Analog Digital Converter Electronics

The main read-out system which will be used is based on the read-out electronics developed for the ALICE experiment. Some modifications are necessary in order to adapt it to the expected output signals from the new micro pattern gas amplification systems. The schematic layout of the system is shown in Figure 1. In the following, the different aspects will be presented.

Figure 1: Schematic view of the front-end electronics and the read-out system.

Mechanics and Connectors

Since the effective area occupied by the front-end card (FEC) per channel is significantly larger than the smallest pad size to be investigated, which is 1x4mm², the FECs can not be attached directly to the pad plane. Instead, they will be mounted on an electronics crate, which is attached to the endplate, and will be connected via Kapton cables to the pad plane. The electronics crate, like the chamber itself, will rest on the rails, since the endplate won't be able to hold its weight. The high density connectors will be mounted directly on the back side of the pad plane.

In the large prototype, not all pads will be equipped with read-out electronics. However, it is reasonable to assume that it should be possible to reconstruct a high momentum track through the full diameter of the chamber. The starting point is that at least 2000 electronics channels are needed. Defining a "road" of 1x4mm² pads through the chamber would in the case of a 2000 channel system give a 19mm wide "road".

Front-End Electronics

The read-out electronics for the large prototype should offer a performance that is better than that of the final TPC, so that the compromises that may become necessary for the multi million channel final system can be studied and optimized. In order to meet these requirements the front end electronics will contain a programmable charge amplifier ASIC, called PCA16, and digitization up to 40MHz sampling rate by a modified ALTRO chip, as developed for the ALICE experiment.

The charge preamplifier (PCA16) is programmable in terms of rise time, gain, polarity, decay time. Test results of a non-programmable prototype preamplifier show that the performance of the chip is up to the specifications or better. Especially, the noise is much below the specifications and the dynamic range is significantly wider. The final, programmable PCA16 chip has recently been produced. In case of problems, a fall-back solution is foreseen which allows to set the parameters manually by jumpers/dip-switches. 200 PCA16 chips have been delivered, which satisfies the need for the EUDET project.

The ALTRO chip has been developed for the ALICE TPC to be operated at 10MHz with 10 bit resolution. It offers a large flexibility and can be used as a general purpose AD-converter for a multi-channel system.
The sampling frequency needed for the digitization depends on the characteristics of the pulse from the gas amplification system. In order to accurately reconstruct the pulse shape to extract the charge deposited on the pad, a few points on the rising edge of the pulse are needed. A modified version of the ALTRO chip with 40MHz sampling rate has been developed. With this sampling frequency, one can measure rise times down to around 50ns. Presently 125 chips of 40MHz sampling rate are available, which corresponds to 2000 channels in total. These are intended to be used for the EUDET project.
The internal memory of the chips can store up to 25μsec drift time at this sampling rate. The maximum drift time in the 60cm long prototype is 15μs assuming a drift velocity of 4cm/sec. Thus, the chip storage provides a sufficient margin for different gas choices (drift velocities).
Further, the ALTRO has a flexible sample by sample subtraction of pedestals accompanied by a powerful suppression of zero data. This will reduce the requirements on the data acquisition (DAQ) system.