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Adaptive biasing offsets temp drift

Posted: 16 Oct 2002 ?? ?Print Version ?Bookmark and Share

Keywords:adaptive biasing? temp drift? thermal drift? potentiometer? sensor?

The main objective of an electronic circuit that deals with sensors is to provide a "clean" signal to a control unit that behaves in a directly proportional way to the physical quantity to be measured. Typically, a signal from a sensor goes through signal conditioning followed by A/D conversion. Each stage contributes its own error to the measured signal in an additive manner. While noise is usually being eliminated immediately with a low-pass filter and the digital conversion error will be kept within an acceptable range, selecting the highest needed resolution, the sensor's thermal error is either being ignored or, more likely, compensated in the digital domain.

As long as the number of detectors in the system's periphery is relatively small, the thermal compensation in the digital domain seems to be acceptable. However, for an increasing number of sensors, the system's reliability and performance will be drastically reduced. Therefore, adaptive sensor biasing is the key to solving this problem.

Consider the case of a bridged pressure sensor. In this type of design, four sensors that convert pressure to a resistive value are arranged in a Wheatstone bridge configuration. A reference voltage is applied across one set of opposite corners and any change in the resistive values is registered with voltages at the other two corners. Ideally, when no pressure is occurring, the four resistances will be equal and the Wheatstone bridge will be in equilibrium, so that there will be no voltage output. However, ambient thermal effects will create a small imbalance in the system, leading to noise in the measured voltages.

Therefore, the challenge will be to design a system solution that compensates for the thermal drift dynamically, so that the circuit's output shows no voltage across the whole temperature range when no pressure is being sensed. In addition, the circuit's output should show a constant value across the whole temperature range for a given pressure value.

The difference between the two voltage outputs of the Wheatstone bridge must be resolved into two quantities: the voltage representing the pressure that is being sensed and an offset voltage. For a given constant temperature, the voltage difference will vary over some range, called the span, and will begin at the offset voltage. Two additional circuits need to be added to the Wheatstone bridge to compensate for changes in the span and offset values. Each circuit block is dynamically controlled by a variable resistance created by a digitally controlled potentiometer. Pot1 and Pot2 are the analog resistor arrays of the DCP with resistance values "a" and "b."

As long as the sensor works within an isothermal environment, the sensor's offset and span will stay within the expected range; "a" and "b" therefore will stay constant. Once the temperature changes, these wiper positions will have to be retuned in order to keep the sensor's output within a stable characteristic. In such a case, the adjustment criteria for "a" and "b" will be provided by an external temperature sensor and the on-chip EPROM registers that are part of the DCP. The DCP performs a lookup table operation on the typical differential voltage vs. pressure and temperature characteristic of the sensor, which has been previously stored in the EPROM.

Although this analysis has been performed based on a pressure sensor circuit, it can be said that the major principle (thermal drift of sensor's offset and span) are applicable to any other sensor circuit in a similar way. Using Xicor's XDCP solution of an adaptive, retunable potentiometer will add to those applications an additional degree of freedom.

- Axel Kleinitz

Staff Member

Xicor Inc.

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