EP-ESE Electronics Seminars

Utopia: A nine decade femtoampere current to frequency converter and its application in radiation monitoring

by Evgenia Voulgari (CERN)

13-2-005 (CERN)



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The detectors that are used at CERN for radiation monitoring are ionization chambers whose output current can vary from 2 fA up to the microampere range.
The need to digitize input currents over a wide dynamic range of 9 decades led to the design of the Utopia (Ultra-low picoammeter) ASIC.
The architecture of an asynchronous current to frequency converter through charge balancing was fully exploited since for lower radiation and equivalently for lower detector current, the measurement time can be increased. 
By active leakage current compensation, the front-end is able to measure current as low as 1 fA. The addition of a second reference charge injection circuit gives the possibility to extend the upper measurement range up to the microamperes by keeping a constant acquisition time.
The Utopia (Ultra-low picoammeter) 2 ASIC was fabricated in AMS 0.35 μm technology and has been fully characterized for its low current performance at the Swiss Federal Institute of Metrology (METAS). 
The chip is able to digitize currents over more than 9 decades, from 1 fA up to 5 μA. The guidelines set for the design of the ultra-low leakage current converter can be used in any system that targets femtoampere current measurements.

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