Current-mode up-converting D/A-interface for mobile communication transmitter applications

Abstract

A low voltage low power current-mode D/A interface with smoothing filter and upconversion mixer is presented. The D/A-converter is a binary weighted current-switching converter with 8-bit resolution. The uni-directional D/A-output currents are converted to bi-directional input currents for a differential current-mode 1 MHz low-pass filter by a common-mode feedforward circuit. The smoothing filter and current gain amplifier use high-gain current amplifiers using dynamically biased low distortion transconductance output stage. The filtered and amplified current is then fed to a current-switching mixer with off-chip loads. The filter operates with down to a 3 V supply voltage while the D/A-converter can operate even with 2.5 V supply. The harmonic distortion of the filter is typically lower than -50 dBc and thus more than 8-bit spurious free range can be achieved by oversampling.