Pressure sensing is the second most common form of measurement in industrial process control, with its popularity being exceeded only by temperature sensing. Resistance-based bridge circuits are widely used to translate the physical pressure into an electrical output that can be used by the control system. These outputs are usually small and thus require amplification, filtering and analog-to-digital conversion before they can be used in measurement and control systems. Consequently, designers must have a full understanding of the signal-conditioning requirements of industrial pressure sensing applications.
Pressure is sensed by the motion of mechanical elements, such as diaphragms, Bourdon tubes, bellows and capsules, all of which deflect when pressure is applied. The deflection causes a change in the resistances of a strain gage. The most popular pressure sensors use strain gages in a Wheatstone resistance bridge configuration in which all four elements are variable, thus providing optimal linearity and sensitivity. When pressure is applied to the diaphragm, two gage elements of the bridge are subjected to tension; the other two elements are subjected to compression. The corresponding changes in resistance are a measure of the incident pressure.
The bridge is excited by a constant voltage or current, producing an electrical signal. This signal is generally small and is subject to noise, offset and gain errors. Before the bridge output can be digitized, it must be amplified and offset to match the span of the A/D converter and filtered to remove noise. Although the signal-conditioning block could be built with operational amplifiers and discrete circuit elements, integrated instrumentation amplifiers save on parts cost, circuit-board area and engineering design time.
In typical pressure sensor applications, a resistive bridge outputs a differential signal-with a span of tens or hundreds of millivolts that is proportional to the applied pressure and the excitation voltage applied to the bridge. In order to accurately resolve this small differential output voltage in the presence of the high common-mode voltage, an instrumentation amplifier's ability to reject common-mode signal is essential.
For example, 12-bit resolution calls for an LSB of less than 25 microvolts (100 mV/4,096), or about 100 dB below the 2.5-V common-mode level.
Manual bridge compensation
Wheatstone bridges in pressure sensors are typically manually compensated to remove their offset and span errors. This requires the instrumentation manufacturing process to include steps to trim the offset, offset temperature drift, span and span temperature drift. Trimming out these errors is time-consuming and expensive.
Alternatively, the offset can be adjusted by applying a programmable dc voltage from a D/A converter to the reference pin of an instrumentation amplifier. Offset correction is required since the offset would otherwise reduce the available dynamic range of the A/D converter.
Gain uncertainties in pressure sensors make gain adjustment a requirement in most instrumentation amplifier-based systems. This was traditionally done by adding a trimming potentiometer in series with the external gain resistor of the instrumentation amplifier. To achieve higher levels of performance over a wider temperature range, system designers turned to software-controlled gain compensation.
Integration to the rescue
An example of an integrated zero-drift digitally programmable signal conditioner that provides a complete path from sensor to A/D converter is shown in the figure. It includes an instrumentation amplifier fashioned from three auto-zeroed amplifiers (A1, A2, A3), digital trimming potentiometers to adjust two stages of gain, a D/A converter to adjust offset, open- and short-circuit detection circuitry, an output clamp to protect the control system and a provision for a low-pass filter.
To avoid loading the sensor's bridge, the differential inputs feature high impedance and low bias current at both terminals (VPOS and VNEG). Auto-zero techniques minimize offset and offset drift by continuously correcting for amplifier-generated dc errors. This results in a 10-µV maximum input offset voltage and 65 nV/°C maximum drift over a temperature range of – 40°C to 125°C.
Gain, ranging from 70 to 1,280, is programmed in steps of less than 1 (with better than 0.4 percent resolution), via a single-wire serial interface, by adjusting the gains of the two stages individually. Using certain specific processes, the gain setting can be locked in place by blowing polysilicon fuses. The first-stage gain is trimmed from 4.00 to 6.40 in 128 steps by a 7-bit-code that adjusts both P1 and P2; the second-stage gain is set from 17.5 to 200 by an eight-step, 3-bit-code that adjusts P3 and P4. The adjustment values can be temporarily programmed, evaluated and readjusted for optimum calibration accuracy before the settings are permanently fixed.
An integrated zero-drift digitally programmable signal conditioner provides a complete path from sensor to analog-to-digital converter.
Source: Analog Devices Inc.
An 8-bit D/A converter provides a programmable offset, which can be used to compensate for offset errors in the input signal and/or add a fixed bias to the output signal. This bias is used, for example, to handle bipolar differential signals in a single-supply environment. The output offset voltage may be set with a resolution of 0.39 percent of the voltage difference between the supply rails. Like the gain, the output offset can be temporarily programmed, evaluated and readjusted, and then can be permanently set by blowing the fuse links.
Single-supply operation has become an increasingly desirable characteristic of modern sensor amplifiers. Many of today's data acquisition systems are powered from a single low-voltage supply. The AD8555 operates from single-supply voltages of 2.7 V to 5.5 V. The output of amplifier A4 swings to within 7 mV of either supply rail.
Fault detection protects against open, shorted and floating inputs. Any of these conditions triggers a circuit that causes the output voltage to be clamped to the negative supply rail (VSS). Shorts and floating conditions are also detected on the VCLAMP input. An external capacitor can be used to implement a low-pass filter to limit the dc-to-400-kHz output frequency range.
Sensor bridge signal conditioning
Systems engineers would prefer that all pressure sensors with the same part number exhibit nearly identical performance. Typically, off-the-shelf sensors do not meet such requirements with adequate accuracy. One way to achieve consistency from sensor to sensor is extensive trimming during the manufacturing process. If the behavior of these sensors is repeatable over temperature, a better way may be to use the new generation of programmable amplifiers to provide amplification, gain setting and trim, offset setting and trim, and clamping all established digitally.
Such devices can be used to compensate for offset and gain errors in bridge-type sensors, as well as provide an indication of sensor malfunction. They enable adjustments using software, making compensation using trimming potentiometers an outdated art in the manufacturing environment. In the many cases where sensors are used in harsh and crowded environments, measurements benefit from the wide temperature range and space-saving package size of this integrated solution, which is housed in a 4 x 4-mm lead-frame chip-scale package (LFCSP).
Because it is capable of driving very large capacitive loads, it can be placed close to the sensor and at a distance from the signal processing circuitry. Its high levels of programming flexibility and dc accuracy distinguish it from all other solutions.
Reza Moghimi () is an applications engineer at Analog Devices Inc. (San Jose, Calif.).