at 5 V power lines stage of a circuit that generate digital signals with less than 100Mhz. When you use a common mode choke for a ...
3k views

### How to find the cutoff frequency of an underdamped second order low pass filter?

I've learned that for a first order low pass filter (LPF) the 3 dB frequency is one where the amp's gain is (surprise) -3 dB below the DC gain. However, on a second order LPF that is underdamped we ...
78 views

### 3-pole LC filter design

When designing a 3-pole LC filter with a ferrite bead as inductor L, using capacitor values intended for a LC filter with an inductor (not a ferrite) produces a large 'dip' or 'bounce' in its ...
1 vote
285 views

### How can I only let a specific frequency pass in an AM receiver?

I wanted to create an AM receiver and I know partly how to demodulate the signal. I know I need to use a low pass filter to get the low frequency audio signal, but I don't know how to only get data ...
175 views

### Transient analysis of the low pass RC filter

I have designed a low pass RC filter and trying to simulate using transient analysis. I can't figure out the magnitude of the input and output waveform. What's the time delay between the input and ...
68 views

### Capacitor "blocking DC" in a low pass filter [duplicate]

If we imagine a basic lowpass filter, the AC components from the non perfect DC from VIN passes the capacitor and down to ground, meanwhile "pure" DC flows to VIN. If we take a basic closed ...
1 vote
2k views

### How do I know to choose a first order filter vs a second order filter?

I have a DC signal coming off of a battery. The Arduino is reading the voltage. The issue is that the voltage signal contains a lot of noise (120hz frequency). I am planning on building a first order ...
1 vote
46 views

### How can I calculate filter parameters between RF switch and antenna?

In the schematics en.MB1389-WL55JC-highband-E02_Schematic, there's a Pi filter following the RF switch before it links to the SMA connector for the antenna. Upon calculation, I found that the cutoff ...
30 views

### Raspberry Pi DAQHAT: Overvoltage issue

I am working on development for a piece of HV test equipment, where I need to monitor the voltage output for a circuit that is actively arcing. I am measuring the voltage off of a Raspberry Pi DAQHAT (...
100 views

### High R for low frequency op amp integrator

I have to design an integrator for very low frequencies (corner frequency around 0.5 Hz). Are there any reasons why I cannot choose a really high resistor value (say 5M Ohm) to keep the capacitor ...
66 views

### Negative capacitance circuit without relaxation oscillations

Wikipedia gives the following circuit as a negative capacitance circuit: I was curious if I would be able to create a low-pass resistor-negative-capacitance filter, analogous to an RC low-pass filter,...
75 views

### PWM Low-pass filter for LCD contrast pin with a potentiometer?

As a software developer, electronic engineering isn't my strong suit, so I'm struggling to understand some simple concepts regarding this circuit. This is a unique circuit I have encountered that is ...
1k views

### RC Laplace and time domain

Another basic question to understand. It is well known that the transfer function of a low pass RC filter in Laplace domain is $V_o(s)/V_i(s)=1/(1+sRC)$. Now sticking to this format and taking ...
54 views

### Low-pass filter with a load

how can be the voltage of a low-pass filter be mathematically described when a load is connected? Is it possible to ignore the effect of the load on its voltage as something like this circuit can be ...
1 vote
111 views

### How to design an inductor for the output of a low-pass filter for a 10 kVA sinusoidal UPS

The problem is that I have a 10 kVA sinusoidal UPS to repair and after replacing all the IGBTs and other components, I noticed that the low-pass filter output inductor is getting very hot at around ...
4k views

### Is a 100 Ω/1 nF RC low-pass filter enough to protect a microcontroller I/O pin from ESD?

While researching ESD protection on this site, I came across this question. The answer by Peter Smith contains a comment by @Ale..chenski: Just a nit pick: if you can afford 100R and 1nF to ground on ...
1 vote
140 views

### Loss of 2nd order behavior on 2nd order low-pass filter

I have a 2nd order Sallen-Key Low-pass filter, cut-off frequency of 170Hz. When attached to a perfect source, this is the bode plot. As expected, 40dB/decade response. The low-pass filter is used as ...
9k views

### Passive Pi Low Pass Filter LC

I'm trying to use a simple pi low pass filter using two capacitors and one inductor. I've found the formula to calculate the value of the capacitance and of the inductance using the cut off frequency,...