Comparison Study of DAC Realizations in Current Input CTΣΔ Modulators

摘要

In this brief, a comparison between five different DAC realizations is presented for an equivalent multi-M Omega range resistance in a digitizing current sensor, namely resistor, current source, pseudo-resistor, T-network resistor, and switched-capacitor-resistor (SCR). These DACs are needed to realize current input continuous-time sigma-delta modulator (C-CT Sigma Delta M) readouts to sense currents in the pA range. All DAC realizations are discussed concerning disadvantages and advantages and are implemented, laid out, and simulated on transistor level in a 180nm CMOS technology. The comparison is carried out on the area consumption, required circuit overhead, noise performance, inherent anti-aliasing filter (AAF) quality, and two sources of non-idealities (clock jitter and ISI). It is shown that the SCR-DAC - due to its decaying waveform and its inverse proportionality of the equivalent resistance to its capacitor size shows the best compromise over the other methods and achieves the best performance in the smallest area.