Beyond Rail-to-Rail Compliant Current Sources for Mismatch-Insensitive Voltage-to-Time Conversion

This brief presents beyond rail-to-rail (BR2R) compliant cascode current sources to markedly improve the compliance voltage and output impedance of the extensively used wide-swing cascode current source. The proposed BR2R sources employ a bootstrapping technique to linearly charge a load capacitor to beyond the supply rails <inline-formula> <tex-math notation="LaTeX">$V_{\mathrm{ dd}}$ </tex-math></inline-formula> or <inline-formula> <tex-math notation="LaTeX">$gnd$ </tex-math></inline-formula> while maintaining an improved output impedance over an equivalent wide-swing cascode source. A process and mismatch-insensitive differential voltage-to-time (DVT) converter employing these BR2R sources is fabricated in 65-nm CMOS and occupies <inline-formula> <tex-math notation="LaTeX">$0.021~\text {mm}^{2}$ </tex-math></inline-formula>, while dissipating <inline-formula> <tex-math notation="LaTeX">$47~\mu \text{W}$ </tex-math></inline-formula> at 1 V. The measured BR2R DVT signal to noise and distortion ratio is 50.2 dB, compared to 38.7 dB for the wide-swing cascode-based DVT within a 2-MHz input bandwidth. The DVT achieves a common mode rejection ratio of 35.1 dB for a 0.4–0.6-V input common-mode range.