Download Mixed Signal Devices and Sensors Generic Sensor System Block

Mixed Signal Devices and Sensors
By: Aaron GL Podbelski, Cypress Semiconductor
Sensor design in embedded systems has traditionally required analog experts to interface with the sensors, but the use of
mixed signal products relegates embedded sensor design to a typical EE. Previously, interfacing with an analog sensor
required the use of external amplifiers and filters to conditional the signal properly before digitizing the signal in a
microcontroller’s ADC. By utilizing a mixed signal controller, the amplifiers and filters are integrated, allowing the designer to
program each component in the signal path. This integration simplifies the design and reduces the time and complexity of the
system.
Filter
Amp
Sensor
ADC
Digital
Interface
Processing
Core
HOST
E.g. UART,
Modbus,
etc
Actuator
Control
E.g. PWM
Standard
Microcontroller
PSoC
Generic Sensor SystemBlock Diagram
Figure 1: basic sensor signal path, and the increased functionality a mixed signal brings to the solution
Cypress’ PSoC allows users to integrate much more functionality than a traditional microcontroller. At the heart of PSoC is a
microcontroller core, but what allows for such increased ability is PSoC’s configurable digital and analog blocks. In these
blocks the user can place User Modules to select the functionality they want out of the PSoC such as counters, PWMs, UART,
SPI, amplifiers, filters, ADCs, and DACs. The fact that PSoC has this ability gives it all the functionality competitive
microcontrollers, but with added analog functionality and completely user configurable. The PSoC also has the ability to be
dynamically reconfigured, which means that User Modules can be swapped out and be replaced by different ones on a time
division basis. This can allow a user to consume more than 100% of the resources on the PSoC. The configurable digital and
analog blocks of the PSoC allow it to be the ideal choice for systems utilizing sensors.
Each sensor has a different output signal and range. The output signal can be voltage, current, resistive, capacitive, or
frequency based, but few standards exist, and they are only used in specific industrial systems. Since a majority of sensors
output a low level voltage based signal, this article will assume a voltage signal is being used. Note that this article simplifies
some concepts and the component selection process for efficiency’s sake.
Mixed Signal Devices and Sensors
Published in Electronics Components World (http://www.electronicscomponentsworld.com)
Page 1 of 3
January 2009
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The output of a sensor can be as small as several mV and as large as several volts. In order for the signal to be properly
digitized it needs to be large enough for the ADC to effectively read the signal. In the easiest case, the sensor will output a
high amplitude signal that does not need to be amplified. But most sensor signals will need amplification. As an example, a
typical type K thermocouple outputs 41 µV/°C, which needs to be greatly amplified if the user would like to read 1°C
granularity. When selecting an amplifier, the designer must take the ADC resolution into account to ensure that signal is
amplified enough to obtain the desired granularity. Selection of an amplifier is mainly based on the amount of gain needed.
The gain of an amplifier can be between 1x amplification to several 1000x amplification. The gain of a basic operational
amplifier is set by network of resistors. An alternative to an operational amplifier is a programmable gain amplifier (PGA)
which is a device where the gain is set digitally. PSoC has several types of integrated PGAs which can be used individually to
amplify the signal or cascaded together for increased amplification.
In all systems some noise will be imparted on the sensor’s signal. The noise can come from a number of sources including
board layout, radios, thermal noise, and even the sensor itself. Not only does signal noise will cause inaccurate and unstable
readings by the ADC, but the noise has been amplified which exaggerates the error in the signal. Signal noise can be
quantified as low frequency, high frequency, or a specific known frequency. Most often the noise is high frequency noise. To
remove noise, the signal is filtered using low pass or band pass filters. Normally these filters are implemented using passives
on board. Passive filters can be designed to be relatively simple or extremely complex depending on the system’s noise
requirement, but they are static filters. If any possible sources of noise on the board are not taken into account, a board respin/BOM change may be required. PSoC integrates programmable filters on board. PSoC’s low pass and band pass filters
are 2 pole filters with configurable corner frequencies. The filters can even be cascaded to create a filter with up to 8 poles
which is useful for designs that have strict cutoff needs.
In order to utilize the sensor’s filtered signal within the system it is necessary to quantify the analog signal into the digital
domain using an ADC. Selection of an ADC mainly revolves around the system’s requirements for sampling speed and
resolution. The sampling speed required for the system is related to the sensor’s bandwidth or how often the system needs to
be updated. The resolution required for the system is dependent on the granularity needed by the system to react to the
sensor’s information. The system’s usage model defines this speed and resolution requirement. Example: an accelerometer
used on a phone to change the screen’s orientation might only need a 10-bit ADC that updates several times a second,
whereas a load cell used in a production line might require a 16-bit ADC updating several thousand times a second. When
selecting an ADC the user has the choice of using an external ADC or an microcontroller with an integrated ADC. External
ADCs tend to be higher performance compared to integrated ADCs, but the majority of sensor application’s requirements are
well aligned with microcontroller ADCs. Microcontroller vendors only offer one type of ADC with their microcontroller that has
little flexibility, only the ability to change speed and resolution. The PSoC offers several types of ADCs that be selected and
programmed on the fly as well as having variable speed and resolutions, allowing for an increased amount of flexibility when
compared to a basic microcontroller.
Once the signal has been digitized, the user can integrate the signal with a control system in the microcontroller, or they can
pass the data to a host processor via a communication protocol. The signal path for a sensor may seem simple, but the
implementation can be convoluted. Cypress’ PSoC reduces the complexity of quantifying the sensor’s signal path by
integrating the amplifier, filters, and ADC into a single device. Additionally, the PSoC also has an analog MUX that can be
used to allow multiple sensors to be connect concurrently. The PSoC mixed signal array is an ideal solution to interface with
sensors for a wide variety of industrial and consumer applications.
Mixed Signal Devices and Sensors
Published in Electronics Components World (http://www.electronicscomponentsworld.com)
Page 2 of 3
January 2009
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Mixed Signal Devices and Sensors
Published in Electronics Components World (http://www.electronicscomponentsworld.com)
Page 3 of 3
January 2009
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