Tests on the modm4 (abcd2 metallised p-type chips on the new Oslo barrel hybrid)

We modified the setup and the hybrid in the following way:

- SET UP

clock and commands are ECL signals going from the DRAFT into Jan's PCB (before they were going through the BC96)
floating power supplies for VDD and VCC (previously provided by the BC96)
new PCB that can handles ECL signals and it has just one single ground (before we used the Melbourne PCB)

- HYBRID

the substrate is Alumina (before it was Berillia)
just one ground plane (before it was splitted in AGND and DGND)
power and ground planes thinkness is 8 um gold (before it was 3 um gold)
chip's pads connected to ground (before they were floating and we needed a trick to connect the chip's backplane to ground)
the detector biasing scheme has been modified with the addition of 1K resistor between the decoupling capacitor and the detector backplane (it can be shorted later if we want to restore the previous configuration)

Results for the high current region (preamp = 220 uA and shaper= 20 uA):

- 6 chips connected to 6 cm silicon detector

gain,offset,enc [ps]
s-curves for 2 fC injected charge [ps]
comments: there is no sign of oscillation, but there is a very little deterioration of the s_curves at thresholds < 100 mV

- 5 chips connected to 6 cm silicon detector and chip 6 (the farest from the kapton cable) connected to 12 cm detector

Noise scan at 100 Volts and 1 Volt [ps]
Threshold scan for 2 fc if chip 6 is disabled [ps]
comments: all chips are oscillating for the normal detector bias (100 Volts to achieve full depletion), BUT they seem to work reasonably for biases less than 30 Volts. In the last case we can notice a deterioration of the chips response due to the high detector capacitance (detector partially depleted), but no oscillations. If we switch off the 6th chip (the one connected to 12 cm) we get good results for the remaing 5 chips even at 100 Volts.
We tried to modify the detector bias circuit: in the default configuration the two detector backplane wires are connected together on the hybrid before the resistor and the decoupling capacitor. The bias and guard pads of the first detector (the one that seats underneath the hybrid ) are connected to the ground on the hybrid while the bias and guard pads of the second detector are connected to the ones of the first detector on one side. We modified the bias and guard pads connection of the second detector connecting them directly with the ground on the kapton cable (one of the cleanest ground on the hybrid) and NOT through the first detector. We also connected the module metal frame to ground. None of these attempts gave better results.