Sallen-Key HP filter driving me nuts - Page 1
Forum section: electronics / projects

author: Ice-Tea
author_rank: Super Contributor
timestamp: March 29, 2016, 03:04:52 pm
content: OK, this sums it up:I have been working on a quad, selectable HP filter. For that, I've been using the absolutely greatAnalog Devices Filter Wizard. Needing 60+ dB/decade with a flat pass characteristic I fiddled with the wizard untill it gave me a4th order Butterworth with 2 active stages.That sounds about right, in addition, the graph gives me 80dB, another one tells me to expect 360° phase delay: all good for a 4th order filter. So, I did this 4 times and came up with this:Pretty mych exactly what the wizard gave me. The only difference is the 1:1 buffer at the entrance (to avoid loading the source too much)and the 1K resistors at the output (it's a MBB rotary switch, didn't want two opamps having a go at each other during transit).Works pretty nicely too, except... It's not 80dB. It's 60dB. Digging a bit deeper: the first stage does a fine 40dB job, the second does only 20. In addition, around the cutoff frequency the I get, say, 0.5dB amplification iso attenuation which actually corresponds to the second stage not performing as it should (the first stage is supposed to 'overshoot' a bit, the second is supposed to 'undershoot' to make the cutoff sharper).Sooo... I dunno. I don't get what is happening. Pointers?

author: Kleinstein
author_rank: Super Contributor
timestamp: March 29, 2016, 05:09:53 pm
content: The output is taken from the wrong point. The 1 K resistors (could be a little smaller) should go directly from the OP.

author: Ice-Tea
author_rank: Super Contributor
timestamp: March 29, 2016, 05:14:28 pm
content: I'm going to kill myself.And you, Sir, I own a beer!

author: danadak
author_rank: Super Contributor
timestamp: March 29, 2016, 08:07:42 pm
content: Just a thought but the accuracy and sensitivities of the filter dominated by T sensitivepassives. A FIR implementation in a processor, coefficients changed for the roll off desiredfrequency, easy to achieve.Cypress PSOC has a DFB (Digital Filter Block) in it, as well as opamps, comparators, CPU,DAC, A/D.........Even the lowest ended part, no DFB but a fast multiplier, could handle this.And cutoff a f() of the clock rate, not passives.Just a thought.Do you need linear phase ? Settling time / latency an issue ?Regards, Dana.

author: Ice-Tea
author_rank: Super Contributor
timestamp: March 29, 2016, 08:49:40 pm
content: Actually proposed a digital filter to but the customer doesn't want it.

author: uncle_bob
author_rank: Unknown
timestamp: March 29, 2016, 11:57:01 pm
content: Quote from: Ice-Tea on March 29, 2016, 08:49:40 pmHiMake sure they understand that each one will need to be tested and individually tweaked prior to shipment ....I've spent a *lot* of time doing exactly that, on a number of analog filter designs.Bob

author: danadak
author_rank: Super Contributor
timestamp: March 30, 2016, 11:12:58 am
content: Switch Capacitor filters -http://cds.linear.com/docs/en/application-note/an40f.pdfRegards, Dana.

author: rs20
author_rank: Super Contributor
timestamp: March 30, 2016, 11:28:14 am
content: Quote from: Ice-Tea on March 29, 2016, 08:49:40 pmLike above, my response to this is to ask whether you:Explained the advantages of digitalExplained the disadvantages of analogExplained the invalidity of common misconception about digital

author: Ice-Tea
author_rank: Super Contributor
timestamp: March 30, 2016, 11:43:02 am
content: Quote from: danadak on March 30, 2016, 11:12:58 amThe switched capacitor filters, at least the monolitic blocks, have a range issue. Often the clock is 1/100 or something like that wrt to the filter frequency. With a HP cutoff of 20 Hz that's a bit of a problem. And yes, I explained the advantages of digital, explained the disadvantages of analog and explained the misconceptions. All that pretty much goes out of the window if the reply is "if I don't understand it I don't want it". Truth be told: I'm gathering some components left and right to build a full digital system (not just for the HP filter but for a lot of other functions around it) to demo to him at some point. But as that is a non-paying job that doesn't ewactly have the highes priority.

author: uncle_bob
author_rank: Unknown
timestamp: April 02, 2016, 05:05:35 pm
content: Quote from: Ice-Tea on March 30, 2016, 11:43:02 amHiIf it's going into production with an analog filter, make sure they are set up with a 100% sweep test and adjust as part of their process. That's not going to be cheap... If they don't believe it's needed, do a simulation to show just how bad it will be.Bob

author: Ice-Tea
author_rank: Super Contributor
timestamp: April 02, 2016, 05:53:21 pm
content: They are not terribly fuzzed by the sharpness of the knee or the exact cutoff frequency. Actually, the wizard simulates the influence of component spread and with 1% res and 5% caps it's not all that bad. At least not for their application.

author: uncle_bob
author_rank: Unknown
timestamp: April 02, 2016, 06:49:13 pm
content: Quote from: Ice-Tea on April 02, 2016, 05:53:21 pmHiWhat's likely to happen if you just ignore it:1) The caps all come in on one side of the tolerance band. (or out of the tolerance band).2) The caps have a temperature coefficient (or voltage or humidity or stress or solder heat or...) that the wizard didn't model3) The whole thing shifts to a wrong cutoff *and* to much ripple at once.4) The problem is discovered by the ultimate end customer when the gizmo fails in some odd way5) This becomes apparent after XXX pieces have been shipped to the field (since no testing is being done)You don't want to know how many times I've seen this sort of train wreck happen to people. This doesn't even include the "new materials guy" who discovers that -20% / + 80% Y5V caps cost a lot less money and makes his bonus with a minor change to the BOM (No, that's not a hypothetical, it actually happened).Bob

author: Ice-Tea
author_rank: Super Contributor
timestamp: April 02, 2016, 08:06:49 pm
content: Heheheh... I once worked as a HPL that oversaw production of some high volume Electronics with a Chinese ODM. One of the things I did was toss out all Y5V. You will never any in a design I signed off on.The caps are C0G. If he ever changes them behind my back that's his problem..