LWA352 Digitizers: Design Concept

Abstract

The first stage of digital signal processing for LWA352 will use SNAP2 FPGA boards [1]. Each SNAP2 is expected to be capable of handling at least 64 LWA signals sampled at about 200 MHz, but it is a challenge to get that many digitized signals into the board. Our design will achieve this by using both of the SNAP2's FPGA Mezzanine Card high-pin-count (FMC-HPC) connectors to accommodate daughter cards carrying the analog-to-digital converters. It is the design of those Digitizer Boards that we discuss here.. II. CONSTRAINTS Analog signal processing for LWA352 will be accomplished in a separate rack from the digital processing, and each signal will be connected to the digital rack on a coaxial cable. Available ADC integrated circuits for our sampling rate have differential inputs, so baluns is required on the Digitizer Boards. A survey of available ADC ICs identified only two devices having at least 4 channels and at least 8 bits of resolution at 200 MHz sampling: the Analog Devices HMCAD1511 and the Texas Instruments ADS5296A. The HMCAD1511 has been used in some Casper ADC boards [2], including those of the first- and second-phase implementations of LWA-OVRO. We have selected the ADS5296A because it claims better isolation among channels, has finer resolution (10b), and has additional useful features (including synchonization among devices and register readback). The FMC interface is a VITA/ANSI industry standard [3] that specifies the connectors in some detail, including functional assignments of pins [4]. The SNAP2 closely conforms to the standard pinout, although some pins are unconnected. The standard also specifies the form factor and other details of the daughter ("mezzanine") cards [5], making them rather small; all are 69 mm wide and the longest is 84 mm. This is not enough space for a large number of coax connectors, balun transformers, and ADC ICs. To get 64 signals into the SNAP2, we need 32 per FMC connector, requiring 8 of the ADS5296A quad-ADC devices. We will use a nonstandard form factor, but the width is still constrained by the spacing of the connectors on the SNAP2 board (Figure 1) to a maximum of 76 mm. The SNAP2 is designed to fit a standard FMC card that is 76.5 mm long, but ours will have to be longer, so they will extend over the front edge of the SNAP2.

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