Adaptive Bias Technology to Compensate Sensor Temperature Drift
The main goal of the sensor electronic circuit is to provide the control unit with a pure signal proportional to the measured physical quantity. Usually, the signal from the sensor is adjusted before analog-to-digital conversion. Errors are generated at each stage and are added to the measured signal. Noise is usually eliminated immediately by a low-pass filter, and the analog-to-digital conversion error is also limited within an acceptable range. The main goal of the electronic circuit of the pass sensor is to provide the control unit with a 'pure' signal proportional to the measured physical quantity. Usually, the signal from the sensor is adjusted before analog-to-digital conversion. Errors are generated at each stage and are added to the measured signal. Noise is usually eliminated immediately by a low-pass filter, and the analog-to-digital conversion error is also limited within an acceptable range. By choosing the highest resolution required, the thermal effect error of the sensor can also be ignored, or more likely in the digital Compensate it within the domain. As long as the number of detectors outside the system is relatively small, thermal compensation in the digital domain is acceptable. However, as the number of sensors continues to increase, the reliability and performance of the system will drop sharply. Therefore, adaptive sensor bias technology has become the key to solving this problem. Now take the bridge pressure sensor as an example to illustrate this problem. In this type of design, four sensors that convert pressure into resistance are built as a Wheatstone bridge (see Figure 1). The reference voltage is applied to one set of diagonal corners, and any change in the impedance value will be reflected in the voltage value of the other set of diagonal corners. Ideally, when no pressure is applied, the four impedance values u200bu200bare equal, and the Wheatstone bridge is in a balanced state, so there is no voltage output. However, the thermal effect of the environment will cause a slight imbalance in the system, resulting in noise in the measured voltage. Therefore, our challenge is to design a system solution that can dynamically compensate for thermal drift so that in the entire operating temperature range, when there is no pressure input, the circuit has no voltage output. In addition, in the full temperature range, for a given pressure value, the output of the circuit should be a constant value. The difference between the two voltage outputs of the Wheatstone bridge must be decomposed into two different quantities: the voltage representing the measured pressure and the offset voltage. For a given constant temperature, the variation range of the voltage difference is called the span (Span), which starts from the offset voltage. The Wheatstone bridge requires the addition of two additional circuits to compensate for changes in span and offset values. Each circuit module is dynamically controlled by a variable resistance generated by a digital control potentiometer (DCP). Pot1 and Pot2 are DCP analog resistor arrays, the resistance values u200bu200bare 'a' and 'b' respectively (see Figure 2). As long as the sensor works in an isothermal environment, the offset and span of the sensor will remain within the expected range, so 'a' and 'b' will be fixed. Once the temperature changes, in order to keep the output of the sensor within a stable characteristic range, the position of these sliding contacts must be readjusted. In this case, the adjustment criteria of 'a' and 'b' will be provided by the external temperature sensor and the on-chip EPROM register in the DCP. According to the temperature and pressure characteristics of the sensor, DCP performs a look-up table operation to find out the corresponding typical differential voltage stored in EPROM in advance. Although the above analysis is based on the pressure sensor circuit, the main principles (such as sensor offset and thermal drift of span) can be applied to any other sensor circuit in a similar manner. The XDCP solution using Xicor's adaptive, re-adjustable potentiometer will add more degrees of freedom to these applications.
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