First Year Performance Paper - Section 3.1
3.1 The ATWD and Amplifier Calibration
The DOM ATWD bin readout scales linearly with input signal voltage; however, each ATWD bin has a unique slope (V/ADC-counts) and intercept. DOM-Cal measures this response by varying ATWD bias voltage and applying a linear fit to the resulting amplitudes for each bin (0-127) of each signal channel (0-2) for both ATWDs. Each ATWD bin can be digitized to a depth of 10 bits.
The DOM front-end amplifier gains for each channel are 16x, 2x, and 0.25x, providing complete dynamic range for signals generated by the PMT. The amplifier gains vary due to component tolerances and must be determined precisely for PMT gain calibration. Amplifier calibration is a two step process: The absolute gain of the high-gain channel is determined first, then the lowergain channels are calibrated relative to the high-gain channel. The high-gain channel is calibrated by injecting artificial Single Photo Electron (SPE) like pulses into the DOM PMT input and comparing peak amplitude from the ATWD to the true peak amplitude of the pulse. The amplifier gain is given by the mean peak ratio for a large sample of pulses. ATWD sampling speed is maximized during the procedure to minimize waveform integration error.
To calibrate the lower-gain channels, DOM-Cal uses PMT signals from light pulses generated by a LED on the DOM MB. ATWD data is acquired and calibrated both for the channel with the unknown gain and the previous channel, whose gain was already calibrated. The gain of the next channel is the gain of the previous channel times the ratio of the pulse integrals of the two channels. LED pulse ranges are chosen to avoid ATWD saturation in the higher-gain channel, yet still provide significant amplitude in the lower-gain channel to minimize integration error. Fig. 5 and Fig. 6 show the relation between the gains of two ATWD channels.