An Ultra Low Current Measurement Mixed-Signal ASIC for Radiation Monitoring Using Ionisation Chambers

Measurement of total ionizing dose in a radiation field is efficiently carried out by ionisation chambers. The paper details the design of a mixed-signal ASIC for the front-end electronics of ionisation chambers. A single chip solution for ultra-low current measurement is designed by combining the current processing analog section realized using low leakage thick gate transistors and the data handling digital section implemented using fast thin gate transistors. The design succeeds in limiting the cross coupling between the two circuit domains using deep n-wells and guard rings. The ASIC fabricated in 130 nm technology attains a wide dynamic range of −7 fA to −20μA with maximum error in measurement less than ±4 %. The ASIC occupies an area of 3.52 mm 2 and has a total power consumption of 17.4 mW. The femtoampere range input leakage current of the ASIC contributed primarily by the ESD diodes was found to be varying exponentially with temperature. Dose rate measurements from 5 μ Sv/h to 7.4 Sv/h is demonstrated by interfacing the ASIC to an ionisation chamber.