Multichannel Analog to Digital Converter and Readout System for the Time Projection Chamber

Abstract

The Time Projection Chamber (TPC) project involves the characterization of signals arriving at approximately 20,000 separate detector wires and pad electrodes. The characterization requires measurement of the signal amplitudes in 100 ns time slices over a total time of 20 μs (the total chamber drift time). Charge Coupled Devices (CCD's) are used to store an image of the 20 μs span of signals and the image is "played back" at a low rate (50 μs time slices, i.e., a time expansion of 500:1). Digitization of the 20,000 parallel signals at the output of the CCD's must therefore be accomplished in 50 μs and readout of digital data into the computer system must also be accomplished in this time. Many zeros occur in the data, a fact which reduces the data handling demands on the readout system, but does not reduce the stringent requirements on the digitizer. Therefore, the requirements on the digitizer are highlighted by the need for low cost, low power consumption and by the need to perform 9-bit digitizing in less than 50 μs. Furthermore, a double buffering scheme must be used to permit interleaving digitizing and digital readout cycles. We describe our implementation of this concept and some possible applications for other experiments which require many Analog to Digital Converter (ADC) channels.