High order filters: how much is too much?  - Page 1
Forum section: electronics / beginners

author: NaxFM
author_rank: Regular Contributor
timestamp: December 27, 2022, 09:35:13 pm
content: So, it was time for me to enter the insidious and magic analog realm, and what better way to start with one of the most basic building blocks: the humble low pass filter.For a circuit i need a very steep roll off and for this I need something like a 6th order filter.I was playing in LTspice with the Sallen-key topology using real opamps, and everything works wonderfully with a 6th order filter.I added another filter to make an 8th order filter and, aside of a capacitor which may be a bit too large (but still reasonable), everything worked perfectly.However it was too perfect.I started wondering: can I really add as much filters as I want in the real world? In the simulations everything is good, but what about a real pcb?I'm dealing with very low frequencies, the bandwidth is just 300Hz, so the parasitic inductances and capacitance of the passives should be basically nothing.The opamp I'm using has a bandwidth of 1.5MHz, well above the "50*fc" minimum frequency I found in the art of electronics (fc is the cut off frequency of the filter).It just seems too easy, I'm sure there must something which limits our abilities to keep adding filter stages, but I just don't know what it may be.Is it just the capacitors becoming bigger and bigger or there's something more?When something looks too easy on paper I always start to have doubts...

author: Benta
author_rank: Super Contributor
timestamp: December 27, 2022, 09:59:55 pm
content: In LTSpice you can do anything.In the real world there are:TolerancesNoiseDistortionSensitivitiesDC bias/offsetTemperature drift...I'm sure others will add to the list, this was compiled in 30 seconds.

author: radiolistener
author_rank: Super Contributor
timestamp: December 27, 2022, 10:21:46 pm
content: 7 order filter for about 100 MHz is hard to tune, so I think higher filter order practically not worth the cost

author: Benta
author_rank: Super Contributor
timestamp: December 27, 2022, 10:27:34 pm
content: Quote from: radiolistener on December 27, 2022, 10:21:46 pmThe OP is talking about 300 Hz.

author: nfmax
author_rank: Super Contributor
timestamp: December 27, 2022, 10:28:29 pm
content: As you increase the filter order by adding stage after stage, the Q factor of each added pole-pair grows. How rapidly it grows depends on the filter response characteristic, but it always grows. Eventually, you will run out of OPAMP gain-bandwidth...

author: Benta
author_rank: Super Contributor
timestamp: December 27, 2022, 11:17:25 pm
content: Quote from: nfmax on December 27, 2022, 10:28:29 pmYou'll need to back up that postulate. Sources?

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: December 28, 2022, 01:10:28 am
content: If you get the filter too tight, you get ringing or self oscillation.Happens in radios when using a narrow filter..

author: jonpaul
author_rank: Super Contributor
timestamp: December 28, 2022, 02:45:45 am
content: Signal analysis theory reveals the tradefoff between filter steepness, pôles, and phase, time domain responseSharpest band stop gives worst time domainSee any filter design handbook,use a phase controlled type for fine audio, like be sel or Butterworth and reserve elliptic and tcpschbyschev for non audio instruments.Jon

author: rstofer
author_rank: Super Contributor
timestamp: December 28, 2022, 02:54:50 am
content: You can read about filters, talk about them and even simulate them but until you build one and make real world measurements, you have incomplete knowledge.Considering the low frequencies involved, a breadboard will probably work just fine.This is the kind of experimentation I do with my Analog Discovery 2. It's too bad they jacked up the price and now it is out of range for most hobbyists.

author: BrianHG
author_rank: Super Contributor
timestamp: December 28, 2022, 03:01:35 am
content: If your signal is under 300Hz and you know how to code, you can use a DSP/MCU with a built in ADC and DAC & software engineer your filter. Now, you can go nuts on the height of your 'order'. IE: 9th order and above is possible.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: December 28, 2022, 03:27:29 am
content: Hi,You can use the Monte Carlo analysis tool in SPICE to see the impact of component tolerances.Here I am using ideal resistors and 5% capacitors.{mc(1n, 0.05)}means mc for monte carlo - random selection1n nominal value0.05 = 5% toleranceThis is equivalent to building 50 filters with 5% capacitors and measuring the frequency response of each filter.Regards,Jay_Diddy_B

author: Terry Bites
author_rank: Super Contributor
timestamp: December 28, 2022, 06:47:12 am
content: Its best to work out the filter specifications first. As pointed out too much roll off comes at a price.Only use what you really need. Going for higher orders than you need is poor design.If you want a fully optimised brick wall response, then digital filters are the way to go- plenty of design tools out there.

author: Kalvin
author_rank: Super Contributor
timestamp: December 28, 2022, 08:46:38 am
content: A general note about active filter design: Compose the filters using cascaded 2nd order sections. In that way the effect of the component tolerances will remain in control. There are many filter topologies and references available online that contain information for calculating component values for different filter types.Quote from: Terry Bites on December 28, 2022, 06:47:12 amSwitched-capacitor filters are a hybrid between analog and digital domains, and they can be used to implement various filter topologies quite easily. For some applications SCFs may introduce too high switching noise, though.Take the Mystery Out of the Switched-Capacitor Filter: The System Designer’s Filter Compendium:https://www.analog.com/media/en/technical-documentation/application-notes/an40f.pdfFor example, there are quite many SCFs available that provide 8th order elliptic low-pass filter with steep and well-behaving stopband transition.

author: srb1954
author_rank: Super Contributor
timestamp: December 28, 2022, 10:02:35 am
content: Quote from: nfmax on December 27, 2022, 10:28:29 pmAnd the tolerance requirements on the passive components becomes more and more critical for the stages with the high Q-factor.The effect of component tolerances can be checked by doing a Monte-Carlo analysis in LTSpice. This will show the effects of component variations from their correct design values, which is mostly exhibited as significant deviations from the desired response in the transition region from pass-band to stop-band.

author: srb1954
author_rank: Super Contributor
timestamp: December 28, 2022, 10:08:38 am
content: Quote from: Benta on December 27, 2022, 09:59:55 pmAnd in LTSpice there are no real-world parasitic component effects such as stray capacitances and inductances, supply line impedances, ground track impedances etc unless you specifically include them in the model.

author: MrAl
author_rank: Super Contributor
timestamp: December 28, 2022, 11:01:55 am
content: Hello,One of the hardest things to deal with for a high order analog filter is tuning. As the response becomes sharper it becomes harder to get the cutoff or center frequency just right. Even a passive twin tee notch filter is hard to tune just right and the component values drift with temperature so the values change and that shifts the notch frequency enough to make a really good notch do much less. If you have any doubts, just create a 60Hz passive notch filter (or 50Hz depending on where you live) to filter out the line frequency from an audio amplifier and see how just the tiniest change in a capacitor value or even resistor value can shift the notch so much that you still easily hear the line frequency hum. When you tune it you get really good cut of the line frequency hum, but just let it sit for a few minutes and it comes right back.That's an analog filter for ya. Digital filters are different though. They can be made sharper due the way they work. There are different kinds of digital filters you can look up on the web. They do have their own problems though so you'd have to study them out to get to know how to use one.