Some results from CSC Preamp Prototype tests



The setup Eight prototype preamp channels are connected to the station 2 prototype which is in a cosmic-ray test stand which provides a trigger when two scintillators (one above the prototype chamber and one below the prototype chamber) fire in coincidence. This trigger is sent to the test board which holds the prototype preamplifier and N samples prior to the trigger and M samples after the trigger are digitized and sent to our cosmic-ray test stand readout system.

Some Events (Gee Whiz)
Shown in the plots below are ADC values versus sample number (samples are taken once every 100 nsec) for a set of contiguous strips in a couple of sample events. In the first file you can see 2 strips with non-saturated but also non-pedestal values followed by a strip which reaches saturation followed by another non-zero/non-saturated strip, followed by a strip which basically has a pedestal value. The second plot shows a smaller number of ADC samples, and three strips with the first two showing charge and the third one showing a pedestal value. (Pedestal values are ~channel 3500 and saturation is ~channel 0.)


25 ADC samples from five cathode strips in an event


Another event with fewer samples

Pedestal (noise) measurements
The first plot below shows a pedstal and ADC spectrum for channel 2 from the preamplifier. As can be seen, the pedestal is somewhat wide (~12 channels sigma). Note, again, the pedestal is at the high end of the spectrum and large cathode charge is at the bottom end of the spectrum


Pedestal and ADC spectrum from Channel 2

Since we know that common-mode noise is a large portion of the noise that is currently seen in the preamplifier, I try below to separate the common-mode noise from the "white" noise. The first plot shows the pedestal of channel 2 versus the pedestal of channel 5 and you can see a strong correlation which indicates that a large fraction of the noise is, in fact, common- mode noise. (Just to confuse you, I have plotted 4095-ADC here so that the pedestals are now at the bottom of the spectrum rather than the top.) The next plot shows the difference between these two pedestals, which should give you the non-common-mode noise for two channels in quadrature. The resulting pedestal appears to be ~6.2/sqrt(2)=4.4 channels. Converted to electrons, this is ~(4.4 channels)*(1mV/channel)/(7mV/fC)/(1.6E-4fC/e-) = 3900 electrons.


Pedestal of channel 2 versus Pedestal fo channel 5


Pedestal of channel 2 minus Pedestal of channel 5

Pulse-height distributions
The plot below shows histograms of the charge on the peak cathode strip for all events where a cathode cluster was found, for chamber voltages of 1550 and 1600 volts. Pedestals have been subtracted, which were of the order of 100-200 ADC channels so the top of the ADC spectrum is ~channel 3900-4000 in these histograms. Break-down voltage for this chamber gas is ~1950 V, so these spectra were collected at well below the break-down voltage.


Peak strip distributions for 1550 and 1600 V.

Calibration data
The eight preamp channels were calibrated by putting a calibration card with built-in capacitors between the detector output and the preamplifier and sending step voltages into the capacitor card. The voltages were not especially precise--set with a scope and a standard pulser. Around 150 events were taken at each voltage step, covering the range of the ADC. The plots below show the output ADC value versus the input pulser voltage for all eight channels and the full and lower portion of the calibration data from channel 2. All data look very linear over the range of interest (at least within the errors of the calibration method). A couple of channels show a small flattening at the lowest portion of the spectrum which is probably the onset of pedestal values. The final plot shows 3 channels, lower portion. You can see that the differences between the straight line fit and the data points are similar which either says that the non-linearity of the preamp is uniform or points out the error in the input voltage which was fed simultaneously to all channels.


Calibration data from all 8 preamp channels.
Post-script


Calibration data from all preamp channel 2.
Post-script


Lower portion of channel 2 calibration with full straight-line fit.
Post-script


Post-script
Lower portion of three calibrations.


Calibration data with DAC


A sample set of calibration data taken for channel 3 of the preamplifier.


A sample set of calibration data taken for channel 3 of the preamplifier, showing just the lower portion of the spectrum.


The residuals of a straight-line fit to the calibration data. The residuals are in units of ADC channels where full scale=4095, and the residuals are plotted agains ADC value. Just the portion above DAC=15 was used for the fit. The two plots show three separate measurements taken for channel 3 (top plot) and channel 5 (bottom plot). Note that to get a calibration measurement to better than 1% of the most probably pulse (which should fall in ~ channel 400), the ADC gain must be calibrated to better than ~4 channels.



Last update 11/6/98 by Melynda Brooks