Need help protecting high-impedance input buffer circuit - Page 1
Forum section: electronics / general

author: Singularity
author_rank: Newbie
timestamp: June 25, 2019, 08:50:59 am
content: Hello all, I'll get right to it.I'm working on a design for a battery powered 1G AC/DC voltmeter which can high-impedance sources with 1mV accuracy across a ~+-120V range. I'm using the ADA4530-1 as a voltage follower to feed my ADC (AD7177-2), supply to the opamp is fed by a bipolar +-7.5V source to allow bipolar measurements without the need for biasing of the input. Currently I'm using a 1G(5 x 200M/1.2W resistors air-wired in series to reduce capacitance) along with 50M(2 x 25M/1W series air-wired) shunt resistors to divide the voltage down to within limits of the ADA4530. As I'm trying to measure both 50/60hz and DC simultaneously, I use software to filter the AC out of final DC measurements.The design as is works as intended, but in order to maintain the low noise/simplicity of the circuit there is no protection (other than the high value of the resistors and opamp ESD rating) and this is where the fun starts...I'm trying to protect this circuit from high-voltage pulses on the order of ~20K lasting as long as .25 seconds, if you're wondering why on I would need this I'm attempting to protect the input from electric fences or "hot-fences" the same kind of fences used for deterring livestock. For clarification, only the positive lead will ever touch said fence, and assume the negative lead is placed in the dirt.I realize the power rating of my chosen resistors is rather small when considering the voltages I'm talking about, but I was somewhat limited by board space. At the moment though protecting the circuit has taken priority over the pain of modifying the board. Is it simply a matter of increasing the power rating of the resistors? The ADA4530 has 4kV ESD protection, but with such large voltages I'm unsure exactly what the resistor divider is actually doing as far as dividing the input voltage. I'm going to be adding a spark gap at the inputs for good measure, but other than increasing the power rating of the resistors and said spark gap is there anything else I can do that maintains my low-noise?Thank you in advance, I'm open to all suggestions no matter how intricate as long as they can maintain the accuracy of the circuit and maintain a measurable range of at least +-90V preferably +-120V

author: tggzzz
author_rank: Super Contributor
timestamp: June 25, 2019, 09:11:24 am
content: Gas discharge tube, for examplehttps://www.littelfuse.com/~/media/electronics/product_brochures/littelfuse_circuit_protection_product_overview_product_brochure.pdf.pdfAs well as power rating, ensure your resistors have an appropriate voltage rating and are appropriately linear.

author: MiDi
author_rank: Frequent Contributor
timestamp: June 25, 2019, 10:46:51 am
content: In most (precision) op amps there are protection diodes between both inputs and from inputs to the power rails.For most of them the inputs are spec'd for max. 10mA DC, the esd ratings are more like an energy dump capability.The simplest protection is a series resistor for both inputs, calculated for the maximum voltage between them and if needed to the ps rails.For ADA4530-1 this would mean >= 2MOhms (rated 20kV, 200W) on live input- not economic.Low leakage protection for up to 20kV is a serious task.If the maximum energy and impedance of source that has to be dealt from protection is known, this could relax the requirements dramatically.If the source could only give <=10mA short circuit current, than there is only the first energy impulse left to deal with, the dc current after that could be handled by the protection diodes of op amp.PS: Take a look at the cat ratings for e.g. DMMs, these ratings are mainly clustered for the amount of energy that has to be dumped by protection circuit.

author: Singularity
author_rank: Newbie
timestamp: June 25, 2019, 10:47:53 pm
content: From the Patriot product page, their highest output system outputs 13J and max output voltage is 10.2 kV max, 7.0 kV @ 500 Ωhttps://www.patriotchargers.com/PMX1500.htmwhen selecting a discharge tube (I've never had need of them before now, and have been up for a while now, so I'm getting a little slow mentally) I'm guessing to select based on the DC breakdown voltage as to make sure I don't limit my AC voltage range, so In this case a GDT with a DC breakdown voltage of 110-145V? So a 120V / 50hz AC sine wave has a V/uSec of 120/5000 = 0.024 which is less than the mentioned 1V/usecsomething like the SL1011A145 :https://m.littelfuse.com/~/media/electronics/datasheets/gas_discharge_tubes/littelfuse_gdt_sl1011a_sl1411a_datasheet.pdf.pdf

author: splin
author_rank: Frequent Contributor
timestamp: June 26, 2019, 03:49:43 am
content: The good news is that you might not need any additional protection - with a 50M/1G input divider the input current with 20kV at the input is only 20uA which should be easily handled by the AD4530-1 input protection diodes.Perhaps I'm addressing a typo, but the bad news (highlighted):Quote from: Singularity on June 25, 2019, 08:50:59 amYikes - that's 8.3ppm accuracy - not just DC, but at 50 and 60Hz! You would be very hard pushed to buy a meter that is that accurate at DC for 1 year after calibration - even if you have $15k to buy it. And you probably wouldn't want to leave it in a field connected to an electric fence...An 8ppm @ 60Hz meter is likely unobtanium for any price you could afford.Look at the error sources; let's assume you can calibrate the meter to read 0V with a shorted input and 120V with 120V applied (a 120V < 1ppm reference is expensive)a) Resistor divider drift over time and temperature: the highest value 5ppm/C resistors at Digikey are 25Megohm Caddock USF371 @ $23 each (so $966 for the divider) - but you need to keep the temperature within 1C of the calibration temperature. The shelf life stability is 20ppm/year so you might get a month or two between calibrations (these are very good resistors hence the high cost).b) ADC linearity: the AD7177-2 has a maximum INL of 3.5ppm - which will vary with time and temperature making it difficult to compensate.c) ADC voltage reference: an LTZ1000 reference can achieve < 1ppm/year plus .05ppm/C but not cheap or trivial.d) Input divider parasitic capacitance. As you stated, air wiring will help but probably not enough. According to this:https://www.edn.com/design/components-and-packaging/4423492/Resistors-aren-t-resistorsa carbon film resistor has a capacitance of 1 to 2 pF. So if your divider happens to have (say) 1G//0.2pF (5 x 1pF in series) and 50M//1pF the error at 60Hz is 2533ppm!e) Thermal EMFs. 1mV at the input is only 47uV at the ADC. Thermal EMFs can be many uVs/C when dissimilar metals are in a cct with temperature differences. You should be able to control most of these but the high resistance divider resistors may be a problem.f) Noise: the 1G/50M divider will have a Johnson noise of 5.8uVpp/sqrt(Hz) meaning that the reading rate will have to be somewhat below 1 reading per second - but the high resistance input divider will exhibit additional flicker noise which will likely exhibit significantly higher noise below 1Hz.g) Voltage coefficient of the input divider resistors. Those expensive Caddox resistors are specified at less than .02uV/V or 2.4uV/120V. Other high value resistors will likely have somewhat higher coefficients. If both legs of the divider have matching coefficients then this won't present an error but it may need to be measured or calibrated.You could use a high voltage opamp such as the ADHV4702-1, or a bootstrapped amp, to provide a high impedance input, which would allow a much lower resistance divider to avoid some of the above problems but the other probems remain.

author: Singularity
author_rank: Newbie
timestamp: June 26, 2019, 06:07:03 pm
content: QuoteIf I'm being perfectly honest our use case only requires +-10V @ +-0.001V DC (+-30V @ 0.1V AC) accuracy and in fact the design has been certified and is NIST traceable for that range, once we go above +-30V that DC accuracy does indeed start to degrade but is still within acceptable limits for our use case (we have a very talented programmer and I'm no slouch myself, so a lot of our accuracy comes from careful software filtering/zero calibration across a wide temp range)QuoteWas able to mitigate this problem slightly by using a couple of ntc's in close proximity to the divider as well as having the opamp case coupled to the ADC via a heatsink and we currently don't market this device and we have 5-6 in the field at any one point in time, so the calibration gets checked every month or soQuoteOriginally I decided against this since it is a battery/handheld meter (pelican 1120 case with A LOT of shielding) and I knew I could achieve certification without it, but I have come back to the idea recently as there are a few situations where >1G impedance would be very beneficial and that is damn near impossible to do using a resistor divider with such high values while also keeping our range the same. (A lot of times we have to use this around HVAC transmission lines and the induced voltage on the leads requires being able to handle at least +-120V)With that said, despite not being an analog expert by any means, I'm pretty proud of its' performance. Just goes to show what you can achieve with enough time/research/googling/and most importantly testing.This issue with the electric fences is another matter all together though, I've had to repair 3 so far, usually requires replacing the resistors, opamp, and ADC, one of them needed to have the reference replaced as well. I'm wondering if a better initial cleaning or better humidity control could possibly help as well, just to make sure there are as few paths around the resistors as possible (they're put in an ultrasonic cleaner with alcohol for 1-3 hours, so I'm not sure how to improve on that) and they do have a desiccant pack inside.So even if I do decide to go the bootstrapping route in the future, I still need to figure out something for this fence problem. I'll for sure try using a GDT and increasing the rating of the divider resistors just in case, can you tell me if the SL1011A145 from littelfuse is the proper choice if I'm trying to protect it from (13J energy, 10.2 kV max, 7.0 kV @ 500 Ω, pulse length ~250uSec, 8kV/10uSec rise time)? I probably got it wrong as I've never used these before.

author: 1audio
author_rank: Frequent Contributor
timestamp: June 27, 2019, 05:53:29 am
content: GDT's are great and can dump a lot of power but you need to understand them. Once triggered the plasma inside is a short until the energy is removed. You can add a MOV in series to deal with the "short" in that the MOV will keep a certain voltage which will stop the discharge in some cases. In your case that may not be a problem, except to be sure the operator is protected if the two probes were to short to each other. Also the GDT is not that fast, taking a few uS to arc. MOV's and Zeners are faster. The MOV though will be a resistance and not open and Zeners are more problematic and fragile.You should also check all the internal clearances. Its quite possible that an arc in an odd place or going through to an external return not via probes could be responsible for the unit that self-destructed.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: June 27, 2019, 09:15:04 am
content: An interesting trick used with protection in precision integrated circuits is to actually not connect the protection diodes to ground. Instead, they are connected to the output of another opamp, which is connected as a voltage buffer, which buffers the input line. This output line is then again connected with diodes to actual ground. By doing this, there is no voltage across the protection diodes of the input, and as a result no leakage. Any leakage of the second set of diodes is sourced by the opamp.Since this opamp needs only a low bandwidth and almost no gain (since an error is not a big deal here, as long as the voltage drop over those ESD diodes is low) the opamp can be tiny and as a result also have tiny input leakage, capacitance and area. If a transient event happens, the buffer opamp is too slow to keep up and even if it does, it hits the rails. As a result, the buffered line is no longer at the same voltage as the input, and you have a working set of input protection.

author: iMo
author_rank: Super Contributor
timestamp: June 28, 2019, 02:42:37 pm
content: Something like this?The currents via D4 and D6 should be at ~fA level (DC), imho.The R14=1k could be of an higher value, like 1k-100k, I think.PS: the resistor's values should be adjusted based on real voltages used, the 741 is there as a placeholder only..https://www.eevblog.com/forum/projects/simple-dc-afe-for-adc-chips-with-unipolar-diff-inputs/new/#newPS: added more pictures..