High-Impedance and Low-Noise Op Amps Enable Dry Electrodes in Medical Systems

Introduction

The use of wearable sensors for fitness and health monitoring is growing tremendously. Over the past decade, wearable technology has gained much attention from the tech industry for commercial reasons and has generated interest of physicians due to its potential benefits in the health of their patients. Wearable devices use biomedical sensors to monitor not only activity parameters such as step count or walking/running speed or elevation, but also physiological parameters such as heart electrical activities or blood pressure.

One of the more popular biosensors is an ECG sensor (also called EKG), which records electrical heart activity over a period of time (Figure 1). Each heartbeat is triggered by an electrical impulse, normally generated from special cells in the upper right chamber of the heart. The device that measures and records the electrical activity of the heart is an ECG machine or electrocardiograph. It works by attaching electrodes to the surface of the patient's skin. This recorded activity is then analyzed by physicians and is used to diagnose patients' various heart conditions to help monitor their progress. ECG machines are used extensively worldwide, from hospitals to many healthcare practitioners, to monitor the health of their patients.

A Fully Integrated ECG Solution

A typical biopotential ECG solution is shown in Figure 2. Two lead electrodes are attached to the human body, then connected through an external EMI filter to the ECGP and ECGN pins of the MAX30003. The MAX30003 is a fully integrated biopotential ECG AFE IC, comprised of an input MUX, a fast-recovering instrumentation amplifier, an anti-alias filter, and a programmable gain amplifier (PGA). The output of the PGA drives a high-resolution ADC with the conditioned signal that is then sent to the microprocessor for storage, calculation, and display.

There are two types of electrodes, wet or dry electrodes. By attaching the electrodes to the skin, they receive ECG signals generated by the heart muscle. The clinical use of ECG sensors includes wet electrodes, which need to use a sticky gel to adhere to the body. In contrast with conventional wet electrodes, dry electrodes use a single metal as a conductor between the skin and the electrodes, and do not need conductive gels. Because of the dry electrodes' easy setup, many types of ECG wearables such as wristbands, chest straps, and smart watches use a dry electrode-based ECG sensor. The solution we introduce here is ideal for an ECG with conventional wet electrodes, with an impedance in the range of 10kΩ. However, when used with an ECG with dry electrodes, the impedance of the dry electrodes is 100 to 1000 times higher than conventional electrodes (for example, 10MΩ for dry electrodes vs. 10kΩ for conventional electrodes). With such a high impedance, the ECG signal gets attenuated, resulting in a small signal. To accommodate this high impedance, the input impedance of a biopotential ECG channel should be very high so that the attenuation is kept to a minimum. The MAX40023/MAX40024 high-impedance, low-noise buffers help maintain detectable ECG signals.

To maintain a decent ECG signal within a typical and sufficient range that can be detected and processed, the front end of an ECG channel really needs a high-impedance buffer with low bias current (Figure 3). The low-noise performance of the buffer is also required to avoid adding too much noise into the whole system and to meet the ECG system noise standard. In consideration of a wearable device's battery life, the power consumption of the buffer must be as small as possible.

The MAX40023/MAX40024 family features these necessary specifications, making them the best candidates for medical sensor applications. These low-noise, low-power, low bias-current amplifiers have an input bias current of 5pA (maximum) and consume only 17µA quiescent current per channel. The noise performance is also outstanding, with less than 32nV/rtHz noise floor, and less than 1.5µVP-P in the 0.1Hz to 10Hz bandwidth. The MAX40023 also has an internal EMI filter with an EMI rejection ratio of at least 110dB at 1.8GHz and 2.4GHz. Since most ECG devices have more than one pair of dry electrode leads, the MAX40024 is a dual amplifier in a 9-pin WLP package, which is ideal for space-saving wearable applications.

Conclusion

The MAX30003 is a fully integrated ECG analog front-end IC, which can be easily implemented in ECG measurement with either a conventional or dry electrode. However, to get the best performance out of a dry electrode, high-impedance, low-noise buffers such as the MAX40023/MAX40024 can be used. These op amps, with less than 5pA bias current, help keep the ECG signal at a reliable level for detection. Their space-saving feature, as well as low power consumption, is a good fit for wearable ECG devices.