Shield Connections for Low Noise Measurement (AN-159) - Page 1
Forum section: electronics / projects

author: Tinkerlad
author_rank: Newbie
timestamp: July 10, 2025, 12:08:34 pm
content: Heya brains trust. Long time lurker etc.. Just looking through AN-159 from ADI (amazing work there!)q, and one thing that is mentioned re shields on coax entering into the shielded enclosure is that the output coax shield is not connected.. I am a little confused as to how this is the case as otherwise where is the return current for the output conductor going? See attached figure from the app note.My guess at what is meant is that the output coax connects to gnd at the output opamp then is isolated from all shields it passes through including the use of an isolated bnc bulkhead at the outer container?Is someone able to explain if the outer bnc bulkhead was not isolated, where the currents are travelling that is detrimental to the measurement?

author: moffy
author_rank: Super Contributor
timestamp: July 10, 2025, 12:29:19 pm
content: The schematic shows the output shield connected to the board ground but not the metal of the shield box:"Input and output BNC shieldsconnect to the steel can, while coaxial cable shields tie tothe board ground on both sides. If the output shield connections were to be connected to the steel can as well, aground loop is formed that can pick up stray fields." AN-159-7

author: Tinkerlad
author_rank: Newbie
timestamp: July 10, 2025, 12:32:40 pm
content: Cheers, so does that mean the that the return path for the output ports goes through the outer enclosure up through the stand-offs / input coax shield to get to ground on the board?

author: mtwieg
author_rank: Frequent Contributor
timestamp: July 10, 2025, 01:44:50 pm
content: Quote from: moffy on July 10, 2025, 12:29:19 pmThe way it's written is definitely confusing...At first I thought it was referring to the cables used to connect the box to external equipment. But it's actually referring to the coax cables inside the box.But it's not clear why they make this suggestion. If they sever the shield of the internal output cable, then the return path of that cable's center conductor will be along the walls of the boxes and the standoffs supporting them and the PCB. A much larger loop area than allowing the coax to connect normally. The completed shield path would also form a "loop" with the path through the boxes/standoffs... but so what? I can't see any way for that loop to create issues for the circuit.

author: AnalogTodd
author_rank: Regular Contributor
timestamp: July 10, 2025, 02:32:45 pm
content: The tiniest of ground loops on this circuit cause issues (I designed and built it and wrote the application note). The idea regarding ground loops is that you have coax connections to the internal board for the input and output BNC jacks. If, however, you also connect the shields on those jacks to the metal can, there is a loop through the board, coax shield, and metal can that will pick up signal that then gets amplified through the circuit and throws off the measurement (and can even show up as oscillation on the output). There's a lot that can go wrong with high-gain amplifiers if you don't pay critical attention to the details.The drawing probably could have been better--trying to convey such a fine point regarding how to ground everything wasn't simple. Getting the circuit figured out in the first place was a pain. Once it was built, that was when I realized just how much more work I had to put in to get solid measurements. That was a bitch!

author: Tinkerlad
author_rank: Newbie
timestamp: July 11, 2025, 12:48:42 am
content: Thanks so much for your work on thisYeah right, I guess I never imagined that you could get enough potential across the ground loop in this sort of enclosure that this would form. My background is at a larger scale and for EMC reasons we typically have all shields connected at the outermost enclosure, though sometimes not when we have higher power devices and ground bounce. But we also do not deal with this level of sensitivity either..And yeah I understand how hard it can be to convey, just hoping I can meet the intent on the first go so not to have the issues warned about. I am having a crack at building something similar for the first time so gathering as much info as I can. I am trying to also make the gain a bit configurable (hopefully between about 50dB and 80dB, as I need the dynamic range for the measurement I am trying to make is sometimes on larger levels. Its an interesting balance of keeping a noise floor of the measurement as low as possible while also bringing out the weak signals that are present slightly above thermal noise, alongside those magnitudes greater but close in frequency.So the longer return path for the output signal I guess doesnt matter at these frequencies?Also regarding the shielding, I do not have any mu-metal on hand, but have been contemplating adding some ferrite absorber material that we have left as well.. Will be interesting to see how we go. Environmental noise should be relatively low, as for us this will always be used within a shielded room with all incoming signals heavily filtered. So just the UUT and instrumentation to worry about I hope.

author: AnalogTodd
author_rank: Regular Contributor
timestamp: July 11, 2025, 02:34:58 pm
content: The loop here isn't much, but when you start looking at the signal levels you're trying to measure relative to what you might pick up, they get close to the same magnitude and it can affect your results.The longer return path really didn't make much of a difference, no. We're not talking RF frequencies, the -3dB point for full bandwidth was about 3MHz.I wish you luck with your shielding. I spent a good amount of time learning about what it took to shield against magnetic fields at these low frequencies (get yourself a copy of the book by Henry Ott). We had a screen room where we could do shielded measurements, but it turned out to really be best for RF and not so good at low frequency. Considering part of our problem came from the instrumentation and internal signals radiating from those, it's not an easy thing to fix. I recommend checking your final circuit with a shorted input (I used a BNC jack where I put solder braid from the center to the shield) to ensure you have both low noise and no strange signals getting in.

author: tooki
author_rank: Super Contributor
timestamp: July 11, 2025, 06:10:14 pm
content: Quote from: AnalogTodd on July 10, 2025, 02:32:45 pmI feel like this still struggles to make clear what the intent is. (“If you also connect the shields on those jacks to the metal can…” — do you mean the shield (outer shell of the BNC) of the jack, or the shield of thecable going to the jack?)The text in the application note (below) itself seems clearer to me, so am I missing something?QuoteI interpreted this as follows, but your reply above has made me uncertain:Input BNC center pin -> coax conductor -> PCB signal inputInput BNC shell -> coax shield -> PCB groundInput BNC shell -> steel canPCB signal output -> coax conductor -> output BNC center pinPCB ground -> coax shield [coax shield left floating on this end]steel can -> output BNC shell

author: RoGeorge
author_rank: Super Contributor
timestamp: July 11, 2025, 07:42:24 pm
content: Quote from: Tinkerlad on July 10, 2025, 12:08:34 pmWatch the videos linked in this posthttps://www.eevblog.com/forum/chat/fun-for-nerds/msg5583117/#msg5583117, maybe search for a few other talks by the same guy. The video is only a teaser, and it is about designing good PCB layouts, not about noise and grounding.However, the aspects hinted there will help getting the properly understanding of electricity, not as volts and amps, but from the perspective of the electromagnetic fields. The energy is not transported through the wires (as most EE people are led to believe), the energy goes through air, traveling through the area enclosed by the direct and the return wire path. The wire only acts as a guide for the EM field.That is why, when there is more than one path (for example a ground loop, or a ground plane, etc.) the energy (the signal) might go through other places not expected (again, by other places I'm not talking about wires, talking about the space between the direct and the return paths, which is either the air, the dielectric of a cable, etc.).The rule (when there are more possible paths) is that the current will tend to choose a path that encloses the minimum area between the direct and the return paths. The signal does NOT travel through the wire (or through the established paths that enclose that minimum area from a paragraph ago), the energy of the signal travels through the space between the direct and the return paths, which is mostly empty space, or a dielectric. And all other unwanted EM fields (from other sources), fields that will coexist in that area, will combine and alter the useful signal.Once this idea is assimilated (the idea that the energy of a signal travels through the space between the wires, and NOT through the wires), things like where or when to connect, or to not connect a shield, why to keep the direct and return wires together (with a minimum area between them), why or when a GND plane can do the job (to avoid a costly full metal enclosure), and so on, will become easy to follow. All these apparently ad-hoc measures for anti-noise/low-EMI-radiation/PCB-signal-integrity/etc. will become logical steps.

author: mtwieg
author_rank: Frequent Contributor
timestamp: July 12, 2025, 01:03:24 pm
content: Quote from: AnalogTodd on July 10, 2025, 02:32:45 pmJust for clarification on the shield connections, I modified the drawing showing the shields in blue. Is this accurate? The shield does not contact the two inner boxes? Only one coax cable (the input side, I assume) is partially visible in the photo, unfortunately.Why is the ground loop formed by the output coax problematic, but not the other ground loops formed by the input coax and numerous standoffs?

author: AnalogTodd
author_rank: Regular Contributor
timestamp: July 14, 2025, 01:21:51 pm
content: Quote from: mtwieg on July 12, 2025, 01:03:24 pmYour picture is accurate. When we did the board layout, the mounting holes for it were not electrically connected to ground of the board, so any screws going through and connecting to the boxes doesn't form a ground loop.It all came down to 'star' grounding.

author: mtwieg
author_rank: Frequent Contributor
timestamp: July 14, 2025, 02:02:19 pm
content: Ok, so the intent was for the input coax to be the star ground point.But with the shield gapped on the output cable, the return path for the load current must be a very large loop. See updated diagram with the load current return path shown. In fact, if the board standoffs are not connected to circuit GND, then this makes the return loop much bigger, and includes the shield of the input coax. It seems counterintuitive that this is preferable to having the output cable shield connected and providing the return path. Yes, in this case we have created a "ground loop" inside of the box, but so what?An aside question: why was it decided to not connect the coax shield(s) to the internal boxes? Surely that would be better at rejecting common mode conducted interference. If you wanted to prevent internal ground loops, one could make all the standoffs insulating, so the inner shields would only connect to gnd at one point each.

author: AnalogTodd
author_rank: Regular Contributor
timestamp: July 14, 2025, 08:06:00 pm
content: Honestly, I think what would likely have been better would have been BNC connectors that were electrically isolated from the exterior box in the first place. That way the board could be the star ground point and there wouldn't be a concern about ground loops through the cans nor would there be a concern about long return paths on the outputs. I could then have connected the board to only one spot on the cans and used insulators for the others.The problem was in working with what I could get provided to me by the company in the time allotted along with getting the whole thing written up and published (LTC still tried to keep a lot of the startup mentality even 30+ years along). It took us quite a while to actually figure out some of the effects we were seeing; things didn't match anywhere close to what simulations suggested they should until we started reviewing and correcting second-order effects. Our PSRR measurements were off by nearly 40dB at 1MHz when we first started looking at the part. Once we found the magnetic field coupling from input to output (that took weeks to figure out and understand fully) we were able to match with a much higher degree of accuracy.It was a lot of work done while trying to fulfill other job duties, and I may have missed getting it absolutely perfect. Personally, I was very happy with how it turned out. One of the best things was hearing when the LT3042 was being shown to customers and the noise levels and PSRR were being touted to them, they often asked, "How the hell do you measure that?" The answer was, "I'm glad you asked," and a copy of the App Note was handed over. I always heard good things back from sales and management about the customer response.

author: mtwieg
author_rank: Frequent Contributor
timestamp: July 15, 2025, 12:04:24 pm
content: Quote from: AnalogTodd on July 14, 2025, 08:06:00 pmSo I'm guessing the purpose of the shields was only to block magnetic fields (which happens regardless of if/how the shield is connected to the circuit) and not electric fields (which does depend on the shield being connected somehow)?QuoteTo clarify, I'm not saying there's any "problem" with the design of this amplifier or test fixture setup. I'm not confident any of my suggestions would actually improve its performance (especially figure 18, accurate measurements of anything on the order of 100dB is incredibly difficult). My point is that the app note doesn't explain itself well to readers. Especially its treatment of ground loops.For example near the end the app note explains why putting large local bypass capacitance on the DUT input ends up degrading PSRR measurements. This is counterintuitive, so the app note gives a detailed and coherent explanation. That's great, that's useful to readers. But for ground loops, the app note just cautions against them repeatedly without any explanation why. As if the mere presence of a closed loop angers the analog gods, resulting in measurements being "corrupted" in some way. This isn't helpful to readers, it just promotes the "black art" perspective on analog electronics, or makes engineers paranoid about the use of profane shapes in their designs (or, in cases where the mark of the beast is unavoidable, they try and placate the gods by peppering everything with ferrite beads/common mode chokes).

author: AnalogTodd
author_rank: Regular Contributor
timestamp: July 15, 2025, 01:03:37 pm
content: Quote from: mtwieg on July 15, 2025, 12:04:24 pmYes, that is the case. Electric fields show up as DC signals (because it comes from a stationary charge) and are filtered out by the high-pass functions in the circuit. Magnetic fields exist from moving charge and impose current on nearby conductors.QuoteThank you for the honest feedback. Describing the difficulties caused by ground loops isn't something that we (myself as the writer and also more senior engineers reviewing it) felt could easily be covered in an application note like this. That was why I listed references like the book by Morrison. It's not necessarily a paragraph or two that explains the problem, it could take entire chapters. I'm over thirty years into my career in analog design and I still make mistakes in connecting circuits to 'ground' (as many will understand, there can be potential between any two points on a board that are labeled 'ground').When I went to write this application note, one of the first things I did was to go back and read a LOT of Jim Williams' app notes. He had a great way of putting his ideas forward that pulled readers in and helped them understand his circuit. Even though I worked with him for nearly two decades before he passed, I still wanted to get some of that voice in my writing and I felt the best way was to know how he wrote. When I passed this along to other senior engineers to review before publication, they commented how I got a lot of that flavor but added in more theory and how that theory was held up in what I saw on the bench. If you go back and review AN-83 (I did the circuit on that while Jim wrote it up) one thing that never got explained was why the filters used were fourth-order Butterworth. That needed explained in my version.Analog electronics isn't so much a "black art" as it is learning what will work and what will bite you in the ass and making appropriate choices going forward. Even more important is learning how to debug and find root cause for any issue you're banging your head against--that's what I would consider more of a "black art".