Featured Medical Application - Heart Rate Monitor
A
heart rate monitor
can be a useful tool for anyone interested in exercise because it allows a person to manage the intensity of a
workout. This is important because personal fitness goals often require exercise to be maintained at some target
heart rate.
Heart rates
are typically measured either optically, such as with finger-worn devices, or using a chest strap containing
electrodes. Raw sensor data likely requires filtering and precision amplification before A/D conversion and
processing. A microcontroller processes the signal and controls any interface and auxiliary peripherals.
Major subsystems include:
-
An analog front end (AFE) which, depending upon design considerations, may include
precision/instrumentation amplifiers, filters, ADCs, and multiplexors. This can be implemented
discretely or as an integrated solution.
-
A microcontroller, to control peripherals and extract a heart rate from the signal(s)
-
I/O communication
-
Power management
Power efficiency is a primary consideration when designing battery powered devices. Choosing an appropriate
low-power microprocessor can extend battery life.
Regardless of the optical or electrical nature of the sensor itself, the signal will be subject to
significant noise from sources such as 50-60 Hz line power interference or body movements. However, a
well-designed filter may help mitigate these error sources.
Chest Strap
LNA
Coin Cell
INA
Wireless
PMIC
V
REF
ADC
Processor
MCU
LED Driver
Voltage
Level
Shifter
Wireless
Display
ESD
RF
Heart Rate Monitor Block Diagram
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This design is for reference only. The design, as well as the products suggested, has not been tested
for compatibility or interoperability.
Low Noise Amplifiers for Heart Rate Monitors
Amplifiers
have enormous voltage gain, use feedback to operate, and can be classified in different ways. They
can be identified by the device they are intended to drive (e.g., headphone amplifier, speaker
amplifier), the frequency range of the signal (e.g., RF, Audio), and by the function that they
perform (e.g.,
low noise amplifier
, inverting amplifier, power amplifier.)
Coin Cell Batteries for Heart Rate Monitors
Batteries
are a portable, wireless means of storing energy via the use of self-contained chemical cells. They
can be for one-time use and discarded, or recharged and reused. In essence, a battery is an energy
storage device, but can only store and release electricity as direct current. Direct current is a
flat line at a given amplitude (until it declines as it is exhausted), versus alternating current,
which is a sinusoidal wave.
Instrumentation Amplifiers for Heart Rate Monitors
Amplifiers
have enormous voltage gain, use feedback to operate, and can be classified in different ways. They
can be identified by the device they are intended to drive (e.g., headphone amplifier, speaker
amplifier), the frequency range of the signal (e.g., RF, Audio), and by the function that they
perform (e.g., low noise amplifier, inverting amplifier, power amplifier.)
Wireless for Heart Rate Monitors
Wireless technology
enables the transfer of information over short or very long distances without cables. The term
"wireless" most often refers to telecommunications.
Wireless communication
is possible using a wireless transmitter and corresponding receiver. A wireless receiver refers to
the receiving end of the information transfer and requires less energy to operate than the active
transmitting portion where the transfer originates.
Power Managment ICs for Heart Rate Monitors
A
Power Management Integrated Circuit
(
PMIC
) is a special-purpose IC that provides one or more power management related functions. These can
include voltage regulation, DC/DC conversion, battery management capability and more. Many PMICs
offer an I
2
C and/or SPI bus interface, and some might provide additional features such as an integrated touch
screen interface.
Voltage References for Heart Rate Monitors
A
voltage reference
produces a constant level of voltage over time regardless of load, changes in power supply, or
temperature. Voltage references are used in power supplies, analog-to-digital converters,
digital-to-analog converters, and many other applications where voltage levels must be maintained at
a steady level. Without a voltage reference, precision is greatly affected and may render the device
inoperable. Voltage references can vary greatly in performance. A voltage reference for a power
supply might hold its output to within only a few percentage points off of its nominal or stated
value; however, a voltage reference to instrumentation-level standards are measured in parts per
million regarding stability and precision to the nominal or specified value.
ADCs for Heart Rate Monitors
An Analog-to-Digital Converter (ADC or A/D converter) measures the magnitude of an input analog
signal and converts it to a digital number that is proportional to the magnitude of the voltage or
current. An ADC often converts signals collected from the real-world to digital signals for
processing. One of the more important specifications of an ADC is the resolution that it offers,
which is the number of discrete values (represented in bits) that the ADC produces in relation to
the analog signal it is converting. The more bits, the higher the resolution. A higher resolution
yields a more accurate approximation of the analog input.
Processors for Heart Rate Monitors
The term "MCU" or "
processor
" refers to an electronic device that performs computational functions and carries out the
instructions of a stored program. Other terms for processor are microprocessor, central processing
unit, and digital signal processor. Essentially, the processor refers to "the brains of a computer."
MCUs for Heart Rate Monitors
The term "
MCU
" or "processor" refers to an electronic device that performs computational functions and carries
out the instructions of a stored program. Other terms for processor are microprocessor, central
processing unit, and digital signal processor. Essentially, the processor refers to "the brains of a
computer."
LED Drivers for Heart Rate Monitors
LED drivers are used in many applications, but in displays they are a constant-current source
commonly used to power LEDs for screen backlighting. LEDs are current-driven devices whose
brightness is proportional to the magnitude of forward current flow. Desirable features for an LED
driver are tight regulation of current, high efficiency, PWM dimming, overvoltage protection, load
disconnect, small size, and ease of use.
Voltge Level Shifters for Heart Rate Monitors
Level shifters
, or
level translators
, are needed because voltage levels continue to migrate to lower values to support new, low-power
high-performance applications. With this change, system incompatibilities arise as technologies
evolve at a different pace. If two devices are to interface reliably, the output driver voltages
must be compatible with receiver input thresholds. For this condition to be met in mixed voltage
systems, a level, or voltage translator is often required.
USB Transceivers for Heart Rate Monitors
USB
is a standard connection interface between computers and digital devices. A
USB transceiver
is a physical layer device that prepares data for transmission and then sends to, and receives data
from, another transceiver. The transceiver detects connection and provides the low level USB
protocol and signaling. The term "transceiver" indicates an implementation of both transmit and
receive functions. It transmits and receives, encodes and decodes data, provides error indication,
implements buffers to stage data until it can be managed, and adjusts for the clock rate from the
serial stream on the USB SuperSpeed bus to match that of the "link layer" higher up on the
communication stack.
Displays for Heart Rate Monitors
LCD
means "liquid crystal display." It is an electronically driven flat panel screen that orients liquid
crystals within the panel in a direction that blocks or transmits light coming from behind the
panel. LCDs are a low cost, energy efficient visual display that can be controlled in segments or as
individual pixels, in shades of black and gray, or in full color. LCDs have most commonly replaced
bulky cathode ray tubes in televisions and computers and are available in all sizes. Liquid crystals
were first discovered in 1888, but were first put into common use in the early 1970s as electronic
digital-display watches.
ESD for Heart Rate Monitors
Electrostatic Discharge
(
ESD
) is an instantaneous electric current that flows from a higher to a lower voltage potential without
warning. One of the more well-known causes of ESD is static electricity, which is created when
insulator surfaces rub together. Permanent damage can occur to semiconductor devices that are
exposed to ESD. An ESD current waveform has an extremely fast rise time. ESD protection chips can be
very effective but consume board space and add some amount of capacitance to an I/O line.
RF Amplifiers for Heart Rate Monitors
Amplifiers
have enormous voltage gain, use feedback to operate, and can be classified in different ways. They
can be identified by the device they are intended to drive (e.g., headphone amplifier, speaker
amplifier), the frequency range of the signal (e.g.,
RF
, Audio), and by the function that they perform (e.g., low noise amplifier, inverting amplifier,
power amplifier.)
Micro SD Card Slots for Heart Rate Monitors
An
SD card slot
is a connector for a type of flash memory card developed by the SD Card Association (SDA). SD
connectors are designed to accept the asymmetrical card shape of
SD memory
, a feature which prevents the card from being inserted upside down. SD cards come in three sizes
(microSD being the smallest) and while an adapter can enable a smaller sized card to interface with
a larger-type card slot, it is generally desirable to use a connector slot of matching size when
product compactness is a consideration.
USB Receptacles for Heart Rate Monitors
USB plugs and receptacles
are meant to reduce human error by their unique shape; they fit together in only one way. USB plugs
and receptacles come in Type A (typically connecting to hosts or hubs) or Type B (typically
connecting to devices) and 3 sizes: standard, mini, and micro. Type A plugs always face upstream,
Type B faces downstream.
Application Notes and Resources
