Simulation a second order low pass filter with ADA4940 - Page 1
Forum section: electronics / beginners

author: oyu
author_rank: Newbie
timestamp: January 14, 2022, 08:43:24 am
content: Hello,I am trying to simulate a second order low pass filter using ADA4940-2 Fully Differential ADC Driver as show in the following circuit attached.I' am getting the trace as shown in the second attached picture. I' am using RC filter to cut at 3Khz and i am not sure about my filter response and i don't understand why it started at-20dB not at 0dB.Could someone help me please to understand my trace.Thank you!

author: Benta
author_rank: Super Contributor
timestamp: January 14, 2022, 04:56:07 pm
content: That's not a second-order filter, it's two first order filters in series.That's a very inefficient way of building active filters.The amplitude rise at low frequencies is due to your input coupling caps.

author: rstofer
author_rank: Super Contributor
timestamp: January 14, 2022, 06:51:21 pm
content: https://www.electronics-tutorials.ws/filter/second-order-filters.html

author: Benta
author_rank: Super Contributor
timestamp: January 14, 2022, 07:54:18 pm
content: Quote from: rstofer on January 14, 2022, 06:51:21 pmGreat link... NOT!Just the page start lays the groundwork:"Second Order (or two-pole) Filters consist of two RC filter sections connected together to provide a -40dB/decade roll-off rate."A bit further down:"Most designs of second order filters are generally named after their inventor with the most common filter types being: Butterworth, Chebyshev, Bessel and Sallen-Key."Lovely. That's when I navigated away from the page. I can't even imagine who writes such claptrap.

author: SilverSolder
author_rank: Super Contributor
timestamp: January 14, 2022, 08:47:05 pm
content: The input capacitors/resistors are a high pass filter.Then, the two low pass filters in series becomes a second order low pass filter.Try running the sweep frequency from 1Hz to 10KHz, and this might all look more clear.

author: Benta
author_rank: Super Contributor
timestamp: January 14, 2022, 08:58:33 pm
content: Quote from: SilverSolder on January 14, 2022, 08:47:05 pmAgreed, and what I said earlier.Concerning the "second order" part: yes, it's technically a second order filter with two real poles. I just prefer to call it "cascaded first-order filters".That's a wasteful way of building a second-order filter, where you'll get much better performance from a filter with complex conjugate poles.

author: SilverSolder
author_rank: Super Contributor
timestamp: January 14, 2022, 09:17:11 pm
content: Quote from: Benta on January 14, 2022, 08:58:33 pmI don't want to divert the thread too much, but does the response actually depend on how a second order low pass filter (LP2) is implemented? I would have thought that the response of a LP2 would be the same, no matter how you implement it?

author: Benta
author_rank: Super Contributor
timestamp: January 14, 2022, 09:34:42 pm
content: Quote from: SilverSolder on January 14, 2022, 09:17:11 pmI have a "sticky" post at the very top in the "Projects" section, which should give some answers.Yes, it matters a lot. When the frequency increases towards infinity, there's no difference.However, when we talk about "proper" second-order filters (complex-conjugate poles), it's a different story.Whether it's Butterworth, Bessel, Optimum, Chebyschev or Intermediate filter responses, the frequency and phase response in the pass-band, at the cut-off point and in the roll-off band are significantly different.

author: SilverSolder
author_rank: Super Contributor
timestamp: January 14, 2022, 09:40:29 pm
content: OK, I see what you mean now.But you could implement a Butterworth LP2 as two cascaded LP1, or a single LP2 built around a single op amp, and the response would be the same? (Just more wasteful with two amps, as you said)Reading your sticky now...

author: Benta
author_rank: Super Contributor
timestamp: January 14, 2022, 09:53:32 pm
content: Quote from: SilverSolder on January 14, 2022, 09:40:29 pmNo, you can't. Two cascaded LP1cannotprovide a Butterworth response.

author: SilverSolder
author_rank: Super Contributor
timestamp: January 14, 2022, 10:51:02 pm
content: I'm still not understanding. I simulated a Sallen Key butterworth filter, and a simple cascade of two passive RC stages, and the responses are so close that you can't even see any difference in LTSPICE.What am I overlooking?

author: Benta
author_rank: Super Contributor
timestamp: January 15, 2022, 11:31:22 am
content: Well, yes.You designed two filters with identical characteristics, so the simulation will be almost identical.But the crux is: none of them are Butterworth filters.Try with:C1: 33 nFC2: 10 nFR1: 4.22 kohmR2: 18.2 kohmAnd do the simulation. This is for a cutoff frequency of 1 kHz.

author: Terry Bites
author_rank: Super Contributor
timestamp: January 15, 2022, 05:00:56 pm
content: If you really need to create a filter with this sort of driver,you could use this configuation:

author: SilverSolder
author_rank: Super Contributor
timestamp: January 15, 2022, 05:29:17 pm
content: Quote from: Benta on January 15, 2022, 11:31:22 amI see what you mean now. With those values, the filter depends on the gain of the op amp and cannot be implemented passively.In fairness, the original poster's circuit did not have that kind of characteristic!Re. Butterworth, this is a pretty common characteristic in loudspeaker crossovers, so it should be implementable purely passively?

author: Benta
author_rank: Super Contributor
timestamp: January 15, 2022, 07:22:11 pm
content: Quote from: SilverSolder on January 15, 2022, 05:29:17 pmOf course.The point is, that you need both positive and negative 90 degrees phase shift for a proper second-order response. With passive filters, this is only possible using capacitorsandinductors.With active filters it can be done using only capacitors.

author: SiliconWizard
author_rank: Super Contributor
timestamp: January 15, 2022, 07:41:38 pm
content: Note that you should plot 'V(out_hi) - V(out-lo)' if you want the gain to mean anything here.

author: Benta
author_rank: Super Contributor
timestamp: January 15, 2022, 07:59:15 pm
content: Quote from: Terry Bites on January 15, 2022, 05:00:56 pmAs the OP never returned, it seems our answers are uninteresting.

author: SilverSolder
author_rank: Super Contributor
timestamp: January 15, 2022, 08:02:36 pm
content: The discussion has taken on a life of its own, lol!I found you can actually implement the characteristic you quoted earlier using a cascade of stages - the secret is in the positive feedback!

author: Benta
author_rank: Super Contributor
timestamp: January 15, 2022, 08:08:44 pm
content: Sure you can.If you have too many opamps in stock, it's a good way of getting rid of them. Lots of topologies out there.But at least you now have a real Butterworth response.

author: SilverSolder
author_rank: Super Contributor
timestamp: January 15, 2022, 08:41:59 pm
content: Quote from: Benta on January 15, 2022, 08:08:44 pmI'm not totally convinced about Butterworth needing this kind of response, though. Isn't the definition just "maximally flat"?For example, if you have a loudspeaker cross-over network, the behavior of the phase around the crossover point (-3dB) is important. The complex congjugate filter we've been looking at above has 90 degree phase shift at the 3dB point. Probably not ideal, if the midrange also has 90 degrees in the opposite direction at that point!So perhaps for some applications, it is better to give up the sharper knee of the amplitude curve, in exchange for better phase behaviour.

author: Benta
author_rank: Super Contributor
timestamp: January 15, 2022, 08:52:24 pm
content: If you want a different discussion, please start a new thread. This has gone too far off course now.

author: Benta
author_rank: Super Contributor
timestamp: January 15, 2022, 10:22:16 pm
content: To the OP (should you return):This would be a much better filter. Configurable to any response (Butterworth, Bessel etc.). Input buffering is missing, I'll leave that to you,

author: mawyatt
author_rank: Super Contributor
timestamp: January 15, 2022, 10:54:27 pm
content: Quote from: SilverSolder on January 15, 2022, 08:02:36 pmYes, using the interstage op amps isolates the RC networks. If you follow the last opamp output in the cascade with a series R and shunt C you can create a 3rd Butterworth LP with all equal value Rs and all equal value Cs that has a radian corner frequency of 1/(RC)If you interchange the all the Rs and Cs it now becomes a 3rd Order Butterworth HP filter with a radian corner of 1/(RC)Best,

author: Benta
author_rank: Super Contributor
timestamp: January 15, 2022, 11:06:20 pm
content: PLEASE! PLEASE! PLEASE!Read the OP and stop this. Start A NEW THREAD!Thank You and excuse me for shouting.

author: SilverSolder
author_rank: Super Contributor
timestamp: January 16, 2022, 12:29:30 am
content: Quote from: mawyatt on January 15, 2022, 10:54:27 pmThe thing I'm not sure about is - what is the definition of a Butterworth filter?One article I've been reading says that a Butterworth response out of a Sallen-Key requires R1=R2, C1=C2, and a Q of 0.71 - but I'm new to all this...Article:https://www.sciencedirect.com/topics/engineering/sallen-keySorry @Benta, the OP has disappeared so the thread is now officially temporarily hijacked!

author: mawyatt
author_rank: Super Contributor
timestamp: January 16, 2022, 02:56:21 am
content: The 3rd Order Butterworth Polynominal is S^3 + 2*S^2 + 2*S + 1.Best

author: Benta
author_rank: Super Contributor
timestamp: January 16, 2022, 11:31:30 am
content: Quote from: SilverSolder on January 16, 2022, 12:29:30 amOK, I can go with that, but it makes it difficult to find the thread later.

author: Benta
author_rank: Super Contributor
timestamp: January 16, 2022, 12:42:01 pm
content: Quote from: SilverSolder on January 16, 2022, 12:29:30 amI have a feeling you're mixing things up.Transfer function and filter topology are two completely different things.The transfer function is a complex polynomium that describes the relationship between input and output of a linear circuit. In other words: it's mathematics.Specifically on analog filters, different mathematician have found optimizations for different behaviour:Butterworth_ maximally flat amplitude response in the pass band.Bessel: maximally flat phase response in the pass band.Chebyshev: maximum steepness at cut off.Sallen-Key, MFB, state variable, are names for active filter topologies (schematics) and say absolutely nothing about the transfer function.As an example, a Sallen-Key can be Butterworth, Bessel or whatever, that's just a question of the component values.The same is true for passive RLC filters, that can be L, T, Pi etc.It's essential to keep these two things apart.

author: SilverSolder
author_rank: Super Contributor
timestamp: January 16, 2022, 12:43:56 pm
content: Quote from: Benta on January 16, 2022, 11:31:30 amI think it's worth a separate thread, you are right.

author: mawyatt
author_rank: Super Contributor
timestamp: January 16, 2022, 01:27:45 pm
content: Quote from: Benta on January 16, 2022, 12:42:01 pmBessel is NOT maximally flat Phase Response but maximally flat "Group Delay". Group Delay is the first derivative of Phase, thus Bessel has a Linear Phase Response.Chebyshev do NOT have maximum steepness at cutoff, that filter is called an Elliptic or Cauer.Best,

author: Benta
author_rank: Super Contributor
timestamp: January 16, 2022, 01:59:42 pm
content: Quote from: mawyatt on January 16, 2022, 01:27:45 pmYou're technically right, if you define that "flat" means horizontal.Concerning Cauer/Elliptic: I was under the impression that we're talking about second-order, low-pass filters with complex conjugate poles. Cauer also needs zeros.I'm not sure that nitpicking brings more clarity.

author: SilverSolder
author_rank: Super Contributor
timestamp: January 16, 2022, 02:17:47 pm
content: I created a new topic here:https://www.eevblog.com/forum/beginners/putting-the-butter-into-butterworth-what-makes-a-filter-a-butterworth/msg3944956/#msg3944956

author: SilverSolder
author_rank: Super Contributor
timestamp: January 16, 2022, 02:21:54 pm
content: Quote from: Benta on January 16, 2022, 12:42:01 pmThis is probably lesson 1 to understand filters. Copy to other thread? Might help other n00bs...

author: mawyatt
author_rank: Super Contributor
timestamp: January 16, 2022, 02:41:59 pm
content: Quote from: Benta on January 16, 2022, 01:59:42 pmFrankly, I'm not sure that incorrect information brings more clarityBTW maximally flat for Butterworth and Bessel filter terminology refers to maximally flat at the origin. This means the derivative is identically zero at the origin, these are the extremes in magnitude and group delay respectively.Best,