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, 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.