Method to Reduce Quantization Error in Direct Interface Circuits for Resistive Sensors

Reading resistive sensors and converting their information to digital values is a matter of great interest in practical applications. Direct Interface Circuits (DICs) perform this task efficiently through a resistance-to-time-to-digital conversion. The main benefit of this type of circuit is the simplicity of its design and the accuracy of the results. However, one of its drawbacks is that quantization errors in the measurements seriously compromise accuracy if the information is not converted using a high-frequency clock or with high capacitor values. To avoid this situation, this article presents a new type of DIC with the same passive elements as a classic DIC, but which uses a high value resistor in the final instant of the resistance-to-time conversion. The circuit has been implemented using a commercial FPGA with the capture module operating at different clock frequencies. The results show that relative errors in measuring resistors of up to $33~\Omega $ decrease to 1.56% when operating with a 12.5 MHz timer.