An Introduction to Biopotential Measurement Challenges
In this new precision low power blog series, we will take a deep dive into the challenges of making biopotential measurements. We will also address some of the common topics that frequently come up, such as:
Designing a precision low power signal chain that maximizes battery life
Measuring small signals in the presence of various interferers
Understanding electrodes and difficulties making good contact with dry electrodes
Common Mode Rejection Ratio (CMRR) and Right Leg Drive (RLD)
Implementing electrode/lead-off detection
The first bullet is an important one to highlight and is not limited to healthcare wearables. Applications such as environmental monitoring, field instruments (electromagnetic flow, level, pressure, and temperature sensing), electrochemical gas sensing, and pH measurement can all require precision at low power. So, what makes this challenging? Going from higher voltage bipolar supplies to a single 3.3V or 1.8V supply greatly reduces the amount of headroom available and limits the gain you can apply. Battery-powered applications are typically space-constrained, so PCB area is at a premium. Additionally, tradeoffs between power, noise, bandwidth, and measurement sample rates must be considered.
Low power designs need a system level approach to truly optimize battery life. Fortunately, Analog Devices is providing recommendations, tools, and support for complete precision low power signal chains to assist with this.
The lines between fitness-based wearables and healthcare technology continue to blur with the introduction of products that can detect atrial fibrillation (AFib) and even sleep apnea. The past two years have further emphasized the need and urgency for medical-grade devices that can monitor the health of patients in the home, enable early detection and prevention of diseases, reduce costs, and communicate with doctors remotely. Biopotential measurements can further be leveraged in a variety of other applications such as augmented reality (AR), virtual reality (VR), authenticating user identity, or even controlling a prosthetic limb. Regardless of the application, the challenges of acquiring these signals remain the same, and this series will aim to address the pain points in your design. If a topic is not covered, please feel free to ask your questions on our dedicated EngineerZone page for Precision Low Power Signal Chains.
Figure 1 – Biopotential Signals sensed by electrodes and their sources [^1]
Biopotentials are electrical signals generated by the electrochemical activity of various cells within the body. When these cells are stimulated, an action potential is produced. Action potentials are a way of transferring information between adjacent cells, and the accumulation of these can result in biopotentials typically measured. Figure 2 shows the electrical activity within the heart and the more recognizable ECG biopotential signal that can be measured with electrodes on the skin. This 3D Heart Animation explains this in more detail.
Figure 2 – Waveforms for specialized cells in the heart and corresponding ECG signal [^2]
The difficulty with measuring these biopotentials is the presence of other undesired interferers. DC to low frequency can contain large offsets due to the electrode half-cell potential and electrode mismatch, slow-moving baseline wander, 1/F noise, and motion artifacts. Note that these signals are not common mode and appear in series with the biopotential signal. At higher frequencies, the common mode signal to worry about is the 50/60Hz from the AC mains. Sometimes other biopotentials can even be an interferer such as EMG when measuring ECG or ECG when measuring EEG. Figure 3 shows the overlapping frequency ranges of these biopotentials and interferers.
Figure 3 – Example biopotential signal amplitudes, frequency ranges, and interferers [^3]
In the next blog, we’ll discuss how to design a signal chain to measure biopotentials while managing interferers, including AC and DC coupled solutions.
[^1]: Neuman, M. R. “Biopotential Electrodes.” The Biomedical Engineering Handbook: Second Edition. Ed. Joseph D. Bronzino Boca Raton: CRC Press LLC, 2000. [^2 ]:Malmivuo, J. & Plonsey, R. Electromagnetism Principles and Applications of Bioelectric and Biomagnetic Fields. Oxford University Press, 1995. [^3]: Ha et al. “Low-power integrated circuits for wearable electrophysiology.” Wearable Sensors: Second edition. Ed. Edward Sazonov Academic Press, 2020.