Help with ADC Input Protection - Page 1
Forum section: electronics / beginners

author: Trinity
author_rank: Contributor
timestamp: July 16, 2014, 12:45:19 am
content: I need some advice on protecting an ADC input pin.Here is a simplified sub-circuit of the area in question:Under normal conditions, Vtest is off, and the load sees periodic pulses of 300V.A test routine is periodically called to determine the resistance of the load. When this happens, the main 300 V source is turned off, and the 4V test source is enabled. (It's really a 5V source going through a diode with a 1 V drop to protect the test source from the 300V.)At this point, the load becomes half of a voltage divider with R1 being a known 300k leg. The ADC measures the voltage at the dividing point, and then the resistance of the load is calculated.The load resistance can be anything from a direct short to ground to a full open.During main operations, the ADC has to be protected from the 300V, so I've set up a Schottky protection circuit - but this is the first time I've ever worked with this type of protection circuit.So here's what I'd like help understanding:1. Is this circuit suitable for what I'm trying to accomplish?2. How do I choose the value of R2? What are the practical limits?3. How do I choose the specifications of the schottky diodes?Sorry for the long post, and thank you in advance for any of you who've even read this far!

author: nuno
author_rank: Frequent Contributor
timestamp: July 16, 2014, 02:01:05 am
content: I can't fully answer your question, but here's a start.When you have Vmain ON at 300V, current will flow from Vmain through R2 through D2 into V2. Vadc will be at V2 + Vdiode(fw), so IR2 = (Vmain - V2 - Vdiode(fw)) / R2.How much current do you want to see going through R2 and D2? Small means less heat of R2 and D2, less load on Vmain and less current into V2, but also affects the ADC reading because higher R2 means it takes more time to charge the sampling capacitor and wil make the input more sensitive to noise.There's also a caveat with this kind of protection, related to the current going into V2. Can V2 take it? V2 or loads also connected to V2? If it/they can't, Vadc will go above V2 + Vdiode(fw) and the "protection" is defeated.On another subject, 4V into >300K is a very small current; are you sure your diode will drop 1V?

author: Trinity
author_rank: Contributor
timestamp: July 16, 2014, 04:02:44 am
content: Quote from: nuno on July 16, 2014, 02:01:05 amThank you very much for clarifying those issues. I understand much better now.Does that mean that the schottky diodes don't need to be rated for a voltage much higher than 5V (i.e. at least 10V for derating)?Quote from: nuno on July 16, 2014, 02:01:05 amHmm. I'll have to look into that. Is there a better solution for protecting the ADC?Quote from: nuno on July 16, 2014, 02:01:05 amYou're absolutely right. The data sheets only go as low as 20 mA, and for that, it's about a 0.5 V drop. Would I be correct in assuming that current in the uA range isn't shown because it wasn't tested, and not because the diode won't work with current that low?This is the data sheet for the diode in question:http://www.onsemi.com/pub/Collateral/MURS360BT3-D.PDF

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 16, 2014, 04:12:21 am
content: Another gotcha: leakage. Junction diodes are typically preferred over schottky diodes for protection, because the leakage is so much smaller. Especially check the high temperature leakage on them -- mA for schottky, uA for junction.You may also want a JFET or MOSFET type op-amp to buffer the input, just because, when you're talking hundreds of kohms, even a slow ADC sampler input starts looking iffy. Good start would be something like TLV2371.For diodes, look at BAT54S (dual) for logic inputs or 1N4148 (individual) or BAV99 (dual) for analog. No way should you ever, ever be protecting a small-signal input with a 3A diode, there's just no reason and no way. Size the resistor to suit the purpose.Tim

author: mij59
author_rank: Frequent Contributor
timestamp: July 16, 2014, 04:32:00 am
content: Hi,What's the value of R_load ?How long are the pulses of Vmain ?

author: Trinity
author_rank: Contributor
timestamp: July 16, 2014, 04:43:11 am
content: Quote from: T3sl4co1l on July 16, 2014, 04:12:21 amCan you elaborate on the type? I thought schottky diodes were a type of junction diode.When I look for components, the primary categories seem to be general, rectifier, PIN, schottky, TVS, varactor, and zener. Is the type you're talking about one of those?Quote from: T3sl4co1l on July 16, 2014, 04:12:21 amThe reason I wasn't concerned about using a buffer here is because it's a DC test. I can wait until the voltage settles, so this seemed to be an acceptable way to drop the component count.Quote from: T3sl4co1l on July 16, 2014, 04:12:21 amCan you clarify this point?For this circuit, the load on the 5V test source is minimal - within the mA range. Having a forward current capability of 3A is more than an order of magnitude of headroom. I took the critical parameter to be the breakdown voltage. I needed to protect against 300V, so I chose a diode rated to 600V.Why is this not suitable?

author: Trinity
author_rank: Contributor
timestamp: July 16, 2014, 04:46:27 am
content: Quote from: mij59 on July 16, 2014, 04:32:00 amThe resistance is different during main operations than it is during the low voltage resistance test, but it can be anything from a dead short to a full open.As for the pulses of Vmain, they are approximately 1 ms in duration with a maximum duty cycle of 1.5%.

author: nuno
author_rank: Frequent Contributor
timestamp: July 16, 2014, 09:53:39 am
content: QuoteYes.QuoteA "safer" method is, in my opinion, to use a zener diode (or similar; I've never tried this for an analog input). There's another caveat I forgot to mention about the diode from GND and to VCC protection method, which is, it only works as long as the ADC circuit is powered up (V2 is connected). If you have V2 OFF and the 300V pulses are there, there's no protection for the ADC chip and other circuitry powered by V2; Vadc will not be clamped to ~V2. You must also make sure V2 always powers up before the 300V source.The zener has the disadvantage that you probably can't use all of the dynamic range of the ADC (well, maybe if you can find a really good zener, or use something like a TL431 or so, I never really thought about this, if it's possible), because it needs to cap the voltage at V2 tops but at the same time it will start conducting before Vadc reaches a maximum value and this will affect your readings at the top end of the input range (the zener will pull a little bit of current and this will make an extra voltage drop at R2, and if R2 is very high like 300K, the effect can be big).QuoteDoesn't matter what is the voltage drop at uA, because you know it will be very small, way less than your desired 1V.

author: KerryW
author_rank: Regular Contributor
timestamp: July 16, 2014, 12:50:05 pm
content: Since Vadc is limited to -0.3V to +5.3V, you don't need high voltage diodes. BAT54S would work fine. Since the Voltage is always positive, you don't even need D2, bit I would leave it in anyway.One potential solution would be to open the line on the left side of R2 and insert a constant current source. An LND150 depletion mode MOSFET with the gate & source tied to R2 and the drain to the junction point would form a constant current source of 1 to 3 mA that could withstand 500V. You could then drop the value of R2 to 1K. The ADC would see a source resistance of ~2K.

author: rob77
author_rank: Super Contributor
timestamp: July 16, 2014, 12:54:03 pm
content: it's quite hot here , so my brain doesn't work correctly... but...are you measuring with the ADC only when the 300V is off ? because during the 300V pulse you'll see just the max reading from the ADC.and definitely i would place a 5V1 zener accross V2 and make sure V2 is below 5V1 - more exactly just below the knee point of the zener (or the other way around - choose a zener with a knee point just above V2) - to keep protecting the ADC during absence of V2.

author: rs20
author_rank: Super Contributor
timestamp: July 16, 2014, 02:16:21 pm
content: Quote from: nuno on July 16, 2014, 02:01:05 amCan you elaborate on this? I'm presuming here that the supply voltage of the ADCisV2, so if current through the diode raises the voltage on the V2 net (assuming consumption on that net can't be guaranteed to be greater that the tiny current we're speaking of), then the supply voltage of the ADC will raise right along with Vadc, and the over-volt of the ADC input with respect to its supply won't happen. Now, if the voltage on the V2 net rises so high that things get damaged, then sure, that's a problem, but that's not what you said and that's not likely either.

author: nuno
author_rank: Frequent Contributor
timestamp: July 16, 2014, 09:48:08 pm
content: Quote from: rs20Well, that would be good if ADCs were usually powered by a much smaller voltage than their maximum rating, but that's not usually the case, is it? So, if you have a 5V ADC powered at 5V, and with the typical Maximum Absolute rating of 5.5V or 6V, that's a really small margin for Vadc and V2 to raise compared to what a 300V source is able to do.QuoteThe V2 net, as I wrote above, doesn't have to raise a lot for the ADC supply (and most probably the rest of V2's load) to raise above max absolute ratings.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 16, 2014, 09:57:41 pm
content: Quote from: Trinity on July 16, 2014, 04:43:11 amYes... there's always a junction somewhere.... but more importantly, it's a Si-Si, P-N rectifying junction (with metallic junctions elsewhere to eventually connect to normal wires). Whereas schottky are usually made with PtSi, or something weird like that, that makes a rectifying (rather than metallic) Schottky junction.QuoteSchottky and varactor are the special cases in this list: schottky are chemically and electrically different. Varactors are junction diodes, but with a characteristic doping profile to achieve the characteristic (and specified) capacitance, and probably special layouts to improve Q factor.The rest are all PN junction diodes, designed for, and controlled for, various properties.Zeners are optimized for reverse (true (low voltage) Zener, or (high voltage) avalanche) breakdown under continuous operation; TVSs are optimized for peak current handling. (Both may work well in the opposite role, you just don't have the manufacturer's assurance that they'll work reliably under those conditions.)General purpose rectifiers range from simple diffused PN diodes, to good old PIN diodes at high voltage (the 'I' region being necessary to achieve the high breakdown voltage). They can indeed be used as PIN diodes for RF purposes, but probably won't be optimal (or specified) in series resistance or recovery time. High speed rectifiers are optimized through doping profiles, patterning and carrier lifetime controls (dopants like gold or platinum, or electron irradiation); forward voltage drop and reverse breakdown voltage trade off against recovery speed.All rectifiers are avalanche capable, but are rarely specified for operation that way; in the old days, junctions were made with poor process control and purity, and often suffered from fatal breakdown (hence, textbooks up until the 70s or so always recommended using a resistor and capacitor in parallel with each diode, when connecting strings in series). You might even get away with, say, using a 1N4007 as an ~1100V zener diode, but I wouldn't count on it for a real design.So, from that space of diodes, you can pick whatever's best for the problem at hand. A too-big diode exhibits way more leakage and capacitance than a small-signal circuit should have to deal with. Always use components best proportioned for the task: just because you can use a humongous power transistor for controlling milliamperes, doesn't mean you should -- and there are probably very good reasons against it that you haven't considered.QuoteWith a 300k source resistance, you're counting on >>30Mohm equivalent to get anywhere near even 7 bits of ADC reading, let alone 10+ bits typical of microcontrollers' internals, or even better external units. Or from 5V, leakage well under 100nA.The MURS360 is 3.0uA max at room temperature, so you're already screwed on that -- by a factor of >30. The typical case might be better, but considering the typ. and max. at 150C aren't very far apart, the typ. at room temperature probably isn't much better (1.5uA?).Now, that is at 600V. And you could extrapolate the graph (very unusual to see a graph of maximum allowed values -- very good of On Semi to provide that!) down to, say, 3nA at a couple volts, which is now a factor of <30 better than needed, not bad. But if it heats up at all, you're screwed again. So it's still very dubious, and if you want more bits of accuracy, you don't have a choice.What's the maximum current it needs to handle, anyhow? 300V / 300k = 1mA. So it's 3.5 orders of magnitude oversized, not counting ESD.A typical solution would be a BAT54S, but this does 2uA leakage at room temperature already. Like I said, schottky isn't the way to go here. BAV99 is rated for 25nA max at 20V, 25C -- now we're talking. And even at elevated temperature, the typical isn't above 100nA until 70C or so.By the way, leakage currents do cancel, to some extent, but this depends on matching between the two diodes, which is not guaranteed. Very likely, one is down by half of the other, so you still end up with +/- half the total leakage. It also varies nonlinearly with voltage, which will cause a much more insidious error to your ADC measurement than a simple straight-line constant current error would!An even smaller diode would be beneficial, so you could get guaranteed leakage down in the single digit nA. But now you start to run afoul of possible ESD problems, depending on how and where your circuit actually connects -- if the high-voltage end of the 300k is available to the outside world, then as much as a 10kV spark (assuming the resistor doesn't break down in the process -- an axial resistor might actually be okay, but an SMT, doubtful) could deliver no more than 33mA, nary a scratch. But if the ADC end is exposed to the world, you better watch out, because, come winter time, even a casual perusal of fingers on PCB can develop thousands of volts, and the sparks, tens of amperes peak. Just by operating current, you might not even need external diodes at all, you could probably get away with the diodes inside the ADC chip -- but that's not good design practice, and definitely not good practice when it comes to ESD.Tim

author: Trinity
author_rank: Contributor
timestamp: July 17, 2014, 02:10:46 pm
content: Quote from: nuno on July 16, 2014, 09:53:39 amI had originally set up a zener diode protection circuit, but I read a few people mention that while this is great for protecting against voltages that are significantly higher than the zener rating, when you get very close, the zener can settle at a voltage different from it's precise rating.Is that the effect you're describing?

author: Trinity
author_rank: Contributor
timestamp: July 17, 2014, 02:13:06 pm
content: Quote from: rob77 on July 16, 2014, 12:54:03 pmYes, when the diagnostic routine is run, main operations are suspended.Quote from: rob77 on July 16, 2014, 12:54:03 pmIt's been mentioned that a zener would corrupt the ADC readings in the upper range. I assume that if I add one in addition to the schottky diodes, that I would have the same problem?

author: Trinity
author_rank: Contributor
timestamp: July 17, 2014, 02:22:45 pm
content: Quote from: T3sl4co1l on July 16, 2014, 09:57:41 pmPlease forgive me, but I'm getting lost here. I don't understand what you're pointing out.Where is the 30 Mohm coming from,how does the 300k resistance affect the ADC reading, and what is the issue is with the leakage current?Edit: Okay, I see what the problem is with the leakage current.I'll make a seperate post about a diode I found that might be useful.Quote from: T3sl4co1l on July 16, 2014, 09:57:41 pmWhoa, I'm wondering if there's been a miscommunication somewhere. You're talking about the maximum current the diode needs to handle with regards to the 300V, but the diode in question is the one not shown between the 5V test source and R1. I originally made a mistake and thought the diode would have a 1V drop, so to simplify the schematic, I just showed a 4V source - but as I explained earlier, it's really a 5V source going through the 600V, 3A diode in question.Now, you're correct that the 3A rating is far larger than it needs to support, but it's there to protect the 5V test source, not to protect the ADC, so I don't understand why it would affect the ADC readings.

author: rob77
author_rank: Super Contributor
timestamp: July 17, 2014, 02:24:36 pm
content: Quote from: Trinity on July 17, 2014, 02:13:06 pmzener would corrupt the ADC reading when connected to the ADC input directly. i'm suggesting to place accross the V2 power rails ( a zener with a knee point just above the V2 rail). the role of that zener is to protect the adc when V2 is not present - you will end up with Vshottky + Vzener on the ADC input when V2 rail is not there. furthermore the zener would help the V2 rail to absorb the current , therefore minimizing the risk of lifting the V2 rail by the small current flowing into it through the shottky.

author: rs20
author_rank: Super Contributor
timestamp: July 17, 2014, 02:57:08 pm
content: Quote from: nuno on July 16, 2014, 09:48:08 pmOK, sure, but that's not what was written in your original message at all -- just thought it was worth clarifying. Also, it's the load on V2 that has to be guaranteed toconsumethat current (I realise that you briefly allude to this, but it's the absolute key question), not whether V2 can "take" it (unless there's a widespread ability for regulators topulla voltage down that I'm not aware of). Worst comes to the worst, a 5k resistor across V2 (or maybe a zener with a sharp enough knee) will make this problem go away entirely along with normal decoupling caps, even if you can't make any guarantees at all on power consumption in the rest of the circuit (any voltage dividers? power-on LEDs? quiescent currents of everything else?).

author: Trinity
author_rank: Contributor
timestamp: July 17, 2014, 06:24:12 pm
content: Quote from: rob77 on July 17, 2014, 02:24:36 pmOh, that's beautiful. Yes, I will absolutely do that - thank you.

author: Trinity
author_rank: Contributor
timestamp: July 17, 2014, 08:24:42 pm
content: I found this diode here:BAV199W,115http://www.nxp.com/documents/data_sheet/BAV199W.pdfIt has a typical leakage current of 3 pA, and a maximum of 5 nA.One catch I see is that it has a forward voltage drop of about 1V.Is this a good choice for me to work with? And if so, what's the best way to deal with the voltage drop?The ADC has an absolute maximum of Vdd + 0.3. I'd like to keep it no higher than 5.1 V if I can.If it helps, I have access to a 3.3V rail as well.

author: nuno
author_rank: Frequent Contributor
timestamp: July 18, 2014, 12:48:02 am
content: Quote from: TrinityNot exactly the same effect. Below is the "regulator" quadrant of the curve of an old zener, a 4V7 or 5V I can't remember. At around 3.7V it is already pulling some current, maybe 50uA or so. At 3.5V it may be pulling already, let's say, 10uA. Now think of how much voltage those 10uA would be droping on the 300K R2... will it influence your ADC reading?

author: nuno
author_rank: Frequent Contributor
timestamp: July 18, 2014, 12:56:08 am
content: Quote from: rs20V2 can be a battery and absorb that (or part of that) current. It's marked just "V2". So yes, it is V2 or the load on V2 that must take the diode current for the protection scheme to work.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 18, 2014, 12:58:37 am
content: Quote from: Trinity on July 17, 2014, 02:22:45 pmNo, that was in regards to the protection diode(s): 300V on 300k is 1mA. So the diodes don't need to be big.If I misread what part you intended to use there, that's fine, the rest is still true.Similar reasoning applies to the 5V source, though: for example, suppose someone suddenly slammed 300V on the load again. That transient, coupled through the junction capacitance (and reverse recovery), can cause big problems upstream.Tim

author: Trinity
author_rank: Contributor
timestamp: July 18, 2014, 04:02:17 am
content: Quote from: T3sl4co1l on July 18, 2014, 12:58:37 amHow do I protect against this? Is it just a matter of choosing different specs for the protection diode?

author: Trinity
author_rank: Contributor
timestamp: July 18, 2014, 04:19:40 am
content: Okay, guys. Here's a revised version:I added a Zener and bypass caps to V2, and dropped the resistance going into the ADC.How is this looking now?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 18, 2014, 05:36:32 am
content: 20k looks awfully low just for a sensor, that's 15mA and 4.5W at full voltage! You want to reach a suitable compromise between accuracy, leakage and tolerable loading (dissipation). That's what engineering is, optimization.Ah, the 5V source has a diode and a 400k resistor? Then that diode can be very small as well; I don't think you'll find a 1N4148 in 400V+ ratings (aside from specialty high voltage rectifiers), so a UF4004 or better would be fine. Or if you don't care about having 400k loading the circuit under test, just leave it hooked up all the time, no diode whatsoever... 1mA backflow into the 5V source probably isn't the worst that can happen, and might do nothing at all, or need only a little work to be reliable (like the zener to keep the supply voltage from running away in case of excess current). You could just as well bias the ADC input with a resistor and connect that, in turn, to the load; even use just one diode, so the ADC measures the "high" end of the diode. You lose a diode drop, but I'm guessing now that you're not going for 10+ bit accuracy or anything.Again, you're counting on everything settling to the expected voltage; I don't know what exactly you're looking to get from it, but the accuracy isn't looking good. You might just use a comparator instead of the ADC (well, technically it's still a 1-bit ADC) to do a dumb go/no-go condition, for instance.Tim

author: Trinity
author_rank: Contributor
timestamp: July 18, 2014, 05:54:24 am
content: Quote from: T3sl4co1l on July 18, 2014, 05:36:32 amGood point. I brought it up to 100k, and that puts the current through it at about 3 mA, and the power at about 870 mW.Quote from: T3sl4co1l on July 18, 2014, 05:36:32 amThe 400k resistor needs to be there for the test as it controls the range that's measured.Quote from: T3sl4co1l on July 18, 2014, 05:36:32 amWell, the accuracy for this purpose is somewhat flexible. However, is improving the accuracy a matter of optimizing the component parameters, or does it require a different circuit?Also, is the accuracy going to be off by a constant bias? If the error response is linear, it should be easy enough to account for it.Remember that during the test routine, the 300V source is shut off and will present as an open.

author: Trinity
author_rank: Contributor
timestamp: July 19, 2014, 01:18:31 am
content: Alright, so if the current design isn't suitable, let's talk op-amps.If I put an op-amp in unity gain configuration to drop the input impedance into the ADC, I still need to protect the input to the op-amp.But the solutions we've discussed still all introduce a bias current into R2. Does that not degrade the accuracy anyway? If I use an op amp, what's the best way to protect the input?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 19, 2014, 06:24:20 pm
content: Alright, what are you actually doing?

author: Trinity
author_rank: Contributor
timestamp: July 19, 2014, 06:58:59 pm
content: Quote from: T3sl4co1l on July 19, 2014, 06:24:20 pmReplied by PM.