Efficient Offline Outer/Inner DAC Mismatch Calibration in Wideband ΔΣ ADCs

Distortion due to feedback DAC mismatch is a key limitation in ΔΣ ADCs for wideband wireless communications. This article presents an efficient frequency-domain mask-based offline mismatch calibration method of both the outer DAC and the inner DACs in a ΔΣ ADC. The test stimulus for the calibration is a two-tone signal near the band edge. To avoid the need for high-performance signal generation, a frequency mask is applied to void the stimulus signal and its phase noise. In this way, the method is robust against distortion and jitter in the stimulus signal, which therefore could be combined from two low-quality signal generators. The two-tone band-edge signal has the additional benefit that the number of needed samples of the excitation signal is very modest because as many intermodulations as possible contribute to the calculation of the mismatch errors of the DACs. Experimental results confirming the calibration method are obtained from a prototype chip, designed for an 85MHz signal bandwidth in 28nm CMOS technology. A two-tone stimulus around 78 MHz is applied to calculate the mismatch of the outer DAC and the inner DAC with only 68K samples. With the DACs calibrated, an SFDR improvement of 28.1 dB is achieved for a single-tone input at 5 MHz, while for a two-tone input around 71 MHz, the IM3 is improved from -63.6 dBc to below the noise floor (<; -94.1 dBc). This illustrates the effectiveness of the approach.