Abstract
Direct Interface Circuits (DICs) carry out resistive sensor readings using a resistance-to-time-to-digital conversion without the need for analog-to-digital converters. The main advantage of this approach is the simplicity involved in designing a DIC, which only requires some additional resistors and a capacitor in order to perform the conversion. The main drawback is the time needed for this conversion, which is given by the sum of up to three capacitor charge times and their associated discharge times. This article presents a modification of the most widely used estimation method in a resistive DIC, which is known as the Two-Point Calibration Method (TPCM), in which a single additional programmable digital device pin in the DIC and one extra measurement in each discharge cycle, made without slowing down the cycle, allow charge times to be reduced more than 20-fold to values around 2 µs. The new method designed to achieve this reduction only penalizes relative errors with a small increase of between 0.2% and 0.3% for most values in the tested resistance range.
Highlights
Electrical magnitudes of sensors can currently be read using a wide range of circuits, includingDirect Interface Circuits (DICs), which comprise several features that make them suitable for multiple applications
A DIC performs a magnitude-to-time-to-digital conversion through a programmable digital device (PDD) and a few additional elements
In order to study the proposed DIC and subsequently compare its results with those based on a classic calibration method such as Two-Point Calibration Method (TPCM), both circuits were implemented using a Xilinx field-programmable gate arrays (FPGAs), the Spartan 6 XC6SLX25-3FTG256 model
Summary
Direct Interface Circuits (DICs), which comprise several features that make them suitable for multiple applications. A DIC performs a magnitude-to-time-to-digital conversion through a programmable digital device (PDD) and a few additional elements. Reading a sensor with a DIC produces a digital output that can be processed directly by the same PDD as used for the measurement, without the need for analog-to-digital conversion. Only a few simple extra elements are required, such as resistors, capacitors, transistors, or triggers [3]. Such simplicity means that this type of circuit is suited for portable applications, where both the number of components and their consumption are very important. The calculation capabilities of a PDD connected to a DIC allow them to function as smart sensors, pre-processing information from the sensor and reducing the workload in subsequent high-level processing and decision stages [17]
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