Tuning refers the setup steps required to accurately calibrate an FE-I4 module. The tuning of FE-I4 modules is done in the same way in both USBpix and RCE systems, with the exception that the RCE can process multiple numbers of FE-I4 chips simultaneously, depending on system configuration.
Figure 9.11: Illustration of the RCE system configured to readout ATLAS FE-I4 pixel mod- ules.
To compare the response of different sensors, the behaviour of the attached FE-I4 readout chip has to be accounted for. The calibration of the front-end has to be well understood in order to define the threshold of the discriminator as well as the relationship between injected signals charge to ToT. In the following, some of the standard scans [169, 170], which are included in both the USBpix and RCE systems, are described.
Analogue and Digital Test: Via the external charge injection circuit, a defined charge can be injected several times directly into the discriminator (Digital Test) or the amplifier (Analogue Test) of each pixel. These tests show whether the analogue and the digital parts of a pixel cell work as expected, comparing the injected charge with the output. If the readout pixel is fully functional, it should read back the same number of hits as were injected. Figure 9.12a and Figure 9.12b shows results from analogue and digital tests respectively, performed with the ATLAS RCE system. In each case 50 charges were injected. Any variation in the number of hits read out indicates a none uniform pixel response.
GDAC Tune: The purpose of this tuning is to set the global discriminator threshold and feedback current values for the whole FE-I4 chip, to the value specified by the user. An algorithm uses an iterative process to automatically vary many parameters, while
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OCCUPANCY Mod 25 at A1-1
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Figure 9.12: Results for (a) analogue test and (b) digital test for every pixel on an FE-I4b readout chip. In both cases 50 charges are injected and read back using the RCE system
injecting and reading back all the pixel thresholds and comparing them to the user defined target. The parameters that provided the best outcome are then stored.
TDAC Tune: The purpose of this tuning is to fine tune the threshold for each pixel. This is done in the same way as the GDAC tune, but uses smaller steps and individually tims each pixels operating voltage.
FDAC Tune: The purpose of this tuning is to adjust the in-pixel ToT response to a certain charge for every pixel i.e. ToT = 10 at a charge of 20 ke. It is done is a similar way to the GDAC and TDAC tunes but instead focuses on calibrating the ToT to charge for each pixel.
TDAC and FDAC tunes are performed several times (>2 each), in an iterative way. This allows the system to find a compromise between the best parameters from each type of tuning.
Threshold Scan: This scan is a precision test that determines the precise discriminator threshold in each pixel without altering any parameters in the configuration of the FE chip. A given charge is injected into the analogue part of the FE multiple times. This procedure is repeated with different charges. The ideal outcome would be a step function with no hits if the injected charge is below threshold and if above threshold the number of charge injections is equal to the number of hits. However, due to noise effects, charge values close to the threshold will sometimes cause a hit and sometimes not. Figure 9.13 shows results from a threshold scan with the RCE system. These histograms allow for the determination of module tune quality. The quality of a tune is specified by the user.
However, important features include a threshold with a narrow distribution around the target value with low noise (also with a narrow spread). Figure 9.13b and Figure 9.13d show some good quality tune results. Some noisy pixels/channels can be seen and would be masked out before operation. Any noise channels are then masked out
ToT Scan: This scan injects a known amount of charge, and reads back the ToT response for each pixel. Typical output histograms are given in Figure 9.14.
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Thresholds Module 25 at A1-1
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(b) (c)
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Figure 9.13: Results from a threshold scan of a tuned FE-I4b module. (a) 2D threshold
histogram showing each pixels tuned threshold. (b) shows every channels
threshold, indicating noisy pixels. (c) shows the spread of the threshold of every pixel with a Gaussian fit performed to determine the average module threshold. (d) the noise of every channel and (e) the spread of noise for every pixel with a
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Figure 9.14: (a) 2D colour map indicating each pixel’s Tuned ToT (Time over Threshold). (b) shows every channel’s tuned ToT indicating channels with drastically different values and (c) shows the spread of tuned ToT values with a Gaussian fit performed to determine the average module ToT value.