A Digitally-Controlled Fully Differential Low Noise Current Source

In this paper a digitally-controlled fully differential low-noise current source is presented. It was developed for sensor excitation, however, the concept is applicable wherever a precise digitally-controlled current source is required. The current source is based on a two-stage cascaded Delta Sigma modulator using a current-steering digital-to-analog converter and a novel current amplifier that generates a scaled replica of the output current of the DAC. The D/A conversion is done by 64 unary current cells that are controlled by a timing optimized switch driver. To reduce the flicker noise the current cells are chopped using two banks of current cells, that are activated alternatingly. The proposed fully differential current amplifier uses only a single operational amplifier to deliver a filtered and scaled output current. Thus the typically required intermediate transimpedance amplifier is eliminated, which results in an increased noise performance and a reduced power consumption. The current source has a simulated total harmonic distortion of 99.5dB and a simulated signal-to-noise ratio of 116dB. The proposed circuit was implemented in a 180nm CMOS technology and occupies an area of 1.35 mm2.