Frequency-Based Sensor Interface with Dynamic Offset Compensation

Abstract

Sensor interfaces need to be robust and accurate for many applications. This is more challenging for sensor systems operating in radiation environments because the mismatch between components grows as a result of the absorbed total ionizing dose (TID). In frequency-based sensor interfaces, the frequency drift of the voltage-controlled oscillator (VCO) can create dynamic output offset, gain, and linearity errors unless a calibration algorithm is included. In this paper, a digital intensive dynamic offset cancelation technique is proposed for an open loop VCO-based sensor to digital converter, which is achieved by making periodic adjustments to the average center biasing voltage of one of the VCOs in a differential architecture, in effect to make their center frequencies match. A simulation of the behavioral model of the proposed architecture was developed and hardware implementation of the whole system was performed on an FPGA by emulating, with digital modules, the characteristics of the two VCO outputs modulated with differential inputs. The results showed that the output offset error was reduced from around 5% to 0.1% for a relative oscillators’ drift close to 10% of the tuning range, and the SNDR is relatively maintained when subjected to variable relative VCO drifts.