MTL Annual Report

Switched-capacitor circuits can be used to implement many analog systems such as ADCs, DACs, filters, amplifiers, and integrators.  In earlier phase of this research, a reconfigurable switched-capacitor system is proposed to implement different analog systems.  A prototype system has been fabricated that shows basic reconfigurability to implement a pipe-lined ADC and a switched-capacitor filter.  A second prototype system has been designed that utilizes highly reconfigurable blocks. Figure 1 shows the block diagram of such systems.  The building blocks have the same functionality and can be programmed to implement a multiplier or an integrator with a reconfigurable coefficient.  Such a system has many applications such as in software-defined radios and rapid prototyping of analog circuits.

Figure 1: Block diagram of reconfigurable zero-crossing-based analog circuits. Each configurable analog block can be programmed to perform an integration or multiplication. The connection of blocks is also programmable.

The design of such systems has had limited success since many different op-amp topologies are required to cover a large performance and configuration space.  Recently, ^[1] and ^[2] proposed zero-crossing-based (ZCB) circuits to design ADCs.  ZCB circuits replace the op-amp in traditional switched-capacitor circuits with a combination of a current source and a zero-crossing detector.  ZCB circuits are well suited for highly reconfigurable system since their power consumption scales with the operating frequency and required SNR. In addition, ZCB circuits benefit from technology scaling due to their more digital circuit-like operation in contrast to conventional op-amp based circuits.

The system is designed to operate at different speeds while the power consumption is kept at the optimum level.  A key challenge in design of this system is to keep the cost of programmability low in terms of additional power consumption and performance degradation.  Several innovative circuit techniques have been used to address this issue.

1. T. Sepke, J. Fiorenza, C.G. Sodini, P. Holloway, and H.-S Lee, “Comparator-based switched-capacitor circuits for scaled CMOS technologies,” in IEEE International Solid-State Circuits Conference Digest of Technical Papers, San Francisco, CA, Feb. 2006, pp.220-221. [↩]
2. L. Brooks and H.-S. Lee, “A zero-crossing-based 8b 200MS/S pipeline ADC,” in IEEE International Solid-State Circuits Conference Digest of Technical Papers, San Francisco, CA, Feb. 2007, pp. 460-461. [↩]