Current-based measurement technique for high sensitivity detection of resistive bridges with external balancing through control voltages

We present a novel approach based on differential measurements of dc currents with very high sensitivity suitable for the detection of very small variations of resistors in Wheatstone full-bridge configurations. External control voltages allow for the compensation of the bridge unbalancing avoiding the need of changing its elements so making the solution suitable for integrated sensor systems. The proposed current-based measurement technique has been implemented through three different circuits, in transimpedance configuration and without the use of any further amplification stage, employing only two active blocks that allow for a very high integration level. The main characteristics of these solutions, developed both in current-mode (CM) and voltage-mode (VM) approaches, have been preliminary evaluated through PSPICE simulations so validating the new approach. The results have demonstrated the capability of the presented circuits to reveal, with linear responses, relative variations of the bridge resistors lower than 10-6% with respect to their nominal values, providing detection sensitivities up to 150 V/% (i.e., 500μA/%) with the CM solutions and 1750 V/% in the case of the VM scheme. Moreover, experimental measurements have also been conducted by implementing the new technique on breadboard with discrete commercial components confirming the performances of the proposed approach. In this regard, a detection resolution of resistive variations as low as 0.00008%, corresponding to about 0.001Ω, has been achieved in the measurement of only one resistor of the bridge with an improvement of the detection sensitivity of a factor 500 with respect to the standard basic resistive bridge.