AD5781 Pmod Xilinx FPGA Reference Design

The AD5781 is a single 18-bit, unbuffered voltage output DAC that operates from a bipolar supply of up to 33 V. The AD5781 accepts a positive reference input range of 5 V to VDD − 2.5 V and a negative reference input range of VSS + 2.5 V to 0 V. The AD5781 offers a relative accuracy specification of ±0.5 LSB maximum, and operation is guaranteed monotonic with a ±0.5 LSB DNL maximum specification.

HW Platform(s):

• Spartan-6 LX9 Microboard (Avnet)
• Nexys™3 Spartan-6 FPGA Board (Digilent)
• PmodDA5 (Digilent)
  

System: Microblaze, AXI, UART

The bit file provided in the project *.zip file combines the FPGA bit file and the SDK elf files. It may be used for a quick check on the system. All you need is the hardware and a PC running a UART terminal and the programmer (IMPACT).

• Spartan-6 LX9 Microboard (Avnet)
• Nexys™3 Spartan-6 FPGA Board (Digilent)
• PmodDA5 (Digilent)

• Xilinx ISE 14.4 (Programmer (IMPACT) is sufficient for the demo and is available on Webpack).
• A UART terminal (Tera Term/Hyperterminal), Baud rate 115200 for the Avnet LX-9 Microboard or 9600 for the Digilent Nexys™3 Board.

Extract the project from the archive file (AD5781_<board_name>.zip) to the location you desire.

To begin, connect the PmodDA5 to J4 connector of LX9 board, pins 1 to 6 (see image below). You can use an extension cable for ease of use. Connect the USB cable from the PC to the USB-UART female connector of the board for the UART terminal. The board will be programmed through its USB male connector.

To begin, connect the PmodDA5 to JB connector of Nexys™3 board, pins JB1 to JB6 (see image below). You can use an extension cable for ease of use. Connect the USB cables from the PC to the board, one for programming (Digilent USB device) and one for the UART terminal (FT232R USB UART).

Start IMPACT, and double click “Boundary Scan”. Right click and select Initialize Chain. The program should recognize the Spartan 6 device (see screenshot below). Connect an oscilloscope to the output of PmodDA5 and then program the device using the bit file provided in the project *.zip archive, located in the “sw” folder (../ad5781/sw/AD5781.bit).

If programming was successful, the Main Menu will apear in your UART terminal, as seen in the picture below. There are 2 options:

• Press [f] to select Fixed Value Mode.
• Press [w] to select Waveform Generation Mode.
• Press [p] to select Programmable Ramp Signal Generator.

Fixed Value Mode allows entering a value between 0x00000 and 0x3FFFF, value that will be programmed in the DAC. If the number of input characters is less than 5 (e.g. fff or 768), the [Enter] key must be pressed in order to validate the input. If 5 characters are input, the value is automatically validated (in order to prevent entering more than 5 characters).

Pressing the [q] key at any time exits the Fixed Value (or Waveform Generation) Mode and displays the Main Menu again.

Waveform Generation Mode allows selecting between 4 types of waveforms: Square, Triangle, Sawtooth and Sine waveforms. Changing between the 4 is done by pressing [1] to [4] on the keyboard. Pressing [q] at any time will return to the Main Menu.

Run Ramp Signal Generator allows generating a programmable ramp signal. This mode can be used to test the PmodDA5 using a Digital Multimeter.

Enter time step allows setting a time step between 100 and 5000 ms.

Enter increment size allows selecting an increment size that suits your design. Values can vary from 0x00000 to 0x3FFFF.

Run Ramp Signal Generator will start generating the desired output.

The reference design is a simple SPI interface used to communicate with the PmodDA5. The software programs the device sending 16-bit data values read from the keyboard input or from predefined look up tables. The user has the ability to select between two modes: a Fixed Value Mode or a Waveform Generation Mode. Communication between the user and the board is done via UART.

    Pin 15P = +12V
    Pin GNDA = GND
    Pin 15N = -12V
    

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