Bridge-temperature measurement allows software compensation
Bridge transducers are notoriously temperature-sensitive. When the temperature changes, almost everything else varies with some parameters increasing and some decreasing. The TCS (temperature coefficient of span) is generally negative for piezoelectric pressure sensors, and the TCR (temperature coefficient of resistance) is positive. In other words, as the temperature rises, the sensitivity decreases, and the resistance of the bridge increases. In general, the TCS and TCR are close to each other. This fact has in the past motivated designers to add both active and passive external components to achieve some measure of temperature compensation. However, the calibration of such systems can be tedious; the resulting performance, problematic. Piezoelectric-bridge manufacturers have tried to ease the problem by equalizing the coefficients in their manufacturing processes. Another approach is to simply measure the temperature of the bridge and then use a µC to compensate in software, given certain basic properties, such as the bridge resistance at 25°C and the TCR. This approach is effective, but concerns that the temperature you measure may not be the real temperature of the bridge hamper it. For instance, placing the temperature sensor relative to mechanical attachment of the strain gauge has a crucial bearing on the accuracy of the reading. It's not unusual to see errors of 1°C or more in such situations. The idea presented here (Figure 1), suggested in Reference 1, is to determine the temperature of the bridge by measuring the voltage across the bridge as a result of a known excitation current flowing through it.
RREF, reference voltage VREF, and op amp IC1 determine the excitation current, which equals VREF/RREF. The differential bridge output, seen between terminals OUT(+) and OUT(–), feeds directly into channel AIN1 of the AD7706 ADC. This signal is a pseudodifferential input with respect to the ADC's Common input and can accept full-scale signals as low as 20 mV while providing full 16-bit performance. The AD7706 has three pseudodifferential inputs, all having the Common input as reference. The Common input connects to the midpoint of the bridge and serves a reference point from which to make all the necessary measurements. Computing the voltage across the bridge entails two additional measurements. Input channel AIN2 measures the voltage from the top of the bridge to its midpoint (Common terminal), and input channel AIN3 measures the voltage from the midpoint to the bottom of the bridge. You can then compute the voltage across the bridge. Assume that the resistance of the bridge is independent of the pressure under measurement at least to the extent that the error is small compared with the measured results.
The AD7706 is a complete, 16-bit (with 14-bit maximum integral nonlinearity) delta-sigma ADC intended for dc and low-frequency ac measurements. Because its power consumption is 1 mW maximum at 3V, you can use the device in loop-powered, battery-powered, and local applications. The on-chip programmable-gain amplifier has gain settings of 1 to 128 to accommodate both low- and high-level analog inputs without the need for external signal-conditioning hardware. You can find additional information about the IC at http://products.analog.com/products/info.asp?product=AD7706. (DI #2576)
Paillard, Bruno, "Temperature compensating an integrated pressure sensor," Sensors, January 1998, pg 36.