## 3 Results and Discussion
### Measurements with Sensor  
The third optimised SALT prototype with a biased UT sensor connected was characterised with the same set of measurements as discussed previously. A large sensor capacitance presents the most difficult conditions for the operation of the ASIC, and therefore, only results from the largest 12 pF/strip UT sensor (Type A) [40] are presented here (other sensor types were also tested).  
Figure 13: Baseline noise Root Mean Square (RMS) versus channel number before (Raw Noise) and after (MCM Noise) MCM correction; sensor not connected.  
The curves obtained from the trim DACs scan are presented in Figure 15. They are more smooth in comparison to the results without sensor due to the larger noise, which is also seen in the baseline noise histograms shown in the Figure 16.  
The left plot in Figure 16 shows the baseline noise histogram obtained for the default values of the trim DACs corresponding to the one in Figure 12. In both figures, the spread looks purely random between channels. The plot to the right shows the same noise after the trim procedure. It is clearly seen that the noise is very homogeneous between channels.  
The distribution of noise as a function of channel number (with a sensor attached) is presented in the Figure 17, before and after MCM subtraction. As expected, the noise RMS is higher than for the bare chip and is between 0.8-1 LSB. This is due to sensor capacitance. Presented results show the same values of noise before and after MCM subtraction, which  
Figure 14: Pulse shapes for about 20 ke input charge and 128 channels obtained from the ADCs outputs. The sensor is not connected for this measurement.  
Figure 15: Baseline trim DACs scan for 128 channels after a biased UT sensor is connected.  
not always must be the case. Measurement done with different setups gave the same MCM noise value, while the noise measured without MCM correction could be about 30% higher.  
The response of the front-end to the input signal is studied, similarly as for the setup without sensor, using the internally generated pulse and reconstructing the output pulse shape from the output data obtained with different ADC sampling phases. The results for all SALT channels, after trim DAC calibration, are shown in Figure 18. The shape of the pulse is similar to that obtained without the sensor (Figure 14) but the baseline is not completely flat. Residual effects of the 40 MHz disturbance mentioned previously is still visible, although the amplitude is very small, of the order of one LSB. This observation is in agreement with SALT simulations, which showed that it was not possible to completely remove this effect. Since the disturbance is synchronous with the system clock, it will be seen as a constant baseline and subtracted during signal processing in the DSP. Consequently, its effect on the signal amplitude measurement should be negligible. The pulse amplitude after sensor connection is about 24 LSB, similar as for the bare chip. It shows that for the foreseen sensor capacitance range the output amplitude does not change significantly. Signal-to-noise ratio for the setup with the sensor is above 20 and, as expected, is lower than for the bare chip.  
Figure 16: Baseline noise histogram before applying the trim DAC correction (**left**) and after the trim DAC correction (**right**), both with a biased UT sensor attached.  
Figure 17: Baseline noise RMS versus channel number shown without and with MCM correction with a biased UT sensor connected.