A 15 μm-Pitch, 8.7-ENOB, 13-mcells/sec logarithmic readout circuit for multi-level cell phase change memory

This paper presents a narrow-pitch readout circuit for multi-level phase change memory (PCM) employing an architecture of two-step 5 bit logarithmic ADC. A single-slope-architecture based fine ADC yields a 15 μm-width compact single channel readout circuit for column parallel readout structure. A current-mode 2 bit flash ADC for coarse conversion and the pipelined architecture between the coarse and fine conversion enhance the readout rate up to 13 Mcells/sec. With the enhanced residue accuracy provided by the replica circuit of residue generator, the ADC achieves excellent linearity of 9.96 b (linear ADC equivalent). The integration-based residue generation effectively reduces circuit noise and yields 8.7 ENOB. The prototype chip was fabricated in a 65 nm CMOS process and the measured power consumption from a single channel readout circuit was 105 μW at 13 Mcells/sec conversion rate at 1.2 V supply.