A simple method to measure the resistance of a sensor and convert it into digital information in a programmable digital device is by using a direct interface circuit. This type of circuit deduces the value of the resistor based on the discharge time through it for a capacitor of a known value. Moreover, the discharge times of this capacitor should be measured through one or two resistors with known values in order to ensure that the estimate is not dependent on certain parameters that change with time, temperature, or aging. This can slow down the conversion speed, especially for high resistance values. To overcome this problem, we propose a modified process in which part of the discharge, which was previously performed through the resistive sensor only, is only conducted with the smallest calibration resistor. Two variants of this operation method, which differ in the reduction of the total time necessary for evaluation and in the uncertainty of the measurements, are presented. Experiments carried out with a field programmable gate array (FPGA); using these methodologies achieved reductions in the resistance conversion time of up to 55%. These reductions may imply an increase in the uncertainty of the measurements; however, the tests carried out show that with a suitable choice of parameters, the increases in uncertainty, and therefore errors, may be negligible compared to the direct interface circuits described in the literature.

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Direct interface circuits are a simple, inexpensive alternative for the digital conversion of a sensor reading, and in some of these circuits only passive calibration elements are required in order to carry out this conversion. In the case of resistive sensors, the most accurate methods of calibration, namely two-point calibration method (TPCM) and fast calibration methods I and II (FCMs I and II), require two calibration resistors to estimate the value of a sensor. However, although FCMs I and II considerably reduce the time necessary to estimate the value of the sensor, this may still be excessive in certain applications, such as when making repetitive readings of a sensor or readings of a large series of sensors. For these situations, this paper proposes a series of calibration methods that decrease the mean estimation time. Some of the proposed methods (quasi single-point calibration methods) are based on the TPCM, while others (fast quasi single-point calibration methods) make the most of the advantages of FCM. In general, the proposed methods significantly reduce estimation times in exchange for a small increase in errors. To validate the proposal, a circuit with a Xilinx XC3S50AN-4TQG144C FPGA has been designed and resistors in the range (267.56 Ω, 7464.5 Ω) have been measured. For 20 repetitive measurements, the proposed methods achieve time reductions of up to 61% with a relative error increase of only 0.1%.