Micromachined Pressure Sensors Reach a New Low

Medical Device & Diagnostic Industry Magazine
MDDI Article Index

An MD&DI June 1998 Column

SENSOR TECHNOLOGY

Today's noninvasive respiratory management technologies demand high-stability, ultra-low-pressure sensors.

New automatic inhalers, critical-care ventilators, handheld respiratory monitors, oxygen concentrators, and neonatal monitors are increasing the need for high-stability, ultra-low-pressure sensors that can measure flow pressures as low as ±0.5 in. of water. In addition to high sensitivity at low pressure and small size, today's devices require sensors that demonstrate a high level of offset stability, repeatability, and insensitivity to position.

In the past, most designers relied on capacitive strain gage and micromachined silicon capacitive and micromachined flow-through technologies when designing low-pressure sensors. While many sensing technologies can measure flows within the necessary ranges for medical applications by using down-ranged higher-pressure sensors, low output signals and low signal-to-noise ratios can occur. In some cases, the sensing element is processed for a sensitivity that is lower than the range for which the sensor was originally designed, resulting in excessive warm-up drift, long-term drift, and position sensitivity. The sensor user is usually left to find these poor performance factors the hard way, through exhaustive testing.

A new piezoresistive micromachined compensation technology from Data Instruments, Advanced Silicon Group (Sunnyvale, CA) enables measurements of full-scale pressure ranges, including those as low as ±0.5 in. of water, which were previously unachievable with piezoresistive micromachined technology. The new technique, dynamic-self-compensation, monitors for the presence of common mode–induced errors such as warm-up drift, position sensitivity, and long-term instability, then adjusts the sensors to eliminate them.

A DYNAMIC DESIGN

The dynamic-self-compensation design incorporates both electrical and mechanical coupling of two standard piezoresistive, micromachined pressure-sensing elements (Figure 1). Each sensing element is produced using standard electrochemical etching techniques. Sensing elements are chosen to ensure matched performance and then mounted on a hybrid substrate. Half of each sensor bridge is electrically cross-coupled (cross-connected) to the other to achieve a dual-sensor, single-differential output. Effectively, the sensing elements act in unison, both sensing the applied pressure and providing a differential ratiometric output.

Figure 1. Miniature ±1-in.-of-water dynamic-self-compensation sensors.

Mechanically, the two sensor elements shown in Figure 2 are cross-coupled through internal manifolds, which share positive and negative applied pressures. While one element monitors the pressure applied to the top of the sensor, the other one detects pressure from the bottom, providing a full range of sensitivity.

Figure 2. The two standard piezoresistive, micromachined, pressure-sensing elements are cross-coupled through internal manifolds, which share positive and negative applied pressures.

The piezoresistive micromachined pressure sensors employ four implanted piezoresistive elements in a Wheatstone bridge configuration. As shown in Figure 3, the equivalent circuit contains two inputs (positive voltage excitation and negative voltage excitation) and a differential output (positive voltage signal and negative voltage signal). The same equivalent circuit results from a dual-sensing-element approach, enabling the user to further condition the signals from the differential output using traditional conditioning techniques such as a single instrumentation amplifier.

Figure 3. The equivalent circuit allows users to further condition the signals from the differential output.

Economically, this dual-sensing-element design costs little more than traditional single-element designs. This is because silicon sensing elements remain one of the least-expensive sensing technologies. Produced in bulk form, a typical 4-in. silicon wafer yields more than 1000 die at a cost that is typically less than 50¢ each.

A PERFORMANCE COMPARISON

As previously mentioned, warm-up drift, position sensitivity, and stability are significant concerns when designing sensors. Traditional piezoresistive micromachined sensors exhibit warm-up drift as a shift in the zero-pressure offset from the onset of applied power. In applications where power-on measurements must be taken immediately or where autozeroing before every measurement is not practical, it is important that devices display little or no warm-up drift. As shown in Figure 4, a typical single sensor element exhibits a warm-up drift of 300 µV over a 15-minute interval whereas the dynamically compensated sensor exhibits virtually none.

Figure 4. A typical silicon-sensor element exhibits a warm-up drift of 350 µV over a 15-minute interval whereas the dynamically compensated dual sensor exhibits virtually no warm-up drift.

For portable or field applications, position sensitivity must be minimized to avoid potentially dangerous false readings. For example, a paramedic could experience a dramatic shift in a handheld respiratory monitor's performance simply by moving the device from a vertical to a horizontal position. False readings could result and be interpreted as a dramatic change in a patient's status, precipitating improper treatment.

In a test between a dynamically compensated 1-in.-of-water full-scale sensor and a capacitive 1-in.-of-water full-scale sensor, the dynamically compensated sensor showed significantly less sensitivity to movement (Figure 5). Both sensors tested had identical amplified high-level (V dc) output signals and were oriented such that movement 90° to the direction of gravitational force resulted in ±0.5-g changes in position. The plot shows that capacitive sensing technology exhibited a ±115-mV output shift with simple movement compared to a ±3-mV shift with a dynamically compensated sensor.

Figure 5. The position sensitivity testing results of a comparison between the dynamically compensated sensor and a capacitive ±1-in.-of-water full-scale sensor.

One of the most important low-pressure sensor specifications for any airflow application is the stability of the sensor's zero-pressure offset over a range of temperatures. Figure 6 shows that a traditional low-pressure, micromachined pressure sensor exhibited up to 285 µV of shift from 25° to 70°C and 220 µV from 25° to 0°C. By comparison, the dynamically compensated dual-sensor design shifted by 40 µV from 25° to 70°C and –15 µV from 25° to 0°C. This high level of temperature compensation provides greater stability and repeatability in a dynamic environment where temperature changes are common. Such applications are found in mobile and portable monitors and in hospital operating rooms where equipment has remained cold overnight and must be warmed quickly for immediate use.

Figure 6. The results of comparative zero-pressure offset testing over a range of temperatures for the dynamically compensated sensor and a traditional, micromachined sensor. A stress-isolated, fully signal-conditioned sensor.

IN THE FUTURE

Although much of the need for ultra-low-pressure sensors is for controlling and monitoring patients' breathing, medical monitoring goes well beyond the realm of respiratory management. Of particular interest is the ability to monitor and analyze multiple gases during respiration to determine various metabolic conditions. Monitoring the delivery of anesthetic gases as well as respiratory gases with one monitor can effectively replace conventional multimonitor stations. Advanced algorithms are being developed to use multiple gas measurements in the diagnosis of pulmonary disease and lung rehabilitation. If successful, monitoring respiration to this level will eliminate invasive procedures, thus reducing costs and speeding patient recovery times.

Improvements in sensor stability, sensitivity, and accuracy are enabling continuous monitoring of patient respiratory functions. Semiconductor micromachined sensors, which provide short- and long-term stability at full-scale pressures below ± 0.5 in. of water, can serve this expanding new market. These performance features combined with stress-isolated packaging and integrated signal conditioning will enable airflow monitoring directly from the patient's airway. Ultimately, future sensors will integrate dedicated gas, temperature, and humidity sensing to provide all necessary measurements for total respiratory diagnostics in a single package.

Brian Wirth is director of business development for Data Instruments, Advanced Silicon Group (Sunnyvale, CA).

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