Dual slope direct digital converter for bridge connected resistive sensors

Abstract

A dual slope direct digital converter (DSDDC) suitable for resistive sensor elements already connected in a bridge form is presented. A conventional dual slope analog to digital converter (DS-ADC) is altered with an intentionally introduced instrumentation amplifier (INA) to realize the proposed DDC. The DSDDC accepts bridge connected resistive sensor elements as input and provides a digital output that is proportional to the quantity being sensed by the sensor elements. The results obtained from simulation of the proposed DSDDC indicate a worst case error in the output to be < ±0.03 %. Experimental results from a prototype unit, presented herein, demonstrate the practicality the scheme. Worst case error of the prototype unit was found to be < ±0.07 % with test conducted using standard decade resistance boxes emulating the sensor elements. The worst case error in the prototype was < ±0.05 % when tested with a load-cell made of four strain gauges.