clipping a fast negative pulse - Page 1
Forum section: electronics / projects

author: brumbarchris
author_rank: Regular Contributor
timestamp: May 08, 2019, 09:43:17 am
content: So I have to deal with a big and fast negative pulse, as in the attached picture. The requirement is to reliably and as accurately as possible (nice spec, right?) clip or clamp this pulse to a fixed voltage level (does not need to be adjustable) between -0.8V and -1.4V across temperature (-20C to 60C) and across the parameter variations that the components employed would have.What are your suggestions?Best regards,Cristian

author: OwO
author_rank: Super Contributor
timestamp: May 08, 2019, 09:58:25 am
content: Source impedance? is adding another series resistor acceptable?

author: brumbarchris
author_rank: Regular Contributor
timestamp: May 08, 2019, 10:05:37 am
content: This pulse is coming off of a ~2mH coil which has some 10Ohm DC resistance, when supply current is suddenly cut. We also have a 1k damping resistor in parallel with the coil.

author: dzseki
author_rank: Frequent Contributor
timestamp: May 08, 2019, 10:17:29 am
content: Usualy a diode is used for such application.

author: AndyC_772
author_rank: Super Contributor
timestamp: May 08, 2019, 10:21:15 am
content: Do you mean it needs to be clamped to a fixed, accurately controlled, known level (which happens to be between those limits)?Or, is the requirement simply to keep the voltage within those limits in order to not damage something?If it's the latter, then a fast silicon diode to GND may be all you need. If it's the former, you'll need something much more involved and complex.

author: brumbarchris
author_rank: Regular Contributor
timestamp: May 08, 2019, 11:06:18 am
content: This is what I need, sorry for not making it more clear to begin with:QuoteYes!A simple diode will not do, as it is by far not accurate enough. To put a figure on it, let's say the clamping voltage needs to have a tolerance of +/-20mV, and of course, it has to be within the already stated limits.Best regards,Cristian

author: AndyC_772
author_rank: Super Contributor
timestamp: May 08, 2019, 12:32:47 pm
content: In that case, I'd probably look toward using a fast comparator to control an SPDT analogue switch.When the input is above threshold, the switch output is connected to the input. When it's below threshold, the comparator output flips state and switches the threshold voltage directly through to the output.You'll almost certainly want some input scaling, an output buffer, and an actual protection diode too. Maybe use two diodes in series to give you a worst case (ie. most negative possible) input of ~-1.5V.

author: OM222O
author_rank: Frequent Contributor
timestamp: May 08, 2019, 01:59:57 pm
content: simple solutions include: a schottky diode going from output to ground (with the positive lead (anode) connected to ground) and a snubber RC circuit.Edit: I'm not sure how fast each method is, but they are the go to for this application, you should be able to find suitable parts if you go through a lot of datasheets

author: brumbarchris
author_rank: Regular Contributor
timestamp: May 08, 2019, 02:59:54 pm
content: The variable forward drop on the schottky is far to great for this application. Definitely larger than 20mV, even for the same device, across temperature.Comparator + SPDT analogue switch is a possible option, albeit a bit more IC heavy than the single opamp solution I had in mind, which is depicted in the attached picture. Of course, this needs to use a fast opamp, but also the comparator would have to be fairly fast.

author: OM222O
author_rank: Frequent Contributor
timestamp: May 08, 2019, 03:24:16 pm
content: Quote from: brumbarchris on May 08, 2019, 02:59:54 pmthat solution is also using a diodehow can that be any more accurate? It adds accuracy to the triggering point if that's what you're after, but the output voltage will still vary significantly based on the diode temperature, current, etcThe main questions is: why do you want such a precise negative voltage for? if it's meant to be used for something, then just use a charge pump to create the negative voltage. if it's meant for protection, then you don't need 20mV accuracy?

author: dzseki
author_rank: Frequent Contributor
timestamp: May 08, 2019, 04:09:28 pm
content: Also a question: do you need to actually clamp that pulse or you just need an output voltage as per specified during the pulse?What about a "comparator like" feedback loop using an opamp that drives a shunt transistor, if the voltage is above say 0V the transistor is closed, if the pulse gets negative the opamp and the transistor would clamp the output at the threshold voltage.

author: StillTrying
author_rank: Super Contributor
timestamp: May 08, 2019, 04:47:06 pm
content: There's not enough clear information for sensible suggestions. Such as whether it's the 64V pulse that needs to be clamped, or it just needs to be prevented from getting to the next stage. And if the next stage is only ever going to see a very stable -0.8V then that's easy.

author: OM222O
author_rank: Frequent Contributor
timestamp: May 08, 2019, 05:01:45 pm
content: Quote from: dzseki on May 08, 2019, 04:09:28 pmthe main question still remains: WHY WOULD YOU WANT TO CLIP A PULSE TO A VERY SPECIFIC VOLTAGE WITH LESS THAN 20mv ACCURACY!if a negative rail is needed, this a really bad way of generating it, and if it's done for protection purposes, then the accuracy should not matter

author: capt bullshot
author_rank: Super Contributor
timestamp: May 08, 2019, 05:01:56 pm
content: So the circuit doesn't have to absorb the pulse energy but just to limit the negative voltage to a stable level, and you can apply a high source impedance like that 2k2 resistor shown in your circuit?So one might consider a fast enough Rail-to-Rail I/O OpAmp to buffer the signal. Apply some coarse input protection using diodes, set its negative supply voltage to the desired clamping level and have your output clamped to the negative voltage.Or do you need a signal path without active devices in the non-clamping state, like your circuit? One can speed up the OpAmp by doing some biasing tricks to prevent output saturation in the non-clamping state. This will increase the clamping action at the falling edge of your signal.

author: SiliconWizard
author_rank: Super Contributor
timestamp: May 08, 2019, 06:42:37 pm
content: Quote from: OM222O on May 08, 2019, 05:01:45 pmI'm not sure why either. Especially since the OP mentioned at first that it should clamp between -0.8V and -1.4V. So if 20mV accuracy is needed, does that imply that this clamping voltage is going to be adjustable? Because this is the only way I see that those two requirements would match?

author: OM222O
author_rank: Frequent Contributor
timestamp: May 08, 2019, 07:10:19 pm
content: Quote from: SiliconWizard on May 08, 2019, 06:42:37 pmHe mentioned it's NOT ADJUSTABLE and the value doesn't matter. it just has to be accurate for some reason

author: David Hess
author_rank: Super Contributor
timestamp: May 08, 2019, 09:15:39 pm
content: Quote from: brumbarchris on May 08, 2019, 11:06:18 amQuote from: brumbarchris on May 08, 2019, 02:59:54 pmWhen diodes are used as precision clamps, then usually two are used with one compensating for the voltage drop and change in voltage drop with temperature of the other. This leaves the logarithmic change in voltage with linear change in current (60 millivolts per decade) but you gave no specification for current.If more precision is required and the speed is not too great, then a shunt regulator can be used as a clamp.

author: brumbarchris
author_rank: Regular Contributor
timestamp: May 09, 2019, 09:33:19 am
content: Hello everybody, thank you for your replies and patience.Here are some clarifications:QuoteThe signal is going to be then "signal processed" and eventually fed to an ADC input. As there will be a large number of products that will be manufactured, we will need to account for component-to-component variation between various samples. I guess one of the concerns, something I should have probably mentioned in the first post, is that once the input signal goes above -0.8V it must be not altered or distorted in any way. That is where the actual information that we are interested in is.QuoteAs mentioned in the OP, the level does not need to be adjustable, but it has to be precise. So for instance if you suggest a solution that always clamps at -1.2V+/-20mV that is good. Equally good to a solution that clamps at -0.9V+/-20mV. Of course, the nominal clamping level needs to be known (if it is 1.2V or if it is 0.9V) and reproduce able across samples.QuoteUnfortunately speed is quite high, and I do not think a basic shunt regulator would provide this accuracy, particularly if it is BJT based.QuoteIt is definitely more accurate as the opamp will drive its output so as to compensate any variations in the diode voltage drop until the VOUT node (which in this case is connected directly to the inverting input of the opamp) reaches the same value. AFAIK, the opamp drives its output so as to always keep its inputs at an equal level.QuoteCould you elaborate a bit more on that please? It's "sounds" relatively similar with the opamp based variant that I was considering, and the schematic of which I indicated in one of the above posts.Best regards,Cristian

author: capt bullshot
author_rank: Super Contributor
timestamp: May 09, 2019, 01:07:29 pm
content: See that quick and dirty schematic, hope you get the point. Component choice is up to your requirements, choose R2 to limit the current into the OpAmp input to a level typically lower than 5mA, better 1mA (check the datasheet and appnotes).Anyway, your approach appears rather unusual, for what you describe (accurate and fast measure the input signal above the clamping level) I would consider coarse clamping by diodes and wouldn't rely on a stable clamping level for the rest of the circuitry. Maybe you should re-think the rest of the design once more.Edit: AFAIR, some special OpAmps exist that allow for signal clamping to a defined level (set by a voltage applied to a pin), independent of the supply voltage. I believe, they were mainly used in video applications.

author: OM222O
author_rank: Frequent Contributor
timestamp: May 09, 2019, 01:11:34 pm
content: A basic simulation here:http://tinyurl.com/y4eeqnjdit shows that the negative edge is clipped well, but so is the positive edge due to the supply rails of the op amp (+-15v) so that can be a limitation.Also it has so sink / source considerable current in order to do that (about 30mA which is on the high side for op amps, they're not meant for driving loads)I'm not sure if this will actually work in real life when you build it.

author: David Hess
author_rank: Super Contributor
timestamp: May 09, 2019, 05:09:53 pm
content: There is another way to make a precision clamp with diodes and it is very fast.Use the signal, now low impedance, to drive the input of a diode bridge. Set the top and bottom currents through the bridge to produce the correct clamp levels into the load impedance. Some oscilloscope input protection circuits used this method as shown below in the Tektronix 1GHz 7A29 vertical amplifier.

author: Marco
author_rank: Super Contributor
timestamp: May 09, 2019, 06:31:30 pm
content: That's more a clipped voltage follower, the input does not provide the power and is not itself clipped.The only reason I can think of for the requested circuit is that the flyback voltage on the primary of a transformer needs to be limited to a precise voltage to also generate a precise voltage on the secondary.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 09, 2019, 07:06:26 pm
content: Wait. Why are you clipping the pulse youfought so hard to perfect?Tim

author: brumbarchris
author_rank: Regular Contributor
timestamp: May 09, 2019, 08:24:29 pm
content: QuoteIn order to further signal process it and feed it to an ADC eventually.QuoteHey, nice simulation. But it actually suggests the circuit would work. Clipping the positive side of the signal is not a problem, as I do not need to clip a sinewave, but a mostly negative waveform, as indicated in the OP. The opamp will have to be appropriately selected, of course, speed and current output are critical, offset and other usually important parameters not so much, in this case.QuoteThank you for the suggested schematic, kapitän, but would that circuit be any more accurate than the clamping diodes? Their forward voltage varies quite a lot!Best regards,Cristian

author: Marco
author_rank: Super Contributor
timestamp: May 09, 2019, 09:48:12 pm
content: What's there to ADC if your clipping circuit works?

author: capt bullshot
author_rank: Super Contributor
timestamp: May 09, 2019, 09:57:51 pm
content: Quote from: brumbarchris on May 09, 2019, 08:24:29 pmYes, the "precision" clipping isn't done by the diodes but the OpAmp. The negative supply is e.g. -0.8V, if you use an RRIO amp, the output voltage follows the input as long as it is above -0.8V, but can't go below. The diodes clip to -(0.8V + diode drop voltage) to protect the amp input.

author: OwO
author_rank: Super Contributor
timestamp: May 10, 2019, 03:48:51 am
content: I don't understand why you can't just clip the signal inaccurately first and then clip it again in software after it is digitized.

author: brumbarchris
author_rank: Regular Contributor
timestamp: May 10, 2019, 05:48:52 am
content: QuoteThe remaining slow decaying waveform between -0.8V and GND. That is where the actual information in this signal is.QuoteI'll have to look at that in more detail then, could be a winner!QuoteWe could probably do so, but as the whole system is a bit unblurry now, and we do not know yet what we'll use down the line exactly, we want to make it as easy as possible for it and take the difficulties in the portions of circuits that we are designing now. It might be that later, when we will ourselves know more about these (what type of ADC we'll use, what analog signal processing we will employ before it, how the signal slope varies from product to product etc; we first need to learn some things and perform additional measurements), we could also do what you suggest here.Best regards,Cristian

author: OM222O
author_rank: Frequent Contributor
timestamp: May 10, 2019, 06:07:28 am
content: well then let me tell you that you're building your circuit backwards. the saying would be "making a coat for a button that you found" ... when nothing about your circuit is determined (something as basic as the ADC!) why do you even bother creating analog section for a very specific type of signal? what kind of information are you even trying to look at that can't be gathered from simulations? LTSpice is very accurate in that regard.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 10, 2019, 06:19:18 am
content: This sounds very much like an eddy current sensor, for whatever purpose: material resistivity, thickness, metal detection or identification, etc. You don't need to be cryptic about it, there's nothing special about this.The impulse method never impressed me. In particular, why discard whole microseconds of the strongest part of your precious signal, when you can get the long tail (low frequency) components directly, with a sine wave impedance bridge method instead?A frequency sweep with a DDS and ADC, under MCU control, would be very easy, require no fiddly current source or beyond-the-rails voltages, and as it has no dead bands, and integrates the signal continuously, offers an arbitrarily high SNR.Tim

author: brumbarchris
author_rank: Regular Contributor
timestamp: May 10, 2019, 08:17:10 am
content: QuoteAh, professionals can read between the lines, good catch!I do not want to withhold information, particularly when asking for help. However, as I am myself not very knowledgeable in this field, I would want to refrain from putting forward the wrong information or hypotheses.Besides, theoretically, it is much probable to get pertinent and focused and usable answers when you split the big problem in smaller, simpler chunks (simpler than "please design this eddy current detector for me".)QuoteWell, we have to build and extend whatever experience already exists within our company (not much, admittedly, but more than nothing with regards to the pulse induction principle).Best regards,Cristian

author: Kalvin
author_rank: Super Contributor
timestamp: May 10, 2019, 08:55:13 am
content: It is funny how these toy metal-detectors are still developed and manufactured. Is this due to the lack of competition or what?

author: OwO
author_rank: Super Contributor
timestamp: May 10, 2019, 02:47:33 pm
content: I second the suggestion to use frequency domain methods to make impedance and transfer function measurements. This is very common at RF where network analyzers are used to measure impulse response which gives better results than a TDR.

author: AndyC_772
author_rank: Super Contributor
timestamp: May 10, 2019, 04:01:29 pm
content: Quote from: brumbarchris on May 09, 2019, 08:24:29 pmHang on... what?? You want to precisely clip a signal which is going to be digitised?In that case, your clipping 'circuit' is two lines of code:Code:for (i = 0; i < SAMPLE_BLOCK_SIZE; i++)sample_data = (sample_data < CLIP_LEVEL) ? CLIP_LEVEL : sample_data;

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 10, 2019, 07:56:39 pm
content: Quote from: AndyC_772 on May 10, 2019, 04:01:29 pmAnalog clipping is probably still required to protect voltage ranges, but in the end, digital will be far more accurate than anything else. ADCs don't usually suffer from recovery effects either, so the saturation (to 0x000 / 0xfff) can be used implicitly as well (but, that may be something to test; ADCs fail in all sorts of fascinating ways).It bears noting that the abstraction processonlyworks if the one doing the abstraction, fully understands the specifications of the systems above and below the abstraction interface. This exercise strikes me as an overzealous (and perhaps inexperienced?) abstraction, that would benefit more from a holistic design process instead.That just leaves one thing --Quote from: brumbarchris on May 10, 2019, 08:17:10 am-- the force of "not invented here". Even when the existing background is sparse, there is a strong tendency to stick to what is "known", even if very little is actually known.Quote from: Kalvin on May 10, 2019, 08:55:13 amBeats me -- they're definitely out there. Last time I saw an eddy current analyzer at work, it was being used to check the martensite fraction (and to what depth) of an induction-hardened steel workpiece. A subtle application, but apparently representative and repeatable enough to work. Not that I can recommend anyone just go out and buy an industrial eddy current analyzer for any generic purpose (guessing they're in the $10k price range, as much industrial kit is), but the basic idea in any case is quite tractable, and simplified versions are available for peanuts (like metal detectors) that arguably could be used for the core functionality, and hacked to the OP's goal, without having to invent anything new.Tim