A custom 12-bit cyclic ADC for the electromagnetic calorimeter of the International Linear Collider

A custom 12-bit Analog-to-Digital Converter (ADC) has been designed. It is dedicated to the very-front-end electronics of the electromagnetic calorimeter (Ecal) of the International Linear Collider (ILC). A cyclic architecture, with a resolution of 2 bits per cycle has been chosen. Its compactness allows the use of one ADC per channel which simplifies the layout of the final 64-channel very-front-end chip and preserve the signal integrity. In order to optimize the ratio between the sampling rate and the power consumption, an enhanced design is proposed. It involves the use of a single amplifier shared between two parallel capacitor arrays. The power consumption is then limited to 4mW, and with the use of a power pulsing with the duty ratio of ILC, an integrated power consumption of 0.12 µW is expected. The time conversion of 7 µs obtained with a clock frequency of 1MHz is shorter compare to the 500 µs foreseen for A/D conversion. The building blocks which are the operational amplifier and the comparator have been previously validated in a 10-bit pipeline ADC, but they have been optimized to fulfil the 12-bit resolution and speed performance required. A prototype chip has been layouted using the 0.35 μm CMOS technology of Austriamicrosystems. Measurements show that the Differential Non-Linearity and Integral Non-Linearity are respectively within a ±1.2 LSB range and a ±1 LSB range, with a noise limited to 0.9 LSB@68%CL.