The charge gain was measured by using a known external test capacitance. The external test capacitor was mounted with ground plane shielding to minimize any stray coupling into the preamp input lead. The capacitor has a 10% tolerance and the input voltage was measured to about 5%.
The test board has an internal test pulse wired directly to the asic pins. The coupling from this input into the preamp is dominated by the pin to pin capacitance of the asic packaging so its value isn't well known. The external capacitor was used to calibrate this input.
The pulse height peak channel was recorded for several different test pulse voltages using the external and internal test. The internal test matches the external one if you assume an capacitance of 1.93pF. This is a reasonable value for pin to pin coupling in this setup.
The data shows some non-linearity. The best fit line was done using only the points above 6fC. This gives a slope of 9.53 chan/fC in the region around 16fC where most of the noise width tests were done.
The response to test pulses changes slightly with the APD detector connected and biased. The low gain range appears to have somewhat worse linearity than the high gain range.