A reconfigurable time-to-digital converter based on time stretcher and chain-delay-line for electrical bioimpedance spectroscopy

We propose a time-to-digital converter (TDC) with a reconfigurable time resolution from 0.1 ns to 244 ns using a low-speed reference clock of 32.768 MHz. This TDC combines a coarse counter with two parallel fine TDCs which measure front and back fractional times of the coarse counter. In order to obtain high precision without increasing the reference clock speed, the fractional times are stretched through time stretchers of two fine stages with tunable stretching factor and are quantized through fine counters and chain-delay-lines. The time resolution is tuned by controlling the counting clock speed or the stretching factor. The designed TDC in a 0.18-μm CMOS process quantizes a phase of impedance with less than 0.088° error for frequency range from 1 kHz to 2048 kHz. As the result, this TDC is suitable for phase quantizers in bioimpedance spectrometers.