Pressure Sensing Integration in Portable Devices

A growing number of OEMs are opting to incorporate MEMS-based pressure sensing components into portable device and equipment designs. As a device designer, it's important to recognize that the choice of pressure sensing type within these applications tends to be highly specific to the intended operating characteristics of both the sensor and device itself.

For example, a handheld, battery-operated spirometry device must be able to be turned on and off quickly while continuously recording data over a 20-second period with optimized use of available power. Based on these requirements, it requires a low-voltage pressure sensor which is able to use power only as needed. In another instance, pressure sensors for handheld HVAC monitors, due to their portability and manner of use, cannot be overly susceptible to vibration or position changes, which could ultimately affect measurement integrity.

Among the various types of sensing technologies available for these types of applications, low-voltage die-based MEMS pressure sensors offer some of the greatest advantages within portable devices, including high-reliability performance and extended useful service life. This is particularly important for environments characterized by a wide dynamic temperature range, or where external shock and vibration is present. In addition, these sensors can be isolated from internal device electronics, offering added protection from signal degradation as a result of thermal transfer.

Low-voltage MEMS pressure sensor components are comparatively lower in cost and offer high-quality mass customization and production capabilities. When integrated into assembled portable devices and equipment, they can help achieve exceptional measurement linearity and repeatability, with minimized power consumption and extended service life.

Performance Characteristics

When selecting an appropriate pressure sensor for integration into a portable device, you should understand a sensor's own unique performance characteristics as they relate to device operation requirements, as well as the unique operating conditions of the intended device usage environment and collective potential effects on device performance. Following is an overview of these considerations from a device design perspective:

Sensor Die Design - A typical MEMS pressure sensor is constructed of a body, or "die," and a thin silicon diaphragm with four surface piezoresis­tors, whose resistance changes in response to mechanical stress. They are generally arranged in a bridge configuration and are precisely located on the diaphragm surface to maximize deflection response. In doing so, pressure differential response is maximized across the diaphragm. MEMS pressure sensor quality and performance within an application environment is most directly tied to sensor die quality.

Package Size - By definition, a portable device is characterized by its ability to be easily transportable and with on-demand functionality. This typically calls for compact pressure sensor designs which offer performance stability, low voltage requirements, and which can reliably operate in a lightweight, easily transported package. Thus, when incorporating pressure sensing technologies into portable device designs, compactness is a near-absolute requirement. The space constraints within the devices themselves impose certain limitations on sensing technology options within these types of applications. Sensors must not only operate within a small package, they must also be isolated from the internal device electronics to avoid signal degradation.

While traditional low-pressure ceramics products are still in use to satisfy these requirements within some smaller device applications, they are design prohibitive for portable devices, as size and weight remain major considerations. Equally important is for the sensor to be compact enough that it will not cause stress on the sensor package within the assembled device, as this affects overall output signal accuracy, ultimately effecting overall device performance.

Temperature Variation - Operating temperature variations can also have a direct affect on MEMS pressure sensor offset voltage and output span, and can ultimately affect overall measurement stability. Portable device applications typically require use of a pressure sensor that can reliably operate in moderate temperature excursions of 0 to 50C, though certain operating conditions can require more extensive ranges. Portable oxygen concentrators are an example of a device featuring integrated pressure sensors that are used in relatively moderate temperatures, though some models may require a sensor with wider, industrial-level temperature ranges of -20 to 85C. To meet these varying range requirements, manufacturers frequently look for a pressure sensing technology with either user-adjustable or integral temperature compensation options.

MEMS-based pressure sensing component technologies are commonly offered with customer-applied temperature compensation capability, which allows manufacturers to tailor temperature performance to their own device performance requirements.

Sensor Output and Device Stability - Sensor output sensitivity is another parameter that will impact signal strength at a particular operating voltage. Higher sensitivity devices can typically be operated at lower voltages with less signal degradation. The higher output level of the pressure sensing die used offsets the lower operating voltage, thus maintaining comparable signal-to-noise ratios to those found in previous generation devices.

Power/Voltage Supply Requirements and Warm-up Shift - As most portable devices are battery operated, pressure sensor power and voltage supply requirements have traditionally been 5V, though the general trend has been a move toward 3.3V or lower voltages, to help further preserve product battery life. These lower power requirements facilitate easier customer integration of sensors into finished product designs, with increased measurement stability and performance. This is because the risk of internal self-heating and related offset shifts are reduced.

When considering temperature requirements, warm-up shift is also a concern. The warm-up shift of a device is the effect that power has on device physical characteristics in its warm-up phase. An alternate and preferred approach to reducing supply voltage modulates the sensor supply as required by the system bandwidth. In other words, apply power to the sensor only when needed. This reduces power to the sensor to the time average (duty-cycle) ap­plied and, therefore, reduces warm-up drift.

To help manage power requirements, pressure sensors are offered in both compensated and uncompensated versions. Compensated devices offer lower calibration costs, faster production cycles, lower production equipment overhead and easier design-in capabilities. Uncompensated versions are generally designed to operate at 5V.

Low-voltage pressure sensors offer 1.8 and 3.3V power supply requirements, to facilitate sensor integration into portable device and equipment designs.

Low-Power Sensing Applications

With the design parameter issues listed above in mind, the following examples help illustrate the successful incorporation of low-voltage MEMS pressure sensors:

HVAC pressure transmitter for building monitoring. A portable industrial airflow measurement device is used for on-demand measurements of low airflow beneath HVAC vents within typical office environments or apartment building setups. Typically, this application requires use of a basic pressure sensor with unconditioned, uncompensated millivolt output signal, and provides a raw output signal for the OEM device. Within the intended pressure sensor usage environment, the selected component must offer long-term reliability and stability, as well as relatively good accuracy and low environmental media sensitivity. The device application environment itself is typically characterized by modest temperature variations and humidity. In these types of applications, the requirement for low warm-up shift is also important, as the device needs to operate with stability soon after powered on. Position sensitivity is less important, as the device itself is specifically orientated under duct work. The signal-to-noise ratio (or noise floor) of these sensors must be very low, as very small air pressures are being measured. Low power consumption, due to battery or current loop operation, is also a significant consideration.

Medical breathing apparatus. An example of an application requiring a higher degree of accuracy and performance can be found in medical breathing apparatus used within critical patient care applications. Device designs must be highly rugged, as well as offer high accuracy and reliability within demanding environments. As medical breathing devices are employed within hospital, urgent care and other clinical settings, they can be subjected to ongoing high levels of shock, vibration and g-force pressures, as well as wide output ranges. The demands placed upon devices within their intended usage environment would require OEMs to specify a millivolt output or amplified pressure sensor, fully calibrated and temperature compensated. Low position and shock sensitivity are also requirements.

Also required for this type of device is the integral amplification of the pressure sensor. The amplified pressure sensor component typically houses an onboard ASIC (built-in amplifier with compensation), allowing control of the millivolt output sensor gain, noise and compensation. Amplified devices are scaled to fall into the input range of a common analog-to-digital microprocessor without additional gain. The amplified pressure sensor can be thought of as an accurate, compensated device with an amplified output signal that is more plug-and-play for the OEM. This is typically required when the customer's analog-to-digital converter does not have a built-in gain feature.

This type of application typically uses a compensated millivolt pressure sensor. Compensated millivolt low-voltage pressure sensors are calibrated to both zero and span and are temperature compensated, to ensure accurate output signal over a specified operating temperature range. A compensated device is typically used in an application were accuracy is a priority and the OEM relies on the pressure sensor manufacturer to provide all temperature compensation and calibration within the pressure sensor itself. In this case, a manufacturer typically requires a clean, low-noise output signal. The OEM would typically provide an amplifier or ASIC somewhere on their PCB to increase the mV output signal.

Tim Shotter is director of new products and Dan DeFalco is marketing manager at All Sensors Corp.