How close to 10A 100kHz toroidal SMPS inductor do you dare run HIZ analog signal - Page 1
Forum section: electronics / beginners

author: incf
author_rank: Frequent Contributor
timestamp: April 01, 2025, 11:05:26 am
content: Suppose you had a design withanalog signals with 100kOhm impedance. (heavily filtered at end destination)A toroidal inductor switched at 100kHz with 10A peak currents (part of a SMPS)How close do you dare to run these sorts of analog signals to the inductor?would you run them under the toroid on the same layer? (toroid on top layer, trace on top layer)would you run them under the toroid on the different layer? (toroid on top layer, trace on middle or bottom layer)would you run them along-side / adjacent to a trace or plane feeding the inductor?The signals are heavily filtered at their destinations. A DC offset of 10mV might matter, but "high" frequency AC disturbances might not.I suppose there are two types of coupling. I have a small amount of intuition for parasitic capacitive coupling, but I have very little intuition for parasitic magnetic coupling effects.

author: ahsrabrifat
author_rank: Regular Contributor
timestamp: April 01, 2025, 11:41:59 am
content: Route the analog signal as far from the toroid as possible. A trace running near or under the toroid will experience mutual inductance, especially if it forms a loop.

author: mtwieg
author_rank: Frequent Contributor
timestamp: April 01, 2025, 11:58:16 am
content: Depends entirely on what function the analog signal performs, and what is meant by "HIZ". I could think of a lot of examples, like outputs of transconductance amplifiers, resistor divider outputs (for Vin/Vout sensing), soft start capacitors, etc...If this is a signal which also connects to the SMPS controller, then one would hope the chip designers thought through the pinout to put sensitive signals far from switching nodes.Routing on the opposite side of the board (with power planes in between) is generally the best way to reduce capacitive coupling. If that's not feasible, then adding shunt capacitance and series resistance, or even op amps, in certain places may help, but need a specific example to work with...

author: incf
author_rank: Frequent Contributor
timestamp: April 01, 2025, 12:16:01 pm
content: Here is a sample of some of the signals from (what I perceive to be) most vulnerable to least vulnerable:A remote potentiometer circuit with 100k source impedance used to generate an analog reference/threshold voltage that feeds analog control circuitry (it gets filtered and buffered at it's destination). Related to things like the max output power level as well as various start and stop conditions.A handful low impedance digital signals (eg. reset, enable, etc.) driven by a logic gate running to a remote LEDAn open collector PWM signal related to controlling the SMPS (filtered to become an analog average voltage at it's destination)A low impedance 3.3V supply rail

author: Kleinstein
author_rank: Super Contributor
timestamp: April 01, 2025, 12:48:16 pm
content: For the magnetic interaction it does not matter how high the impedance. Magnetic coupling adds voltage, not current.capacitive coupling would effect a high impedance signal more. There can still be capacitive coupling near the inductor from the coils.Toroidial transformers can have quite different stray fields. This especially with windings going one way to create an effecctive path in parallel to the core.Chances are the filtering of the signal could be enough to redue the 100 kHz interference enough.Ideally I would want some shielding (copper layer on top and the sensitive signal only below) if the signal is sensitive.

author: incf
author_rank: Frequent Contributor
timestamp: April 01, 2025, 01:07:01 pm
content: How would one go about "mentally modeling" the inductive interaction? (or better yet, measuring it)An air core transformer where there are "N turns" of toroid and "1 turn" of PCB trace?My intuition says that the equivalent series voltage generated in a 10A trace or toroid 1mm away from a signal would be quite small and have a high ESR.It would not be hard to fit U.FL connectors and run a test traces to try to gauge the amount of capacitive and inductive coupling. (To me, what does seem hard is "knowing" what is measured is valid, meaningful and translating the measured levels into something like an equivalent current/voltage source)

author: PartialDischarge
author_rank: Super Contributor
timestamp: April 01, 2025, 01:25:35 pm
content: Parasitic capacitive coupling will be your main enemy, for magnetic coupling you need a loop to which the magnetic field can get into.Some solutions to the former:-decrease source impedance-add some electric shielding, via a ground plane in between the inductor and the trace or coaxial cable for the analog signal.-differential signaling

author: incf
author_rank: Frequent Contributor
timestamp: April 01, 2025, 01:33:11 pm
content: Quote from: PartialDischarge on April 01, 2025, 01:25:35 pmThere is always a loop, right?Between any signal and the ground plane is a loop. (a small one)My recollection is foggy. In an air core transformer I suppose the relative areas of the two loops would determine the degree of coupling?

author: PartialDischarge
author_rank: Super Contributor
timestamp: April 01, 2025, 01:39:17 pm
content: Quote from: incf on April 01, 2025, 01:33:11 pmYes, any area in which the magnetic field can get into will induce a current, which in turn will induce a voltage depending on the impedance. So the shorter the trace the smaller this area.In your case I think capacitive coupling will be worse. You can measure with a LCR meter the capacitance between your analog line and coil, then run a simulation to see what the result will be, 100kohm is usually too high for analog signals, enough to deserve a coaxial RG174 cable by itself.

author: TimFox
author_rank: Super Contributor
timestamp: April 01, 2025, 01:46:03 pm
content: Quote from: PartialDischarge on April 01, 2025, 01:39:17 pmAC magnetic flux induces an EMF (voltage) around a loop. That voltage will produce a current through the loop equal to EMF/(total impedance), and that current will produce voltages across each impedance in the loop according to their individual values, adding up to the total EMF.

author: PartialDischarge
author_rank: Super Contributor
timestamp: April 01, 2025, 01:54:59 pm
content: Quote from: TimFox on April 01, 2025, 01:46:03 pmYes the Faraday law refers to an EMF, I should have been more specific, many times I prefer to think of induced currents directly since I always deal with closed loops