From 3 stage LPF to an active filter - Page 1
Forum section: electronics / projects

author: janjohansen
author_rank: Newbie
timestamp: July 30, 2025, 10:49:11 pm
content: I have an passive 3 stage filter (1.5k, 1u to gnd, 1.5k, 2u to gnd, 1.5k, 4.7u to gnd) which filters my 2446hz pwm signal great, but in low voltages <300mV, my pwm signal loses some of its ~22 to 24 bits effective resolution (14 bits (PWM) + 8 bits (dithering) and 4892hz Sigma-delta noise shaping) because of the passive filterCan anyone recommend me an active topology which can help me get better use of me resolution?CheersJan

author: Benta
author_rank: Super Contributor
timestamp: July 30, 2025, 11:20:15 pm
content: What you have is something like a 1st order filter plus some extra filtering. Somewhat undefined."Topology" doesn't really come into question until you've defined the filter response that you need.Amplitude? (passband/stopband)Phase? (impulse response?Your specification of 14, 22 or 24 bits is not relevant for analog filter design.

author: moffy
author_rank: Super Contributor
timestamp: July 31, 2025, 12:10:43 am
content: If you want lower ripple/increased resolution then go for more stages of say a 5th order Butterworth filter with a cutoff frequency of 50Hz, TI has a design tool:https://webench.ti.com/filter-design-tool/filter-type

author: janjohansen
author_rank: Newbie
timestamp: July 31, 2025, 12:29:37 am
content: This is my current filter:1.5 kΩ, 1µF, cutoff ~106 Hz1.5 kΩ, 2 µF, cutoff ~53 Hz1.5 kΩ, 4.7 µF, cutoff~22 HzShould i run buffer after each stage or is it enough with 1 after all? Im after precision, as little ripple as possible but i dont want it to be hard to step through low voltages which i find it to be now, thats why i want something active to take of that issue. The resolution is kinda relevant as i can use my full resolution at low voltages because of my filter. So i wonder which way i should put my focus to do something with these issues. I was hoping someone could push me in a direction for a better active filter

author: srb1954
author_rank: Super Contributor
timestamp: July 31, 2025, 02:28:55 am
content: Quote from: Benta on July 30, 2025, 11:20:15 pmIts still a 3rd order filter because the ultimate roll-off rate is -18dB per octave. However, it is a poorly designed 3rd order filter with a very "soggy" initial roll-off.

author: srb1954
author_rank: Super Contributor
timestamp: July 31, 2025, 03:03:44 am
content: Quote from: janjohansen on July 31, 2025, 12:29:37 amA buffer between each stage would help slightly because there is considerable interaction between the stages degrading the filter performance. However, the 2 stages with the higher cut-off frequency are not optimum as the lowest cut-off stage is doing the bulk of the filtering work.A better approach. whilst still using 3 passive stages with equal cut-off frequencies, would be:Stage 1 1.5k, 4.7uF, cut-off 22HzStage 2 15k, 470nF, cut-off 22HzStage 3 150k, 47nF, cut-off 22HzScaling the impedance up at each successive stage reduces the interaction between passive stages and gives a useful reduction in ripple voltage and a considerable reduction in settling time to the final value.Ultimately, you are probably not going to achieve the desired performance with a simple 3rd-order passive filter unless you reduce the cut-off frequency quite a bit, but this would increase the settling time correspondingly. An active filter might help get a little better performance but, with the high stop-band attenuation required, this will be an extremely difficult task. With required stop-band attenuations of greater than 100dB the limitations of the active components come into play, severely limiting the ultimate stop-band attenuation. Indeed, some of the filter design programs offered by various semiconductor manufacturers don't even allow the design of a filter with greater than 100dB stop-band attenuation; presumably so as to not to show up the performance limitations of the active devices they offer.One approach that would be worth investigating would be alternative PWM schemes that reduce the initial ripple prior to the final filter. There have been several circuits published in the Design Ideas section of EDN that accomplish that. A quick web search should turn up some ideas.

author: radiolistener
author_rank: Super Contributor
timestamp: July 31, 2025, 04:40:34 am
content: Quote from: janjohansen on July 30, 2025, 10:49:11 pmAchieving 22-24 bits of effective resolution (134-146 dB dynamic range) is not realistically possible with passive RC filters due to thermal noise, leakage, and other analog limitations.Moreover, using PWM to achieve such resolution is virtually impossible - it would require a clock frequency in the range of 40 GHz or higher for 24-bit precision, which is far beyond the capabilities of typical PWM hardware. Even with noise shaping, achieving such high dynamic range remains unattainable.In practice, PWM-based DAC typically achieve around 10-12 bits of usable resolution. Even with specialized DAC chip, reaching a true 146 dB dynamic range is extremely challenging. As I know such performance is not available in commercially accessible DAC and may only exist in classified or military-grade systems with high limitations for export.To better preserve your PWM resolution, consider using a properly designed high-order LC low-pass filter instead of a simple RC network.The choice of filter topology depends on your requirements for amplitude and phase response. Bessel filters are preferable when linear phase is important, while elliptic filters offer the steepest roll-off at the expense of phase distortion. Other types, such as Butterworth or Chebyshev, represent trade-offs between these two extremes.Below is an example of the theoretical limit you can achieve with a 7th-order LC LPF for 50 Ω impedance. First picture is frequency response and second one is phase response.

author: mawyatt
author_rank: Super Contributor
timestamp: July 31, 2025, 02:07:47 pm
content: In the analog filter domain Bessel types are Maximally Flat Phase at the origin, while Butterworth are Maximally Flat Amplitude at the origin, everything else is in-between these limits in Phase and Amplitude.If the OP intent is to create a precise DC value from the PWM, Dithering and SD Noise shaping, then consider a DC accurate LPF which employs op-amps but the DC Characteristic Input Offset Voltage and Bias Current are removed from the final result.https://www.eevblog.com/forum/metrology/dc-accurate-low-pass-filter/msg5569985/#msg5569985Best

author: d-smes
author_rank: Regular Contributor
timestamp: August 03, 2025, 12:01:57 pm
content: My first take of your problem with low-amplitude signals was 'lack of damping'. The advice above about using proper Bessel coefficients is a good one. My advice is to use Bessel coefficients for your 3-stage passive filter design and then simulate it with a step-function input and make sure there is no overshoot. This will yield a rather gradual roll-off with frequency but should solve your low-amplitude problems.

author: Cpuch22
author_rank: Contributor
timestamp: August 03, 2025, 12:54:39 pm
content: I use this website for filter design. They have pretty much any type of filter you could want; active and passive of many different orders and topologies:http://sim.okawa-denshi.jp/en/Fkeisan.htm

author: PCB.Wiz
author_rank: Super Contributor
timestamp: August 03, 2025, 07:41:25 pm
content: Quote from: janjohansen on July 30, 2025, 10:49:11 pmThat’s notbecause of the passivefilter, that’s your system noise floor.As already mentioned that is a poor, soft filter.You should test the rise times of DAC filters, not the ultimate roll off.You want the best rise-time to ripple ratio.What exactly is this feeding ?Is this in a feedback loop, or do you need absolute precision?You can add active elements, but each one adds noise, offsets and non linearity.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: August 04, 2025, 05:37:55 am
content: Quote from: srb1954 on July 31, 2025, 03:03:44 amThe early EDN notes on ripple cancel, are mostly mirage.if you compare rise times ofequal ripple operating points, using 2R and 2C you get better rise times from a 2nd order LPF than you do from their 'ripple cancel'.ie the same component count in a simpler circuit, works better.If driving a DAC, you may be interested in both rise time and settling time to some < 1% and for the latter, a filter with a slight overshoot may be faster than a 100% passive RC filter.

author: Ian.M
author_rank: Super Contributor
timestamp: August 04, 2025, 06:43:53 am
content: Yes, seehttps://www.eevblog.com/forum/metrology/pwm-new-article-from-s-woodward/where we simulated and debunked them.