Electronics meets the challenges of patient monitors

The medical market is generally grouped into three major subcategories: home, clinical and imaging. Home medical is predominately lower cost, portable systems that have lower performance requirements. Clinical and hospital grade equipment features generally higher performance and is more expensive. Imaging systems are generally very large mainframe bases systems, with the exception of ultrasound, that is rapidly expanding into portable and cart-based systems.

Most of these systems are analog-sensor measurement systems, but are applied to biometric functions such as blood pressure, body temperature or heart rate. These biometric sensors are designed to measure physical events such as temperature, pressure, light and flow. After measurement, the system converts them into a corresponding voltage or current. The signal is then conditioned and digitized for processing and analysis. On the control side, the signal is converted back to a current or voltage and applied to an actuator to control such things as air flow, oxygen or temperature.

A typical high-end patient monitor system has five basic subsystems: ECG; pulse oximetry; blood pressure: body temperature; and respiration, Figure 1. Typically, the most critical components in each system are the sensor circuits.

Figure 1: Block diagram of a patient monitor.
(Click on image to enlarge)

Blood Pressure
In blood-pressure biometric modules, the most critical function is the pressure-sensor circuit. Here, precision amplifiers are used to detect very small signals from the transducer and amplify them to a level suitable for ADC processing.

This is typically followed by an active filter to limited unwanted noise at higher frequencies. Amplifiers with low noise, low drift and high gain are necessary to minimize measurement errors and ensure accurate readings. See Figure 2 for a system block diagram.

Figure 2: Block diagram of a blood-pressure system.
(Click on image to enlarge)

The most commonly used piezo-resistive silicon-based pressure sensor in medical applications is the Wheatstone bridge. The pressure-sensing element combines resistors and an etched-diaphragm structure to provide an electrical signal which changes with pressure. As the diaphragm moves under pressure, stress is concentrated in specific areas of the silicon element.

The result is a small voltage that changes in proportion to the pressure applied to the diaphragm. This bridge signal is then amplified using precision op amps prior to ADC conversion.

Key questions to ask when recommending an amplifier are: what is the required accuracy and what are the required voltages? Hospital-grade equipment has different requirements than portable home-based systems, while pressure sensors have varying sensitivities and voltage requirements. The amplifier will generally be selected to match the requirement of the sensor. The ISL28127 and ISL28217 are excellent op amps for ±5V pressure sensor amp gain front-ends due to their low noise and low DC offset and drift.

Electrocardiogram (ECG)
There are several precision amplifier and instrumentation amplifier opportunities in ECG applications. Figure 3 shows a diagnostic (or clinical) ECG with up to 12 leads. Key blocks for lead devices are the electrode gain amplifier, high-pass filter (usually 0.5 Hz), low-pass filter (around 150 Hz) and right-leg drive circuit. For ECG, each electrode requires a precision instrumentation amp to extract a very small signal that rides on a 300 mV to 700 mV common-mode voltage.

Typically, this amplifier will use a higher supply voltage to enable a high gain without railing the amplifier in the presence of the high common-mode voltage from the body. This amp can be a discrete instrumentation amplifier or an integrated instrumentation amplifier. Second- and third-stage active-filter amplifiers are needed to set a very specific band (0.5 Hz – 150 Hz) to capture the ECG QRS wave signal. Typically these will be low-noise, 5V amplifiers with good appropriate bandwidth. In addition, low-noise, low-power amplifiers are needed for the right-leg-drive feedback function.

Figure 3: Block diagram of an ECG system.
(Click on image to enlarge)

In multi-channel systems, such as a 12-lead ECG monitor, it is common to multiplex signals into a common ADC. The key typical requirements for the multiplexer (mux) are low on-resistance and low charge injection.

Generally a specific mux is selected to match the voltage requirements of the filter amplifiers and the ADC. It is also common for multichannel ECGs to have automated lead detection to enable multi-configuration operations. Generally, a low on-resistance switch is used in this circuit as well.

Multiplexers like the ISL43681 and ISL43640 are excellent choices for medical devices as they can operate from 2 V to 12 V. In addition, they have low on-resistance of 39 to 60 Ω, which lowers distortion and reduces ‘kick-back’ voltage from the ADC. Their low charge injection of 0.3 pC to 2 pC (picocoulombs) reduces error contribution on charge-redistribution ADCs. They are also low power and available in the very small QFN package.

For 3-lead portable ECG applications, Iow-power instrumentation amps from a CMOS process may be a better selection. Since CMOS inputs naturally provide a high-impedance input, the need for external buffers is eliminated, saving cost. A good choice for the input amplifier can deliver active feedback with a very precise base line compensation voltage, low 1/f noise, extremely low offset, and low drift versus temperature.

Pulse Oximetry (S_pO₂)
Oxygen is carried in the blood in hemoglobin and is one of the key vital signs needing detection. Pulse oximetry takes advantage of the fact the blood absorbs certain wavelengths of light differently when it is oxygenated compared to when it is oxygen-deprived.

Figure 4: Block diagram for pulse oximetry system.
(Click on image to enlarge)

A high-impedance, low-bias-current op amp is needed to process the photodiodes that receive the signal at these wavelengths, Figure 4. The ADC also needs to have the fast throughput of a 16-bit device. The DC and background noise is subtracted out, and the pulsed signals are then scaled. Extensive over-sampling, filtering, and signal processing eliminate noise such as movement artifacts from the small signals, and allow the pulse rate to be measured.

Conclusion
Advanced medical systems now require a wide range of amplifiers, multiplexers, converters, references, interface products and power supply products. The portable instruments need the lowest power and most meticulous battery management beyond the stringent noise and gain requirements. The non-portable instruments need reliable isolation and patient protection features, in addition to their precise measurement capability.

It is difficult to offer the best solution for all situations. Every tradeoff of power consumption, available supplies, required resolution, portability, noise and more will play into the selection of the key components. Understanding the implications of device choice will ensure that the system safely delivers quality for the lifetime of the system.

About the author
Tamara Schmitz grew up in the Midwest, finding her way west with an acceptance letter to Stanford University. After collecting three EE degrees (BS, MS, and PhD), she taught analog circuits and test-development engineering as an assistant professor at San Jose State University. With 8 years of part-time experience in applications engineering, she joined industry full-time at Intersil Corporation as a principal applications engineer.