  The CDEX (China Dark matter Experiment) aims at detection of WIMPs (Weakly
Interacting Massive Particles) and 0vbb (Neutrinoless double beta decay) of
76Ge. It now uses ~10 kg HPGe (High Purity Germanium) detectors in CJPL (China
Jinping Underground Laboratory). The energy resolution of detectors is
calculated via height spectrum of waveforms with 6-us shaping time. It is
necessary to know how sampling properties of a digitizer effect the energy
resolution. This paper will present preliminary energy resolution results of
waveforms at different sampling properties. The preliminary results show that
the ENOB (effective number of bits) with 8.25-bit or better can meet the energy
resolution @122keV of CDEX HPGe detectors. Based on the ADC (Analog-to-Digital
Converter) quantized error theory, this paper will also make a quantitative
analysis on energy resolution in CDEX HPGe detectors. It will provide guidance
for ADC design in full-chain cryogenic readout electronics for HPGe detectors.
