A High-Linearity Digital-to-Time Converter Technique: Constant-Slope Charging

A digital-to-time converter (DTC) controls time delay by a digital code, which is useful, for example, in a sampling oscilloscope, fractional-N PLL, or time-interleaved ADC. This paper proposes constant-slope charging as a method to realize a DTC with intrinsically better integral non-linearity (INL) compared to the popular variable-slope method. The proposed DTC chip realized in 65 nm CMOS consists of a voltage-controlled variable-delay element (DTC-core) driven by a 10 bit digital-to-analog converter. Measurements with a 55 MHz crystal clock demonstrate a full-scale delay programmable from 19 ps to 189 ps with a resolution from 19 fs to 185 fs. As available oscilloscopes are not good enough to reliably measure such high timing resolution, a frequency-domain method has been developed that modulates a DTC edge and derives INL from spur strength. An INL of 0.17% at 189 ps full-scale delay and 0.34% at 19 ps are measured, representing 8-9 bit effective INL-limited resolution. Output rms jitter is better than 210 fs limited by the test setup, while the DTC consumes 1.8 mW.