AN1690.0 Datasheet, PDF (1/3 Pages) Intersil Corporation – Electronics Meets the Challenges of Patient Monitors

Application Note 1690
Electronics Meets the Challenges of Patient Monitors
The medical industry is keeping pace with other major
markets in terms of integration. Single units such as blood
pressure cuffs, ECGs and oxygen sensors have successfully
entered consumer markets, but there is increasing interest in
patient monitors, which encompass all of those monitoring
abilities and more.
The medical market is generally grouped into three major
subcategories: home, clinical and imaging. Home medical is
predominately lower cost, portable equipment that has lower
performance requirements. Clinical and hospital grade
equipment is generally higher performance and therefore
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.
piezo-resistive pressure transducer. For simplicity, several of
these biometric modules may utilize common digital, power
and IO subsystems.
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.
Fundamentally, most of these systems are analog sensor
measurement systems, but 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 5 basic
subsystems: ECG, pulse oximetry, blood pressure, body
temperature and respiration. Typically the most critical
components in each system are the sensor circuits.
Each module uses a different sensor and signal conditioning
circuit. For example, the ECG uses electrodes to measure the
electric pulse from the heart. The pulse oximetry (SpO2) uses a
light-emitting diode and light sensor to measure oxygen
content. Blood pressure is typically measured using a
Most commonly used piezo-resistive silicon 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 that 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 proportional 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 frontends due to their low noise and low DC offset and
drift.
B IO M E T R IC
ANALOG/DIGITAL MODULES
E C G /E K G
IN AMP
SPO2
P R E C IS IO N
OP AMP
PROCESSOR
SYSTEM I/F
& CONTROL
AUDIO AMP
IN T E R F A C E
LEVEL XLATOR
U S B /K B D
MEMORY
BLOOD
PRESSURE
VOLTAGE
REF
BODY
TEMPERATURE
A /D
DCP
USER INTERFACE
TOUCH
SCREEN
CONTROL
AMBIENT LIGHT
SENSOR
POWER
CORE &
I/O POW ER
BATTERY
MANAGEMENT
R E S P IR A T O R Y
SW ITCH/
MUX
DISPLAY I/F VIDEO DRIVER
SYSTEM
FUEL
POWER
GAUGE
OR
C O M M U N IC A T IO N
OR TELEMETRY
LCD
A C /D C
BATTERY
CHARGER
W IRELESS
ETHERNET
FIGURE 1. BLOCK DIAGRAM OF A PATIENT MONITOR
October 28, 2011
AN1690.0
1
CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures.
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