A digitally adjustable sensor signal conditioning circuit for low frequency operation

A fully digitally adjustable sensor signal conditioning circuit (SCC) is proposed herein. The SCC is meant to properly adjust the input signal delivered either by active sensors or by passive sensors (e.g. thermocouples, piezoelectric and/or pyroelectric sensors, etc.). The circuit is meant to be resistively connected to the sensor that delivers a signal containing low frequency components, ranging from direct current (DC) to 2 × 103 Hz. The signal conditioning circuit is mainly based on one or two inverting stages, each provided with the possibility to digitally control both the gain coefficient (GC) and the direct current (DC) offset. The GC of each stage can be adjusted in a range spanning from sub-unitary to supra-unitary values, therefore the signal conditioning circuit can be used to preserve the dynamic range of an analog to digital converter (ADC) in an optimum domain, while avoiding saturation, also when the input signal is within the dynamic range of the SCC's input but out of the dynamic range necessary at output. The circuit is designed to detain a low-pass characteristic, thus eliminating high frequency components; therefore it provides a basic bandwidth control. A digital to analog conversion stage is employed to adjust the DC offset and to run self-calibration and self- test procedures. The proposed circuit is meant as a basic building block, employed to interface a wide range of sensor signals in order to provide optimal levels for the ADC stage, or to be inserted in an analog signal chain, where signal level conversion may be achieved (for example signal expansion or compression from 0-5 V voltage levels to 0-3.3 V levels). Alternatively, the proposed SCC could be inserted into more specialized data acquisition systems providing a digital system with the ability to self-condition the input signal before acquiring it. The developed SCC prototype has been employed for separating efficiently small temperature and magnetic field's intensity (very low frequency) sensor signals from the continuous components and for best fitting the dynamic range of an ADC. An automatic parameters adjustment algorithm, with no loss of the output signal has been implemented.