# Injection of EMI to input pins of op-amp in LTspice

In order to observe variation in output from op-amp due to electromagnetic interference I need to inject EMI signal at the input of a CMOS op-amp.I am trying to do an LTspice simulation on this. Can a voltage source be used for this? Is there any other way to simulate EMI noise in LTspice?

• Can a voltage source be used for this? Sure, or a current source. Does it matter ? Is EMI a voltage or a current ? I think voltage or current is irrelevant. You should think about how the EMI will couple to the circuit and how to model that correctly. Is it common-mode or differential ? Via the supply or ground lines ? No, that is not so easy as just adding a voltage or a current source. Realize that your results and conclusions can never be better than how you modeled the system. A bad model will give bad (unreliable) results. Nov 8, 2016 at 14:24
• It is a common mode signal. Basically a sinusoidal signal with zero mean dc voltage Nov 8, 2016 at 16:08

## 1 Answer

If you want to simulate a EMI injection in spice, develop a circuit equivalent of what your trying to model. EMI can couple into signals via inductance or capacitance. If its a magnetic field then use inductive coupling, if its an electric field, then use capacitive coupling. Or a combination of both might be needed. Here is an image from electronic.nu that shows both methods.

Mutual inductance is between two different wires, loops or coils. A wire on a PCB can pick up EMI, first you need to find the EMI source and model the coupling between the two conductors. There are PCB trace calculators to do this. A harder way is calculating the magnetic field impinging on the trace but this is possible if you know what your source signal is.

A capacitivly coupled surface will also induce noise on a PCB trace. Mutual capacitance between traces on a PCB can be calculated. If its between two plates of metal you can use the capacitor equations.

Then you can plug the values for capacitors and\or inductors (if its mutual inductance you also need the K term to couple the inductors together in lt spice) and model it as a circuit. I will say this, upper bounding/lower bounding on equations for estimates goes a long way because in a lot of cases when calculating its hard to get an exact answer.

• +1 just for the veeeery interesting link. I am struggling to find now all English articles by the same author on EMC... Apr 18, 2017 at 23:33
• Thank you so much for the response. Is there are any other simulating software in which inbuilt EMI noise model is present(like white noise in SPICE). May 12, 2017 at 5:03
• Most spice packages, you could probably run some time varying circuit sims in simulink and or simscape. CST Studio, Comsol and ansys HFSS are 3d field solvers and can run transient simulations. You really need to get the book Electromagnetic Compatibility Engineering by Henry Ott. If you like the answer upvote and mark answered May 12, 2017 at 6:20