1. Test Set-up

The linearity measurements are done with short pulses, and the input stages of the ICECAL are AC coupled, therefore these input signals must be synchronized with respect to the ICECAL integrators and ADC aqquisition clock. One of the channels of a Tektronix AFG3252 is used to provide test pulses (negative) to the ICECAL input whereas the second one is used as a system clock (emulating the 40MHz LHC Clock) for syncronization. The system clock is propagated through the optical link, allowing the front-end to be in synchronization with this clock. The test pulses are generated at 40KHz, in syncronization with the 40MHz clock, and therefore the ICECAL Front-End, which ensures that the test pulses will have no phase drift with respect to the acquisition electronics. The following figures show the test set-up and the related parts of the board tested. On this test the parts concerned correspond to the complete ICECAL acquisition chain with the ADC AD6445.


Figure 1. Test Set-up with main components detailed and ICECAL_V3 Board with parts characterized for this tests highlighted.

2. Total System Noise Distribution with ICECAL and AD6445 Readout

The noise distribution of the whole readout assembly has been measured by loading all the ICECAL Inputs with 50 ohms. The ICECAL inputs, are AC coupled with 100nF capacitors for safety, the SMA connectors on the board are all loaded with 50ohms. Each of the ICECAL Subchannels has been treated separatelly for noise calculations due to the different offset value observed. Each channel/subchanel offset has been compensated with the internal registers configurations to reach similar offset levels. A temporal window of 2.5ms has been taken for noise measurements (1.000.000 Samples @ 40MSPS), and a histogram on the raw ADC Data is performed to obtain the noise distribution and a gaussian fit performed to extract noise sigma value.

The figure bellow shows that the noise sigma for all channels and subchannels remains bellow 6 ADC Counts, no noise increase has been observed when introducing the F.Generator on the chain.


Fibure 2. Noise distributions for the four ICECAL channels, each subchannel is studied independently (first two rows), and raw ADC data (bottom row).

Offset Compensation Used
  Ch0 Ch1 Ch2 Ch3
Ioff_MSB 7 6 6 6
Ioff_LSB0 2 0 3 0
Ioff_LSB1 0 0 2 0

3. Total System Linearity with ICECAL and AD41240 Readout

The linearity measurements have been done with a sweep on amplitude of short negative pulses. The test pulses width is 50ns with a repetition frequency of 40Khz, this configuration allows that two consecutive acquisitions (25ns) are performed each 1000 samples (25us). Since consecutive acquisitions are performed with different subchannels, one sweep per channel is enough to characterize both subchannels. The pulses amplitude sweep is performed between 50mv-1.4V, and a 30dB attenuator is placed just after the function generator, the charge sweep per 25ns windows seen by the ICECAL is therefore 790fC-22pC. Previous to the measurements, the ICECAL delay lines have been adjusted to match the 50ns pulses phase into two consecutive acquisitons (Phase_TH from the delay line channel control register), the integrators capacitors are set as default (Cint_MSB 1 , Cint_LSB0 7, Cint_LSB1 7).

The typical slope (with default Cint Values) for each of the ICECAL channels together with the ADC and drivers shows 7.3e14ADC count per Coulomb, which is translated into approximatelly 1.3fC/ADC Count. Considering a noise level with sigma ~ 6ADC counts, this set-up as it is would roughly allow the identification of 16.4fC pulses (2 sigma amplitude) with saturation around 22pC (1300 Dynamic range per channel).

Regarding linearity the four channels are showing a linearity error << 1%, the residuals jumps observed are due to the F.Generator scale changes during the sweep.


Figure 3. Test done for each channel independently: Linearity test results for the four ICECAL Channels (top) and residual non linearity (bottom). Each subchannel is studied independently. Here the version wiht Absolute error in ADC counts.

The test was reproduced connecting the four Input channels on the ICECAL to the function generator at the same time. Different Cint configurations were used to determine the charge slope and saturation levels for the default configuration as well as for the top and bottom sensitivity levels.


Figure 4. Test done for all channels together: Linearity test results for the four ICECAL Channels (top) and residual non linearity (bottom). Each subchannel is studied independently. Here the version wiht Absolute error in ADC counts.

ICECAL_V3 Configuration
  Max Sensitivity Default Settings Max Input Charge
Cint_MSB 0 1 3
Cint_LSB0 0 7 7
Cint_LSB1 0 7 7
Sensitivity 10.4 fC 16.8 fC 22.8 fC
Saturation 14 pC 22.3 pC 30.3 pC
Slope 0.87fC/LSB 1.4 fC/LSB 1.9 fC/LSB
Dynamic Range. ~ 1.3 x 10e3
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