Time stretcher for a time-to-digital converter with a precisely matched current mirror

This paper presents an approach for the design of a time stretcher based on charging and discharging a capacitor by currents with a ratio equal to the desired stretch factor. The stretched time interval can be measured by using a time-to-digital converter to achieve improved system resolution. Expressions for the current source output impedance and transistor area required to reach a specified linearity and matching are derived. The realization uses wide-swing current mirrors to achieve the required output impedance at an acceptable voltage swing at the capacitor. The derived expressions and the overall design are validated with schematic and extracted simulations in a 0.35 μm CMOS process technology.