“The medical market is generally divided into three subcategories: home, clinical, and imaging. Home healthcare is primarily a portable device with lower cost and lower performance requirements. Clinical and hospital-grade equipment are generally higher performing and therefore more expensive. Imaging systems are typically mainframe-based systems, with the exception of ultrasound, which is rapidly expanding into portable and cart-based systems.
The medical market is generally divided into three subcategories: home, clinical, and imaging. Home healthcare is primarily a portable device with lower cost and lower performance requirements. Clinical and hospital-grade equipment are generally higher performing and therefore more expensive. Imaging systems are typically mainframe-based systems, with the exception of ultrasound, which is rapidly expanding into portable and cart-based systems.
Fundamentally, most of these systems are analog sensor measurement systems, but adapted for 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 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 current or voltage and applied to an actuator to control things like air flow, oxygen or temperature.
A typical high-end patient monitor system has five basic subsystems: ECG, pulse oximetry, blood pressure, temperature, and respiration. Often, the most critical component in every system is the sensor circuit.
Each module uses different sensors and signal conditioning circuits. For example, an electrocardiogram uses electrodes to measure electrical impulses from the heart. Pulse oximetry (SpO2) uses light-emitting diodes and light sensors to measure oxygen levels. Blood pressure is usually measured using a piezoresistive pressure sensor. For simplicity, some of these biometric modules may use common digital, power, and IO subsystems.
Figure 1 Block diagram of a patient monitor
In the blood pressure biometric module, the most critical function is the pressure sensor circuit. Precision amplifiers are used to detect very small signals from transducers and amplify them to a level suitable for ADC processing. This is usually followed by an active filter to limit higher frequency unwanted noise. Low noise, low drift, and high gain amplifiers must be used to minimize measurement errors and ensure accurate readings. See Figure 2 for a system block diagram.
Figure 2. Block diagram of blood pressure system
The most commonly used piezoresistive silicon pressure sensor in medical applications is the Wheatstone bridge. The pressure sensing element combines a resistor and etched diaphragm structure to provide an electrical signal that varies with pressure. When the diaphragm is moved under pressure, the stress is concentrated in specific areas of the silicon element. The result is a small voltage that changes proportional to the pressure applied to the diaphragm. The bridge signal is then amplified using a precision op amp prior to ADC conversion.
The key questions to ask when recommending an amplifier are the required accuracy and the required voltage. Hospital-grade equipment has different requirements than portable home systems, and pressure sensors vary in sensitivity and voltage requirements. The amplifier is usually chosen to match the requirements of the sensor. The ISL28127 and ISL28217 are excellent op amps for the gain front end of ±5V pressure sensor amplifiers because of their low noise and low DC offset and drift.