Questions tagged [sallen-key]

For questions about the Sallen-Key filter topology. Use in conjunction with related tags such as "filter", "active-filter", "high-pass-filter", and/or "operational-amplifier", as applicable.

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

I have designed a 2nd-order active Sallen-Key high-pass filter for use in an audio circuit. An LTspice schematic is shown below, where VDD is 5 V. I have also breadboarded the circuit using low ...
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2 votes
2 answers
96 views

Converting two cascaded first-order high-pass filters circuit to a second-order high-pass filter using Sallen-key configuration

Below is the transfer function of the two cascaded first-order high-pass filters (each with 20.6 Hz cut-off frequency) and their circuit: And then there is the transfer function of the second-order ...
2 votes
3 answers
205 views

Second order active high pass filter

I am trying to design an active filter, which I assume is an active high pass filter according to what I see from the Bode plot. I can get the transfer function from this plot and then I need to ...
1 vote
1 answer
137 views

Add Voltage Bias/Offset to 2nd-Order Sallen-Key Highpass

I need to add a highpass filter between a signal source and an ADC to get rid of high-amplitude low-frequency signals that would cause clipping. I built a second-order Sallen-Key filter and the ...
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0 votes
1 answer
124 views

Butterworth Active Low Pass Filter Design

I'm designing a low pass active filter with a Butterworth response. The passband is DC to 1.5 kHz, so the -3 dB cut off frequency should be around 2-5 kHz to be safe. The topology I've decided to use ...
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3 votes
0 answers
163 views

Which of these three op amps should I choose for my low frequency filter? [closed]

I want to design a lowpass filter-4th order Bessel, so two Sallen-Key low-pass circuits in cascade with a cutoff frequency of 200Hz. I need to choose the op amps for it. I have these 3 op amps to ...
0 votes
2 answers
82 views

How to pass from a double-supply filter circuit to a single supply filter circuit?

So here is the main circuit, it's a lowpass filter-4th order bessel, so 2 sallen-key low-pass circuits in cascade. I already have all the components and transfer functions figured out but i would like ...
1 vote
3 answers
419 views

What's the difference between these two Sallen-Key bandpass filters?

I am thinking about using a Sallen-Key bandpass filter for an audio project I am working on. While trying to understand the circuit, I came across two similar, but slightly different circuits. The ...
1 vote
2 answers
339 views

High pass filter: Overshooting before attenuation

Working with a 2nd order Sallen-Key high pass Butterworth filter with a cutoff frequency of \$f_c=0.5\$ Hz I am having difficulties making sense of a result that I got: After applying at its input a ...
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1 vote
1 answer
287 views

Q of Sallen key low pass filter

During i study Sallen-key active low pass filter, i wondered why formula of Q is 1/(3-A). A is Voltage Gain. So i searched from web, and i found this. When R1=R2 and C1=C2, the Q be 1/(3-A). OK. But, ...
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9 votes
4 answers
513 views

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

I'm building a full analog chorus-effect pedal. My schematic is based on that of a Rockman X100 headphone amp, designed by Tom Sholz. In this schematic, there is a 2nd order low pass filter that looks ...
0 votes
0 answers
135 views

LTspice, Sallen & Key low pass filter + Low pass filter order 1

First of all, sorry for my English, it isn't my native language. I want to cascade a first order low pass filter and a low pass Sallen and Key structure in order to get my bandwidth at 30 Hz (Fc = 30 ...
0 votes
1 answer
168 views

Active low pass filter with cut-off frequency ~4250 Hz lets a 20 kHz signal through

I needed an active low pass filter for my project that can produce an output voltage within the required settling time and ripple voltage ranges. Although I managed to get a good enough output voltage ...
0 votes
0 answers
128 views

Is the system linear, time-invariant and causal? Does the decomposition property hold? Sallen Key bandpass filter

I have this ordinary Sallen Key bandpass filter: - simulate this circuit – Schematic created using CircuitLab I'm being asked, if the system is linear and/or time-invariant and/or causal. ...
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0 votes
0 answers
80 views

Why is the output of my opamp mV?

I would like to use an Si514 as a tuneable oscillator driving later stages with 50 ohm impedance. So I put an opamp on it and attempted to configure it as a saleen-key low pass filter (to round the ...
1 vote
1 answer
132 views

Why does adding a gain to a Sallen-key second order HPF change the amplitude response?

As the title suggests, I have done all the necessary calculations to implement a gain into my second order stage of my 6th order high pass filter but for some odd reason, it would change the amplitude ...
3 votes
2 answers
885 views

Why is the output of a high-pass filter not 0 when the input is 0?

I'm trying to design a 5th order Butterworth high-pass filter built on the Sallen-key topology with a cut-off frequency at 100Hz. For some reason, the output has a DC offset of around 5mV when the ...
0 votes
2 answers
88 views

Help for analog signal chain for 16bits ADC

I work on analog amplifier and filter chain for an internal STM32H753VI 16bits ADC. The signal chain is based on ultra-low noise opamp LT6237 and differential driver LT1994. The sample-rate of ADC ...
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2 votes
3 answers
2k views

How to design a 2nd Order High Pass Butterworth filter with a cutoff frequency of 0.5 Hz? [closed]

c1=c2=10µf, R1? R2? using the unity-gain Sallen-key topology what should I do?
0 votes
2 answers
65 views

Bandpass filter obtained by complementary transformation of lowpass Sallen & Key: what went wrong?

I am considering the following lowpass Sallen & Key filter: My components are the following \$R_1=10 k\Omega\$ \$R_2=10 k\Omega\$ \$C_1=150 nF\$ \$C_2=1.65 nF\$ Now I have performed a ...
0 votes
1 answer
80 views

Measuring experimentally the poles parameters

I have the following Sallen & Key lowpass filter: $$T(s)=\frac{\frac{1}{R_1R_2C_1C_2}}{s^2+(\frac{1}{C_1R_1}+\frac{1}{C_1R_2})s+\frac{1}{R_1R_2C_1C_2}}$$ The parameters values are $$R_1=10 k\Omega$...
0 votes
1 answer
87 views

Bode is plot despite of high quality factor

I'm trying to simulate the following circuit in LTSpice: As can be seen, the Gain $$ G = 1 + \dfrac{R_3}{R_4} = 1 + 1.6 = 2.6 \Rightarrow Q = 2.5$$ Which should give a peak, but I'm getting following ...
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1 vote
1 answer
148 views

2nd order Sallen-Key low pass filter frequency response

I am trying to implement the following circuit in LTspice for studying the frequency response of the 2nd order Sallen-Key low pass filter for a project. Why am I getting a negative peak in the ...
2 votes
2 answers
454 views

Sallen Key Lowpass Filter stops rolling off in simulation

I unfortunately have no pictures of my spice simulation since it's on a different computer, but I am attempting to create a Sallen Key lowpass filter with fc~=555Hz. I used the lowpass filter design ...
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0 votes
0 answers
39 views

Is this circuit a Sallen & Key biquad?

So I came up with the following filter to analyze: My first thought was that this reminded me of a Sallen & Key biquadratic section. However I'm having trouble identifying each component. Taking ...
1 vote
3 answers
199 views

How to use Butterworth Polynomials to determine RC components of a n-th order filter

I am working on a project to design a 7th-order Butterworth filter Sallen-Key topology. I am not sure how to interpret the Butterworth polynomials correctly to calculate each component. I mean how is ...
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0 votes
1 answer
93 views

Chebyshev filter - theoretical vs simulation gain

Question is about the gain peak (or maybe more suitable would be growth) ocurring in higher frequencies (>20kHz) in Chebyshev filter 4-th order in simulation (simulated using Bode Analyzer in NI ...
0 votes
3 answers
163 views

Verification of Sallen Key Design

I have designed this Sallen-Key to give a 2nd order low pass circuit with a cut-off frequency of 150Hz. Another design specification is that R1=R2 and C1=C2. The issue is when simulated on LT-Spice ...
0 votes
3 answers
841 views

Sallen-Key LPF Q factor and gain

Welcome all. I need some advice with Sallen-Key topology. I used Analog Devices tool for my design and I admit it nice quick and easy. However, I am struggling with one detail. When studied control ...
0 votes
3 answers
151 views

Sallen key topology choosing right op-amp

Welcome all. I designed Bessel low pas filter using Analogue Devices filter wizard. It is a fantastic toll saving loads of time. However, like any tool has its limitations. Upon completion my design I ...
0 votes
3 answers
755 views

Differences between 2nd order low pass filters

I don’t know how to design filters, but I need a second order low pass filter. I searched them on the internet and found two schemes. One is Sallen-Key, and the other I don’t know what is called. ...
0 votes
3 answers
451 views

Design of second order low pass filter using passive RC components and opamp

I have an RC low pass filter connected to the non inverting terminal of the opamp. There is a feedback resistor from output of opamp to its inverting pin.Then an RC low pass filter is placed at the ...
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0 votes
1 answer
123 views

Noisy output on active bandpass filter PCB Design

I'm trying to design a 40dB active bandpass filter with a centre frequency of 130KHz and a passband of 80KHz. I don't really know much about filter design so I used the TI filter design tool to design ...
0 votes
1 answer
195 views

Is this transfer function impossible with a low-pass Sallen-key filter?

I was given this transfer function and instructed to use a Sallen-key cascaded with a 1st order filter to achieve it. $$H(s)=\frac{628^3}{(s+628)(s^2+628s+628^2)}$$ The first order transfer function ...
6 votes
6 answers
4k views

Why does this potentiometer in an op-amp feedback path cause noise when adjusted?

I have placed a dual-gang 100k potentiometer in the feedback path of two op-amps that work as a Sallen-Key filters. This controls gain but also the Q factor of the filter (somewhat demonstrated on ...
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0 votes
1 answer
508 views

Sallen-Key Low Pass Oscillator

How to design a Sallen-key Low pass Oscillator that gives undamped oscillations? I've grounded the input of the Sallen-key low pass filter but still can't get any oscillation at the output. I've ...
14 votes
4 answers
3k views

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

Appendix E of The Art of Electronics, 3rd Edition (LC Butterworth filters) starts by saying that "active filters are convenient at low-frequencies but impractical at higher frequencies". They go and ...
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2 votes
3 answers
1k views

Why is the cutoff frequency of this highpass filter with formula different from LTspice?

I used this online tool to play around and design a Sallen-Key low pass filter with a cutoff frequency of 0.3Hz. When I now put the resulting values for R1, R2, C1 and C2 in the formula I get exactly ...
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0 votes
2 answers
620 views

Does this Sallen and Key filter work on a single supply and does the input offset matter?

This is the schematic for a Sallen and Key from the Doepfer Musikelektronik website: simulate this circuit – Schematic created using CircuitLab I intend to run this in a synth, the oscillators ...
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8 votes
5 answers
2k views

Circuit for small level voltage detection (microvolts)

I have been working on a project lately which help in the detection myogenic potentials (of the levels of micro-volts). After studying from several places, I finalized the following test circuit. The ...
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1 vote
2 answers
248 views

Sallen-Key - how opamp input bias current changes cutoff frequency?

As in the title: what exactly is the impact of input bias current in Sallen-Key topology, for example the one below: I read that it can matter significantly, however I did not find anything more on ...
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1 answer
539 views

Input impedance of low pass filter on LTspice must be constant in it's pass band?

I have single supplied sallen-key filter more that have nice 3MHz flat LPF response. I tried to compute the resistance directly by dividing the input voltage by input current which is the voltage and ...
0 votes
2 answers
405 views

What is the difference between a Sallen-Key Band pass filter and a band pass filter?

Am currently doing an assignment on sine wave generation and am using a sallen key band pass filter - i am unsure what the difference between the band pass filter and the sallen-key band pass.
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0 votes
2 answers
823 views

Sallen-Key filter for PT100 RTD measurement

I am going to build temperature sensing system using PT100 sensor (and later Raspberry PI 3). I refer my work to Microchip note AN687 Precision Temperature-Sensing With RTD Circuits. On the page 4 is ...
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0 votes
1 answer
259 views

LTC1966 Sallen-Key post-filtering application question

Just a quick question about an application circuit in the LTC1966 datasheet. Figure 13 on page 18 shows a third order (when considering output resistance of the LTC1966) Sallen-Key filter, with a ...
2 votes
0 answers
333 views

Does Sallen-Key filter need a high impedance load?

I am using the circuit below to monitor the output of a piezoelectric diaphragm, represented by a sine wave generator and capacitor C3. The circuit is powered from a split supply (+5V/-5V) derived ...
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3 votes
2 answers
874 views

Circuit for Inverse Chebyshev or Elliptic LPF

I know I came across a 2 op-amp, 2 capacitor circuit that can be used for a single section of an Inverse Chebyshev (a.k.a. "Chebyshev Type II) or an Elliptic (Cauer) filter. It has a pair of zeros on ...
2 votes
5 answers
829 views

Does the Sallen-key filter require a buffer for low power applications?

I am designing a circuit which takes an 3uV at 10-10kHz input that filters, applies gain and integrates the analogue signal. I'm thinking of using the circuit below for the topology using two stages ...
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1 vote
2 answers
450 views

Sallen-Key topology for Butterworth low pass filter

I performed the AC analysis of the following circuit: This is a simple second order low pass filter satisfying the Butterworth filter requirements. According to me, after the break point there will ...