Current-mode reference shifting solution for MDAC-based analog-to-digital converters

This paper presents a current-mode reference shifting solution for MDAC-based ADCs that eliminates the need for external or internal reference voltage buffers, thus improving the energy efficiency and cost of the overall ADC. The solution consists of using an current source connected to the opamp's inputs, where reference shifting occurs by pulling a current through the capacitor in the opamp's feedback loop during the amplification phase, thus, emulating a voltage shift, ΔV = (IΔt)/C. The amount of necessary current, i.e., the biasing of the current source, is controlled by a replica of the converter's stage, which is followed by an integrator and a biasing circuit, all connected in a negative feedback loop. To illustrate the solution's performance, two 10 bit 100 MS/s pipeline ADCs were implemented: one based on the conventional 1.5-bit MDAC and the other based on the proposed circuit. Simulation results show the functionality of the proposed circuit and results for different wirebond inductances prove the stability and performance of the proposed solution.