Design Of Current Mode Analog-To-Digital Converter

For applications aiming at portable electrical devices and mixed-signal SoC, the supply voltage goes even lower, and the voltage headroom for signal processing and function operation comes smaller. These situations bring great difficulties to the conventional voltage mode design methodology. Providing a possible solution to this problem, researchers give the current mode circuit technique a renewed survey. Current mode technology uses the currents in the circuits as the signal variables, and the whole circuits in current mode are insensitive to the supply voltage, could operate under very low voltage and achieve high operation speed. However, the static power dissipation of the current mode circuit is higher then the voltage mode circuits, since the dynamic range is decided by the bias currents. To solve the problems caused by low supply voltage and also to integrate the circuit design architechture, the research and investigation on current mode technique in depth is necessary.Based on the special advantages of the current mode technique, this paper made a thorough research on the current mode technique in analog integrated circuit design. After detailed survey and analysis on the development of current mode techniques, research was made on current mode circuits'application in analog modules implementation, especially on the basic functional blocks—sample and hold circuits, and current comparators. Based on the study of functional modules, a current mode analog to digital converter was realized. The main contents of this paper include:The general analog modules adopting current mode were designed and the application range of the current mode technique were introduced. Setting the sample and hold circuits and the comparators as the examples, a novel class AB switched current memory cell are proposed based on the analysis of the charge injection effects caused by MOS switches on switched current momery cells, and the error current was eliminated theoretically. One continuous-time current comparator and two switched current comparators were presented according to different applications. Low power dissipation characteristics were realized without sacrificing resolution and speed. One of the power efficient switched current comparators was designed and fabricated in CSMC 0.6μm CMOS process. The testing results of this comparator revealed that the accuracy of the design and the feasibility of implementing general analog modules in current mode.To start with the special advantages of the current mode technique, a differential 8bits, 20MSamples/s current mode pipelined analog to digital converter were designed and fabricated in CSMC 0.5μm CMOS process. Adopting the distributed bias current circuits, the deterioration caused by process mismatch and so on was alleviated. A new fully differential current comparison scheme was presented, and the constraints on power consumption and current directions in conventional scheme were removed. And the proposed scheme in this paper presents simpler structure and lower power dissipation. Based on the basic resistive source degeneration linearized structure, a linearity enhanced voltage to current converter was proposed and make the test and measurement of the designed IADC possible. The measurement results revealed that the current mode analog to digital converter designed in this paper operated properly in a wide supply voltage range from 3.3V to 5V, and in a large quatization range from±64μA to±256μA. The ADC proposed in this paper draws 75mA current from a 5V voltage supply at 256μA reference current input, DNL/INL are -0.005～+0.027 LSB/-0.1～+0.2 LSB respectively. SNR is 42.6dB and the ENOB is 6.8 bits. When the supply voltage turns to 3.3V, the ADC draws 21mA current from power supply at 64μA reference current input and DNL/INL are -0.8～+1.2 LSB/-1.8～+1.8 LSB respectively. SNR is 42.2dB and the ENOB is 6.1bits.