Threshold conversions

The four thresholds – front slow, front fast, rear slow, and rear fast – are controlled by DACs that can be set from 0x00 to 0xFF (i.e., 0 to 255 in decimal). The slow thresholds were calibrated in detail prior to launch, but it is likely that the calibrations have drifted over time.

Front slow threshold

Here is a conversion table of selected DAC settings to keV according to the pre-launch calibration. The calibration is a linear function, and the nominal DAC setting is 0x0C, which is ~3 keV.

DAC	1F	2F	3F	4F	5F	6F	7F	8F	9F
0x0C	2.68	2.66	2.58	2.82	2.61	2.51	2.32	2.63	2.52
0x10	3.49	3.44	3.32	3.59	3.40	3.26	3.09	3.42	3.25
0x20	6.68	6.55	6.27	6.66	6.57	6.27	6.15	6.58	6.19
0x40	13.1	12.8	12.2	12.8	12.9	12.3	12.3	12.9	12.1
0x80	25.9	25.3	24.0	25.1	25.6	24.3	24.5	25.6	23.8
0xC0	38.6	37.7	35.7	37.4	38.2	36.3	36.8	38.2	35.5
0xFF	51.2	50.0	47.4	49.5	50.7	48.1	48.8	50.7	47.1

Front fast threshold

The nominal DAC setting is 0x22, which is ~6 keV.

Rear slow threshold

Here is a conversion table of selected DAC settings to keV according to the pre-launch calibration. The calibration is a linear function, and the nominal DAC setting is 0x30, which is ~20 keV.

DAC	1R	2R	3R	4R	5R	6R	7R	8R	9R
0x30	24.1	23.4	24.0	24.3	24.5	23.9	20.8	19.0	21.1
0x40	27.3	26.4	27.2	27.6	27.5	27.1	24.6	22.5	24.1
0x80	40.0	38.5	40.1	40.5	39.7	39.8	39.9	36.6	36.0
0xC0	52.6	50.7	52.9	53.5	51.9	52.4	55.3	50.6	47.9
0xFF	65.1	62.6	65.5	66.3	63.9	64.9	70.4	64.5	59.6

Rear fast threshold

The nominal DAC setting is 0x45, which is ~20 keV. The step size is ~0.28 keV/step.