This article demonstrates how to detect Electromagnetic Interference (EMI) and apply suitable filters to eliminate unwanted noise from previously recorded physiological data.

EMI Types

There are two main types of EMI which are Conducted and Radiated. Conducted EMI travels through conductors like wires or power lines while Radiated EMI propagates through the air without needing a conductor.

Common Sources of EMI

Within each main type of EMI, the sources can be broken down into Natural and Human-Made.

Natural sources of EMI include lightning strikes, solar flares, cosmic noise, static electricity, atmospheric electrical storms, solar magnetic storms, dust storms, snowstorms, solar radiation and more.

Human-Made sources of EMI include alternating current (AC) hum from the power grid, Wi-Fi and Bluetooth devices, lighting systems, electric motors, television broadcasts, radio signals, medical equipment, transmitters, transformers, inverters, microprocessors, microwaves and more.

Most Prevalent Source of EMI in Physiological Data Collection

Alternating Current (AC) 

This type of noise originates from the power grid and creates 50/60 hertz (Hz) & 100/120Hz noise, also known as mains hum or an electrical hum. In North America, the power grid alternates current 60 times per second or at 60Hz. Everywhere else the power grid alternates at 50Hz. For our purposes, 50/60Hz noise typically originates from AC electromagnetic fields from nearby appliances and wiring. Additional harmonics above 100/120 Hz will be caused by the non-linear behavior of most common magnetic materials.

Conducted EMI is the most common form of EMI that we observe in the ECG/EKG and EMG signals. This is typically observed when a participant is touching an electrical cable or device that is plugged into the AC outlet. 

It’s possible to observe 50/60Hz noise when the MindWare Mobile device is collecting data and charging at the same time. For this reason, it is not ideal to charge the Mobile during data collection, but there is an effective filter available for this common type of interference.

It is somewhat common for universities and hospitals to have specialist research and medical equipment that creates radiated EMI so the interference could also be in the environment at your data collection site.

Shared Impedance Frequencies

The other major source of hum is shared impedances; when a current is flowing through a conductor (a ground trace) that a small-signal device is also connected to. 

In some cases where two or more participants are touching each other, the Impedance Cardiography (ICG) signal can be affected by shared impedances in which the conductor is the body.

Additional Sources of Noise

Conducted and radiated sources of EMI are diverse and include elevators, Wi-Fi and Bluetooth devices (such as laptops, cell phones, and wireless routers/access points), lights, electric motors and generators, television transmissions, radio and satellite signals, medical and research equipment, transmitters, transformers, inverters, microprocessors, and microwaves. 

Detecting EMI In Your Data File

Please watch this below video for a step-by-step tutorial on identifying and filtering EMI. You may also follow the steps below if you do not wish to watch a video.

Step #1 – Open BioLab

Click Cancel if greeted by the Network Device Detection Window or Open File if the BioLab window appears

Step #2 – Open a File and View Waveforms

Step #3 – Static Analysis and Frequency Domain

Right click on the waveform that you wish to inspect for EMI. Select Static Analysis and then Frequency Domain

The Frequency Domain Analysis window will show data in a time series graph, “Channel Waveform” at the top of the screen, and the frequency splatter of the time series data within the “Power Spectrum” graph at the bottom.

Step #4 – Static Analysis and Frequency Domain

There can be multiple spikes across the frequency spectrum. This is due to the properties of electrical harmonics in which a source or fundamental current is being repeated throughout the frequency spectrum. For example, if we observe spikes at 60, 120, 180, and 240 Hz, the source for the spikes at 120, 180, and 240 Hz, is likely the initial spike that occurs at 60 Hz.

Use the second zoom button to click, drag, and release a window around the first major spike at the lowest frequency.

Note: In many cases, filtering out the fundamental (leftmost) peak will dampen the affects of the higher frequency spikes. You will usually only need to apply a filter at the source interference spike for further analysis, even if the subsequent spikes remain.

Once the center point of the fundamental interference has been properly identified, you may click the red Done button at the right hand side of the screen. Then, click the red Exit button on the BioLab File Playback window which will return you to the main BioLab Configuration screen.

Step #5 – Applying a Standard Filter

Click into the Filter Type window associated with the waveform you wish to apply a filter to. In this case, the source interference spike occurred at 60 Hz which is common frequency of interference in North America so a standard filter is available (60 Hz Notch). If you are observing data collected in the rest of the world, you will likely want to apply the 50 Hz Notch filter.

Once the 60 Hz Notch filter is selected, you will notice that a Low Cutoff of 50 Hz and a High Cutoff of 70 Hz have applied. Normally, it is best practice to begin and end the tapering of a filter +/- 10 Hz from the center point of the interference spike.

Open file in BioLab

Standard MindWare BioLab Filters

• 50/60Hz Notch Filter

Band Stop Filter

• Enter a Low Cutoff that is 10 units (10Hz) below the center point of the fundamental noise frequency that you identified
• Enter a High Cutoff that is 10 units (10Hz) above the center point of the fundamental noise frequency that you identified
• File/Save As – label with the low and high band stop frequencies

Using BioLab Filters for Analysis

• Open MindWare analysis application
• File/BioLab Filters…
• Find channel that you applied filter to
• Ensure the filter is applied
• Click Use BioLab Filters button

Signals and Noise

Electrocardiography (ECG/EKG) & Electromyography EMG with 50/60Hz Noise

As stated above, the most common type of EMI that we observe is of the conducted variety originating from AC. This type of noise commonly affects the ECG/EKG and EMG waveforms. For further instruction on how to remove this noise from your data, please refer to the MindWare support article “ECG 60Hz Noise Demo Data”.

Impedance Cardiography (ICG) with Shared Impedance Noise

Revisiting an earlier text, In some cases where two or more participants are touching each other, the Impedance Cardiography (ICG) signal can be affected by shared impedances in which the conductor is the body. 

If your protocol involves participants touching each other and you believe that you are observing noise in the Zo and Dz/dt waveforms, please reach out to us or directly create a support case here so that we can help. In most cases, we will request that you send us one or more of your Mobile devices so that we may offset the constant current such that its frequency does not interfere with the other Mobile/s.