Continuous readout and auto-summation
One of the hallmark features of EIGER is its continuous readout that enables kilohertz frame rates with duty cycles greater than 99%. Every pixel of an EIGER ASIC features a digital counter for noise-free counting of the observed photons during an exposure. A readout buffer accompanies this digital counter in each pixel. After acquisition of an exposure, the state of the counter is transferred to the readout buffer virtually instantly. A subsequent exposure can commence after a short dead time of only 3 microseconds, while the previous exposure is being read out from the buffer. The continuous readout of EIGER with counter and buffer in every pixel maximizes duty cycle while allowing for an electronic global shutter. This ensures that the data for all pixels of an image were acquired simultaneously, in contrast to a rolling shutter that leads to skewed images.
EIGER’s auto-summation mode is a further benefit of continuous readout with high duty cycle. While a single exposure is limited to the 12 bits of the digital counter, auto-summation extends the data depth up to 32 bits or more than 4.2 billion counts per pixel, depending on the number of summed exposures in an image. At frame rates of several hundred Hertz and higher, all counts are captured in the digital counter of a pixel. If lower frame rates are requested, exposures are still acquired at high rates on the pixel level, effectively avoiding any overflows. Exposures are summed to images with the requested frame rate on the fly; extending the bit depth of the data by the number of summed exposures. EIGER’s auto-summation mode maintains a duty cycle greater than 99.7% at all times due to continuous readout with 3 µs dead time.