Understanding Shielding in Low Level DC Measurement - Page 1
Forum section: electronics / general

author: ignilux
author_rank: Unknown
timestamp: December 23, 2020, 04:42:51 pm
content: All-Lately I've taken an interest in making some simple test equipment for measuring low level, low frequency signals. I'm working on a few different things right now, including a AD587JQ-based voltage standard (I guess 10V isn't low level to most), a heated chamber for testing tempcos, and a simple auto-zero amplified current shunt down to single digit nA or hundreds of pA, somewhat like a dumbed-down uCurrent. Anyway, the one question that I keep getting stuck on is how to manage the shielding and grounding of the cables and chassis. Note that I am not asking about guarding, which I am more familiar with.Take the following scenario. The current shunt mentioned above will be housed in a die cast aluminum chassis, and powered from a 9V battery. Naturally, this will be isolated from mains earth. The output will be fed to my 8840A bench meter (also floating, of course) through a two-conductor shielded cable. If, for instance, I was connecting my signal generator to an isolated DUT, I would bond the shield of the coaxial connector to the chassis of the DUT and be done with it. The signal generator has its outputs referenced to mains earth, so the entire signal path would enjoy this protection without any common mode currents. My understanding is that in this application, connecting the shield of the cable to the chassis ononeend of the cable is the proper thing to do. This makes sense. But now I'm not sure what to do. I can't bond the meter end cable shield to the chassis, and I'm not going to hack my meter apart to make that happen. I can't leave the shield and shunt chassis floating, or else it will become an antenna.My gut instinct says to connect the cable shield to the shunt chassis, and leave the meter end open. Then make a connection from mains earth directly to the chassis of the shunt. I'm lucky enough to have recently installed two dedicated 8 ft x 5/8" diameter copper clad steel ground rods outside the window of the lab, and I have #4 AWG solid copper running through the lab to the house ground reference (the breaker panel is tied to ground through the main water supply inlet). Needless to say, I have a very low impedance, very quiet ground connection that is readily available. I would like to leave the shunt circuitry isolated from earth ground, such that I can measure other earth referenced currents. For improving the performance of the shield, is it a good idea to add additional capacitive coupling between the chassis ground and circuit ground (besides what intrinsically happens when putting the board inside)? While it seems entirely overkill, for safety should I place a series GDT + MOV between the two grounds?TL;DR:I'm not used to working on completely isolated circuits. How do I ground the shield and chassis, and what are the best practices for performance and safety? Any and all input is appreciated.Thanks!

author: dietert1
author_rank: Super Contributor
timestamp: December 23, 2020, 06:09:45 pm
content: You can try what works best for you.In low level DC metrology the shield is normally called Guard. The measurement reference point is normally called Gnd or Low. One would make two separate nets of wiring one for all Gnd and another one for all Guard terminals. If you use shielded twisted pair wiring, then you use the cable shield to build the guard net: connect to guard on both ends. If a meter doesn't have a Guard terminal, the cable shield remains open on that end.Then you would interconnect both nets in one point. I often use a 50 Ohm resistor for this. I also use another 50 Ohm resistor to interconnect the guard net with PE. Probably this is against safety rules, but the resistors help to reduce ringing on those nets. We have some isolation transformers to solve the safety issues, for example one from CoNET - a two chamber transformer with a large aluminum plate separating primary and secondary in order to reduce noise and spikes coming in from mains.We also have some instruments with internal connections between Low, Guard and PE, e.g. a Keithley nanovoltmeter and some power supplies. Using those with the described setup requires special attention. For example the Nanovoltmeter can be used with batteries for several hours, separate from mains. Still it has a short between Low and Guard. The HP 3456A DVM provides switches to produce an internal short between Low and Guard. And it has front and back terminals, so it's fairly easy to make a mistake.Regards, Dieter

author: ignilux
author_rank: Unknown
timestamp: December 23, 2020, 06:40:35 pm
content: Hi, Dieter-Thanks for your reply, there is a lot of good information in there. It looks to me like there is some inconsistency between vendors and specific instruments as far as the role of the shield vs guard. It is my understanding that the point of the guard is that it is to be driven from a low impedance source equipotential to the high impedance node being measured, in order to mitigate leakage currents. The shield, on the other hand, is any low impedance node to which interfering signals are shunted, often circuit or earth ground. I.e., the shield is for electric field noise reduction. Twisted pair is similar, but for magnetic field interference. Furthermore, guarding, shielding, and twisting are not mutually exclusive.Do I have these definitions correct?Cheers!

author: dietert1
author_rank: Super Contributor
timestamp: December 23, 2020, 08:42:07 pm
content: Shield and Guard are different names for a Faraday cage. You want to operate your circuit in a space free of external electrostatic field. And without large voltage offsets causing leakage. That's why you connect Low (or High) with Guard in one point.With distributed wiring the Faraday cage is a bit difficult: A cable shield exhibits inductance so the cage works best at low frequencies. Imagine a water surface that has come to rest. When your neighbor turns off some electrical device it's like throwing a stone into that surface. Your Guard net will ring. Maybe you can use your earth rods to your advantage, but there will be inductive wiring again. I am using those resistors so external spikes will partially stay away and what enters will be damped . And i can look at the voltage across those resistors to see what's going on.Regards, Dieter

author: martinr33
author_rank: Frequent Contributor
timestamp: December 25, 2020, 06:13:41 am
content: A couple more thoughts:Shield and guard are not necessarily equivalent, but they have similarities.- Shield is an electrostatic shield surrounding wiring that is connected at a single point to ground- Guard is an electrostatic shield surrounding wiring that is connected to either of ground, or a voltage generated close to the voltage being measured. Useful for high impedance sources.(i.e as Ignilux broadly described).If you have low impedance sources (like voltage standards) you can connect all the shields together and to guard terminals, then connect the shield to a ground at one point. The 3458A has a switch for this - which can cause trouble if you don't know it is engaged and have a second ground point.The nanovolt meters are interesting, as their shield is connected to the instrument chassis, which is grounded. So they are sort-of your defacto ground.However, the cables they come with do not break out the shield - all you get is V+ and V-. In practice, this seems to be just fine.

author: dietert1
author_rank: Super Contributor
timestamp: December 25, 2020, 08:14:15 am
content: I think for low level DC a guard driven by an amplifier doesn't apply. It may apply if you want audio bandwidth for a high impedance signal (triax wiring with guard inside shield),I would also recommend watching EEVBLOG video #1119 Designing a 1kV Isolated Oscilloscope. They explain how isolated front ends help in comparison to more traditional approaches.Regards, Dieter