The behaviour of the WFC3 UVIS and IR Analog-to-Digital Converters

Using UVIS and IR data from the 2004 thermal vacuum tests, we have searched for anomalous behavior in the WFC3’s analog-to-digital converters (ADCs). We find that the ADCs of the CCD channel operate nominally, except for the less significant bits which favor 1’s over 0’s by up to 5%, depending on the ADC and on the input voltage. The IR channel, clocked at double speed, also shows departures in the least significant bits at ~1% level. These departures from ideal behaviour are statistically significant. We discuss the relevance of these anomalies for the scientific quality of the WFC3 data, simulating images affected by biased ADCs at 1% and 2% level. Whereas the occasional anomalous bits corresponding to signal levels around the cosmic ray filtering threshold may represent a source of salt-pepper noise hard to eliminate, for both the UVIS and the IR channels the added readnoise is negligible and does not affect the ultimate sensitivity of the instrument. The advantages of clocking the CCD detectors at double speed to reduce the detector cross talk largely supersede the disadvantages of the measured extra ADC noise.