CDC Preamplifier Conclusion

Results of Studies of the new ASIC Preamplifier

After discovering that the gas mixing system at CMU had malfunctioned, we switched to a premixed 90/10 Ar-CO2 mixture. Using the premixed gas, we were able to reproduce the observed behavior of the old preamplifier. We then proceeded to compare the new preamplifier with the old preamplifier.

• Old vs New with 90/10 premix
• channel-by-channel pedestal adjustment is crucual
• New Preamplifier Threshold Studies

Conclusions of the Studies at CMU

1. The gain of the new preamplifier is about a factor of two smaller than that of the old preamplifier.
2. The noise of the new preamplifier is about a factor of 4 less.
3. We also observed that the output range of the old preamplifier was from 0V to 1V. E.g. we mapped 1V onto ~4000 channels in the FADC. The new preamplifier has an output range of 0V to 0.5V, with 0.5V being mapped onto ~4000channels in the FADC. This change was made to match the new 72-channel 125MHZ FADC.
4. In terms of the desire to increase the dynamic range of the CDC by a factor of two, this was to be achieved by decreasing the preamp gain by a factor of two. However this increase has been lost by the factor of two decrease in the output range of the new preamplifier. At the same operating point as before, the dynamic range of the new preamplifier/FADC-125 combination is the same as the old preamplifier.
5. We have not achieved the desired increase of dynamic range in the system.

Alternative Approaches to Increasing Dynamic Range

There are two components to the total gain of the system. That of the preamplifier, and the gain of the chamber itself. It is possible to decrease the gain of the system by decreasing the gain of the chamber. This is accomplished by decreasing the high-voltage on the wires. However, there are other issues that associated with this that need to be examined.

1. The signal size is dependent on the high voltage, but there is a component of the noise that is amplified, but is not dependent on the high voltage. We feel that this is an important part of the noise. For this noise, decreasing the gain of the preamplifier will leave the S/N the same. Decreasing the gain of the chamber will decrease S, but leave N unchanged---worse S/N.
2. We carried out studies of the 90/10 mixture at several different voltage settings. It appears that it would be possible to get the needed dynamic range by running at the low-voltage edge of the plateau of the chamber. Nominal voltage is about 1450 volts, but going down by ~10-15 volts will probably put us at a reasonable point.
3. Based on this work, it appears that we cannot run at much lower voltage, and that the chamber will need to be run in a narrow (plus/minus 15 Volt) range to get the dynamic range.
4. It will be possible to run the chamber at a lower voltage and achieve the dynamic range needed with the new preamplifier. However, doing so will decrease the S/N slightly and limit running to a narrow range of voltages near the start of the plateau. We will effectively give up varying the high-voltage along the entire plateau as variable in selecting the operating point.