What electrical engineers need to consider when designing from practical experience

     By Gustavo Castro Analog Devices

 

     Q: What should I be concerned about when testing my heart rate monitor prototype? 

 

     A: I recently saw a humorous video on YouTube called “I Can’t Believe We Survived” that showed a lot of activities that were common when I was a kid, but are dangerous by today’s standards. While most of these activities seemed completely irresponsible to allow children to participate in (and some of them could have gotten us in legal trouble), at the time, it didn’t seem like a big deal. However, because people got hurt, sick, or worse, we learned our lesson and became more cautious. This video made me think about the activities we do today and how we will look back on them and wonder “Why did we do that?” Fortunately, electrical engineering is not as dangerous as it used to be (op amps like the K2-W required ±300 V supply voltages, which are no longer needed). However, dangerous situations still occur from time to time. 

 

     Due to growing interest in biomedical instrumentation, my team and I decided to design a portable ECG with the goal of monitoring heart rate to diagnose hard-to-detect arrhythmias. At the time, we knew isolation would be essential to the final product, but we hadn’t really stopped to consider it during the development phase. We also had no idea if the test equipment was isolated or even what type of isolation might be used. 

 

     We quickly discovered that acquiring the signal was the most important first step, so we selected the AD620 instrumentation amplifier, along with a couple of op amps for filtering and right leg drive. For isolation, we powered from a 9 V battery and used a DC/DC converter to generate the ±15 V supplies. We purchased some silver/silver chloride electrodes and used twisted wire between the electrodes and the breadboard to avoid picking up noise. So far, everything has worked fine. Next, the subject had to unbutton their shirt and apply the electrodes… oh, and by the way, we needed to see the output at the other end, so we used an oscilloscope to probe the output. 

 

     Oscilloscopes require their ground lead to be connected to earth. So we grounded the system, violating the isolation requirement. Now we are exposed to the danger of leakage current. Even more frightening, the leakage current could go straight through our chest. Ignorance is bliss, so we connected the electrodes to our bodies and probed with the above oscilloscope and benchtop DMM at the same time. Fortunately, everything is wrong the first time, so nothing happened. If you know a thing or two about leakage current on isolated power supplies, you might wonder: "How can this guy still be alive?" 

 

     Fast forward 15 years. Today, heart rate monitoring has become a mainstream application due to increased health awareness and the proliferation of wearable computing devices. This has led to an increase in developers experimenting with heart rate monitors such as the AD8232, and investigating new high-quality ECG recording alternatives using AD620 successors such as the AD8421 and AD8422. This has also led to an increase in the number of situations where unwary engineers are putting themselves in harm's way. I would like to remind my fellow electrical engineers to be cautious and understand and follow safety guidelines before testing prototypes on humans. There are several resources available in publications and on the web for this purpose. If you have any doubts, you can purchase a commercial ECG signal generator at a relatively low price to avoid the tragedy of "dying before you get started"!