## 4 Experimental Stations
### 2D X-ray Detectors
#### 4.3.4 Pixel Design  
The design of the front-end block of the JUNGFRAU pixel is based on the automatic gain switching principle previously used in GOTTHARD [197] and AGIPD. A schematic view of the pixel circuit is shown in Figure 20. The pixel front end consists of several blocks:  
* a preamplifier with three selectable gains: high (G0), medium (G1) and low (G2),
* an automatic gain switching block consisting of a comparator with tuneable threshold and switching control logic,
* a correlated double sampling (CDS) stage to remove the preamplifier low frequency and reset noise in high gain,
* a storage array for 16 images,
* a buffer needed to drive the column bus during the readout phase.  
The preamplifier gain is variable: a fixed small size feedback capacitor is used for the high gain, while the insertion of two capacitors, \(\sim\)10 and \(\sim\)100 times bigger respectively, lowers the gain to a medium or low value. While the gain setting can be fixed with an external signal, in the normal mode of operation the control of the gain is automatically handled by the front-end circuit itself, pixel by pixel. For this purpose, the output of the preamplifier is monitored, during the integration, by a comparator. When the signal level crosses the threshold the gain switching logic is triggered. The threshold voltage is common for all the channels and is placed at the upper limit of the output range of the preamplifier. The digital logic, based on delay stages and lat  
Figure 19: Mechanical drawing of a 4 M JUNGFRAU detector, showing a possible module arrangement around a small central hole.  
of the feedback capacitors and lowers the gain first to medium, then to low until the output of the preamplifier is brought back into the preamplifier output range.  
In the idle state the preamplifier is kept in reset at the low gain mode, so that all the feedback capacitor are emptied. A few nanoseconds before the beginning of the measurement these capacitors are disconnected so that the gain is set to high. Then the reset switch is opened and the input charge starts to be integrated. The amplifier switches depending on the amount of input charge flowing into the readout channel. The output voltage and the gain (encoded in two digital bits) are sampled at the end of the integration time. Together, these allow the determination of the incoming charge with the help of a gain calibration curve.  
A precharge scheme for the feedback capacitors has been introduced to achieve the requested dynamic range of \(10^{4}\) photons per pixel per pulse even if the maximum available feedback capacitance in a 75 \(\upmu\)m pixel is rather small (\(<\)8 pF); during the preamplifier reset, the output side of the capacitor is not charged to the preamplifier reset level but is set to a tuneable voltage level which is chosen such that the preamplifier output is at the lower end of its range for signals just higher than the switching point. For low photon energy operation, the starting gain of the preamplifier can be increased, by a factor of \(\sim\)2, to HG0, decreasing the noise but also the dynamic range before the first switching. In this case the gain switching sequence is HG0, G1 and G2. As in the GOTTHARD ROC, the CDS stage is bypassed in case of gain switching so that the (higher) noise of the high gain is not added to the (lower) noise of G1 and G2. 16 analogue storage cells are present in-pixel, each one with the corresponding 2 bit digital latches to store the gain information. In burst operation, up to 16 images can be stored on the ROC before readout.