Photon Detector DAQ

The data acquisition of the photon detector is based around the SIS3320 250 MHz sampling ADC from Struck Innovative Systems. It has the ability to sample signal pulses from the detector as well as accumulate the samples during an entire helicity window.

Accumulator mode

During a helicity window of 1 ms (at 1 kHz), all samples are added. At a sampling frequency of 250 MHz this corresponds ideally to 250000 samples per helicity window, but the actual number will be lower due to read-out operations. The number of samples is used for normalization.

There are 6 accumulator registers (threshold2 < threshold1):

• Accumulator 1: digital sum of all samples from start to stop
• Accumulator 2: digital sum of all samples above threshold1
• Accumulator 3: digital sum of all samples above threshold2 and below or equal to threshold1
• Accumulator 4: digital sum of all samples below or equal to threshold2
• Accumulator 5: digital sum of all samples above threshold2 and below or equal to threshold1, and N5 samples before or after
• Accumulator 6: digital sum of all samples below or equal to threshold2, and N6 samples before or after

The accumulator registers are numbered from 1 to 6 in the SIS3320 user guide, but are addressed from 0 to 5 in the firmware.

To reduce dead-time in accumulator mode, the accumulator registers are read-out before anything else. Especially VME bus operations are slow and should be avoided before read-out.

To run in accumulator mode, set setup mode to 1 and stop mode to 0. In this mode it does not make a difference if you use double buffers.

There is an undefined time between the GO event (when accumulator sampling is started by the CRL) until the processing of the first helicity window trigger. This first event of every run has to be discarded. In addition, the number of samples in the first accumulator is required to be within reasonable limits.

Single event mode

During a helicity window of 1 ms one event is sampled and buffered. At the end of the helicity window this event is read out.

Multi event mode

During a helicity window of 1 ms many events are sampled and buffered.