AN1839| Application Note
Maxim/Dallas > App Notes > MEASUREMENT CIRCUITS
SENSOR SIGNAL CONDITIONERS
Keywords: pressure sensors, temperature compensation, calibration, signal conditioning, sensors, digital to analog converter, DAC, pressure sensing, piezoresistive sensors, dacs
Dec 27, 2002
APPLICATION NOTE 1839
Sensor Temperature Compensation Using the Four DAC Signal Conditioning Architecture
Resistive element pressure sensors, most notably the Wheatstone bridge configured piezoresistive devices, have dominated the low- to medium-accuracy pressure sensing industry since the early 1980s. The principal source of measurement error with these devices is changing sensitivity and output offset with temperature. Signal conditioning and calibration provide greater accuracy and lower cost. One of the most effective solutions to this basic requirement is the analog path conditioning architecture using four digital-to-analog converters (DACs) to provide the necessary temperature corrections. The temperature sensitivity of the gain of piezoresistive sensors stems from thermal coefficient of sensitivity (TCS) and the thermal coefficient of resistance (TCR). TCS effects arise from dimensional and stiffness changes in the sensor over temperature. TCS is usually negative (sensitivity reducing with increasing temperature). TCR describes the change in sensor bridge resistance with temperature and is normally posit
ive. Most resistive element sensors are designed to make best use of the opposing signs of these two thermal coefficients. The aim is to produce a sensor with TCS slightly lower in magnitude than TCR. This results in a sensor which, when driven from a constant current source; exhibits a much-reduced total temperature sensitivity and allows external temperature compensation to be easily applied.
Resistive Element Sensors and the Four DAC Compensation Architecture
Resistive element sensors, most notably the Wheatstone bridge configured piezo resistive devices, have dominated the low- to medium-accuracy pressure sensing industry since the early 1980s. The principal source of measurement error with these devices is changing sensitivity and output offset with temperature. Signal conditioning and calibration provide greater accuracy and lower cost. One of the most effective solutions to this basic requirement is the analog path conditioning architecture using four digital-toanalog converters (DACs) to provide the necessary temperature corrections. The temperature sensitivity in the gain for piezo resistive sensors stems from thermal coefficient of sensitivity (TCS) and the thermal coefficient of resistance (TCR). TCS effects arise from dimensional and stiffness changes in the sensor over temperature. TCS is always negative (sensitivity reducing with increasing temperature). TCR describes the change in sensor bridge resistance with temperature and is normally positive. Mos
t resistive element sensors are designed to make best use of the opposing signs of these two thermal coefficients. The aim is to produce a sensor with TCS slightly lower in magnitude than TCR. This results in a sensor which, when driven from a constant current source, exhibits a much reduced total temperature sensitivity and allows external temperature compensation to be easily applied. The resulting condition is portrayed in the graph of Figure 1, which contains normalized temperature responses for bridge resistance (Rb) and pressure sensitivity. The slopes of these two responses represent the TCR and TCS characteristics of the sensor. The third curve in Figure 1 represents the ideal response of sensor bridge voltage (Vb) required to balance the sensitivity curve and produce a sensor with a null temperature coefficient of gain.
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