Wednesday, June 22, 2016

Prj 146 - Dual Channel 40MSPS ADC (Part1)

With the F board (BeagleBone Black Spartan6 LX9 FPGA), I now have the option of using both faster ADCs as well as dual channel variants to support quadrature down converters.  This board uses a LTC2292 which is a dual channel 40MSPS 12 bit ADC.  It is similar to previous LTC ADCs I have used (FPGA variant and non-FPGA).  The schematic of the board is shown below.
Dual ADC Schematic - LTC2292, IF inputs, and sample clock.
Dual ADC Schematic - Voltage Regulators
There are a couple of variants and decisions on the board worth noting.  First, I configured it to use 0.5V full scale rather than 1V and configured the output to be signed 12 bit values.  Second, the analog inputs are similar to previous versions using a center tap transformer with 50 ohm input termination. This comes directly from the manufacture reference design and has worked well in the past.  Finally, a 40MHz CMOS oscillator is used but with a buffer to the FPGA on the carrier board. Separate regulators are used for the analog and digital supply voltages.  Everything is 3.3V and liberally supplied with bypass capacitors and chokes. The Kicad source material for the the board is available on github here. The board is a two layer OSHPark with the layout shown below.
Dual ADC Two Layer PCB Layout
A picture of the first unit assembled and mounted on an F board (mounted on a BBB) is shown below.
Dual 40MSPS ADC board.  Connectors at right are from the underlying F board it is mounted on.  Connectors on the left are from the underlying Beagle Bone Black which the F board is mounted on.
For testing purposes the anti-aliasing low pass filter elements were not populated.  This board is designed to work with a digital down converter VHDL image on a Spartan 6 LX9 (F board).  The construction of this board is straightforward.  The only difference is that with this unit I tried applying solder paste using 22gauge plastic needle with paste syringe.  In previous work I only had the default metal large needle that comes with the paste syringe itself.  Using the fine plastic needles make a huge difference, you can dispense fine amounts of paste and let the needle touch the pads without fear of the metal scraping.

Related:
Prj 145 - Beagle Bone Black Simple LX9 FPGA board (Part 1)
Prj 145 - BBB LX9 FPGA Board Design (Part 2)
Prj 145 - BBB LX9 FPGA Board Construction (Part 3)
Prj 145 - BBB LX9 JTAG Boundary Scan Utilities (Part 4)
Prj 145 - BBB LX9 C++ and VHDL (Part5)

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