Sensors and Transducers in Your PCB Design

Key Takeaways

• From battery-powered wristwatches to pressure measurement, sensors and transducers play an essential role in many devices.
   
• Using sensors and transducers for PCBs enables a wide range of functionalities, contributing to the adaptability and responsiveness of electronic systems.
   
• Choosing the right type of sensors and transducers for your PCB is crucial. Below are some essential considerations.

Sensors and transducers play integral roles in PCBs by enabling electronic systems to interact with and respond to the surrounding environment. Sensors detect physical parameters such as temperature, pressure, light, and motion, converting these analog signals into electrical signals that PCBs process. Sensors are used in various applications, from environmental monitoring and industrial automation to consumer electronics. By integrating sensors onto the PCB, electronic devices can sense changes in their surroundings and make decisions, enhancing functionality and user experience.

Transducers, on the other hand, convert one form of energy into another. In PCBs, transducers transform electrical signals into physical effects or vice versa. For example, piezoelectric transducers convert electrical signals into mechanical vibrations for applications like speakers or haptic feedback systems. Accelerometers and gyroscopes, both transducers, are critical for motion sensing and orientation in electronic devices. Using sensors and transducers in PCBs enables a wide range of functionalities, contributing to the adaptability and responsiveness of electronic systems.

Sensors and Transducers in PCBs

The table below includes a list of common sensors and transducers used in PCBs, along with a description and applications of each type.

Piezoelectric Sensors and Transducers

As the table notes, PCBs often use piezoelectric sensors and transducers. These sensors are rugged and can withstand very high temperatures while remaining sensitive to changes in environmental variables. They are also manufactured in various sizes, making them suitable for mounting on a PCB or as a packaged unit that attaches to a board with copper wire.

How Do They Work

Piezoelectric material used as a sensor converts an applied mechanical load to an electric field, which can then be measured as a voltage. This voltage can be converted to digital data with an analog-digital converter (ADC) on your board. You can then use the equivalent digital number to perform calculations and determine physical quantity.

In the case of an oscillating mechanical vibration, the output voltage also oscillates at the same frequency as the mechanical vibration. A crucial point here is that any vibrating mechanical membrane, which includes a piezoelectric crystal, has a spectrum of natural frequencies, not just a single resonance.

Multiple Frequencies

Furthermore, the equivalent electronic circuit formed by a piezoelectric element has multiple resonance and antiresonance frequencies. If you want to use a piezoelectric element as a transducer in your board, it can be excited at multiple resonance frequencies.

When a mechanical load is applied to the element, the frequencies in the response spectrum will shift to different values, and the magnitude of this shift is proportional to the applied load. This change in resonance is used to gather sensitive biological and chemical measurements, as the change in the vibration amplitude will be extremely sensitive near resonance.

Taking Frequency Response into Account

When working with a piezoelectric transducer, you will need to take this frequency response into account. Changing the frequency of an input AC signal at a constant voltage will eventually cause the acoustic vibration of the transducer to change nonlinearly as you near a resonance or antiresonance. For this reason, you should be mindful of the frequency response if you want your transducer to respond in the linear regime.

Piezoelectric sensor and transducer with lead wires

Placing Piezoelectric Elements on Your PCB

Choose the Right Element

Choosing the right type of piezoelectric element for your device is crucial. Ceramic piezoelectric elements have lower costs and have sensitivities that can reach two orders of magnitude higher than piezoelectric crystals (i.e., quartz), but their sensitivity degrades over time, especially at higher temperatures. Crystals have much better long-term stability and are a better choice for high-temperature measurements.

Suppose you want to use a piezoelectric transducer to measure vibrations at a specific frequency. In that case, you should try to select a piezoelectric transducer with a resonance that matches the particular frequency you wish to measure. You can then use a bandpass filter in your board to remove responses at other frequencies.

Use Filters

If you are measuring across a range of frequencies, the usable range will lie between the first antiresonance and the first resonance. You should use a high pass filter with a cutoff near the first antiresonance (which has a lower frequency) and a low pass filter with a cutoff near the first resonance to control the response range. The response from this sensor will have good linearity to changes in load, while it will be flat to changes in applied frequency for a constant load. This allows the two quantities to be easily separated during measurement.

Avoid Placements Above Ground Planes

Just like the placement of a quartz crystal oscillator for generating clock signals, you should avoid placing the piezoelectric element itself directly above a ground plane, as this creates a center-fed patch antenna. This increases susceptibility to EMI, especially at the ground plane’s resonance frequency. Piezoelectric sensors are generally connected to an amplifier, so any noise in the transducer output will also be amplified. You should try to clean up these signals as much as possible. Some circuit designers will use sequential high and low pass filtering to tailor their passband to just the right frequency range.

Quartz oscillators: one common piezoelectric device

Use Impedance Matching Circuits

Finally, the output impedance from piezoelectric elements is very high (usually 1 MOhm), so an impedance matching circuit should be used at the load to maximize power transfer into/out of the element. There are several options for matching your source and load impedance, one being linear two-port matching network.

Piezoelectric elements also have internal dielectric losses, i.e., the electrical signal is dissipated as heat. With crystals, dielectric losses are low, so thermal management is less of a concern. For ceramics, dielectric losses are much higher, so designers should use some form of thermal management, like a thermal pad or heat sink.

Cadence Can Help Incorporate Sensors and Transducers in Your Design

The proper PCB layout and design software lets you design boards to control and collect data from various sensors and transducers. OrCAD PCB Designer contains the tools you need to design boards for gathering sensor data and much more.