The threshold voltage versus dac setting was measured for three radhard chips after an 1 Mrad irradiation. The threshold voltage is:
V_thres = V_tp - V_tn
The following figure shows a measurement of the threshold voltage (V_tn, V_tp is constant). The offset of chip 2 is due to the pico probe.
The slope is extract from a fit of linear function.
| chip 11 | 0.452 mV / dac |
| chip 2 | 0.453 mV / dac |
| chip 1 | 0.449 mV / dac |
The offset for a dac setting of 0 (lowest value which corresponds to 63) the difference between V_tp and V_tn was very hard to measure. The absolute values depend on the how well the probe tip is placed on the pad and we also observed a drift in time which might be due to the pico probe amplifier. However for the most stable measurements the offset was between 0 mV and 4 mV.