An 8-Bit Voltage Mode Analog to Digital Converter Based on Integer Division

An 8-bit subrange Analog to Digital Converter (ADC) designed in a CMOS TSMC90nm process consisting of a coarse 4-bit and a fine 4-bit ADC stage is presented in this paper. It occupies only 0.04mm² of die area and dissipates less than 22mW without sacrificing speed since the sampling rate is higher than 500MS/s and the achieved SNDR is higher than 40dB. These features have been estimated by post-layout simulations. In ordinary subrange architectures a Digital to Analog Converter (DAC) generates an analog value from the digital coarse ADC output that is subtracted by the input in order to produce a residue that serves as input to the fine ADC stage. In such an approach, the linearity errors of the coarse ADC, the DAC and the subtractor are accumulated. In the present work, a voltage mode modulo-16 integer divider is used at the input of the subrange ADC and a “thermometer to binary encoder” that is driven directly by the comparator outputs of the integer divider is also employed instead of a full 4-bit coarse ADC, leading to an ultra low area and power design with less linearity error sources.