Reminders, ADC performances

There is a general need for spacecraft systems to make use of advances in analog to digital (ADC) and digital to analog (DAC) technologies. In particular, imaging systems such as star trackers and remote sensing instruments require 14,16, or more bits ADCs that have low power consumption, fast conversion rates (>1 MSPS) and good radiation tolerances. Because ADC and DAC have both analog and digital functions, they are mixed-signal devices. Most of state of the art ADC/DACs are processed in CMOS technologies. Therefore, the two radiation issues are total ionizing dose (TID) and Single Event Effects (SEE). Because of the accuracy needed to perform analog to digital or digital to analog conversions, a small degradation of one of the device elements can have a significant impact of the overall device performance. For example, internal voltage reference degradations impact significantly the performances of ADCs [1 Layton dw 03]. Available data show that all ADC/DAC have a low to moderate tolerance to TID [1, 2 Bings dw03, 3 Hopkinson dw00, 4 Black dw98, 5 LTC1657 report, 6 AD7714 report]. ADC and DAC are also very sensitive to SEEs: Single Event Transients (SET) in the analog part and Single Event Upsets (SEU) in the digital part. SETs generally cause small deviations in the device output [7 Turf tns90, 8 Turf tns94], and SEUs may cause large variations of the device output. Lingering errors or Single Event Functional Interrupts (SEFI) were also observed [9 Bee Radecs97, 10 Bee dw98, 11 AD7714 report, 12 Wilson dw94]. The SEU/SET LET thresholds are generally low, below 5 MeVcm2/mg, and proton induced SEU/SET were observed [8 Turf tns94]. Converters are complex devices with different architectures. Each converter type has its specific vulnerabilities. The objective of this document is to survey the existing ADC/ DAC radiation test methods. One of the main challenges of testing high resolution ADCs, DAC is the accuracy of measurements. For example, for a 5 V input range 16 bit ADC, the least significant bit, LSB, represents a variation of the input voltage of about 75 μV.