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Cryogenic Analog-to-Digital Converters Operational from Room Temperature Down to 4.2K
Keywords: cryogenic ADC, ultra-wide-temperature-range ADC, cryogenic CMOS design
Currently cryogenic sensor systems (120 to 4.2K) still lack Analog-to-Digital Converters (ADC’s) in their cold front-end electronics. Contrary to room temperature (RT) systems, the AD conversion is performed on a considerable distance from the sensor’s analog read out (at times up to tens of meters), in the “warm electronics”. This affects signal integrity and requires complex shielding approaches. At present, the only cryogenic ADC’s reported in literature are based on superconductive devices and hence have a maximum working temperature of 10K. Nevertheless to monitor the cryogenic systems at working temperature as well as before and during cooling, an ultra wide temperature range ADC is required. For the design of such converters, CMOS is preferable as it is one of the few technologies functional over this temperature range. It is known that CMOS transistors exhibit anomalies as kink (for T <100K) and transient currents (T<15K), due to carrier freeze out. This irregular behavior is not modeled by standard circuit simulators and causes malfunction of COTS circuitry at extremely low temperature. However by adapting analog circuit architectures, and using our extracted cryogenic transistor models, we simulated, designed, fabricated and tested two cryogenic ADC’s, a successive approximation and a flash type converter. Tests of the ADC’s at RT and 4.2K show very similar circuit behavior and performance.
Ybe Creten, Ph.D. Student
IMEC
Leuven B-3001,
Belgium


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