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DAC8734EVM: Incorrect Wiring or Broken DAC

Braden Huffman
Prodigy 50 points
Part Number: DAC8734EVM
Other Parts Discussed in Thread: DAC8734

I recently followed the Breadboard's tutorial/review for this DAC on youtube (https://www.youtube.com/watch?v=uYl_jqJeGtA), and found it to be fairly comprehensive and straight forward; However, the DAC does not seem to be working properly. We have successfully used the code before to test one of these DACs in the past.

I am testing what is described in the next paragraph with the Teensy connected to a breadboard, using jumpers as connections.

The micro controller in use is a teensy 3.6, and the respective MOSI, MISO, CK and CS pins are connected to the pins on the DAC, as per that tutorial. The 3v3 pin is connected tho the 3v3 pin on the teensy, and the 5 V pin is connected to a power supply which is supplying 5 V (and it says its drawing .01 A of current). A common ground is being used between the power supplies, analog GND and digital GND (I tried separating analog and digital, but it yielded the same result). The two voltages on the pins requiring a positive and negative range are ~+- 16.58 V. These are also hooked up correctly and fall withing the range specified in the user guide.

I have two DAC8734EVM, and I believe that at least one of them is broken. They are acting differently in response to the commands being sent to the DAC. One of them is producing no output, while the other produces spikes that go quickly back to 0 V as seen below.

It spikes whenever a significant change is given. For example shifting between the 0th position and the 20,000th position of the DAC. It is not consistent. Sometimes when switching to 20,000 from 0 it will yield negative spikes, and sometimes it will yield positive spikes. The same can be said from switching to 0 from any other significantly greater number. I would expect to get an analog voltage from the DAC instead of the spike. The jumpers shorting the pins are as they came in the box for both DACs. Would it seem to you like the DAC is broken, or does it sound like I did something wrong in the wiring? If the DACs might be broken, is this an issue likely to have been caused by something I did earlier?

  • 0
    TI__Guru 53623 points

    Hi Braden,

    Can you confirm the current that is being consumed by the device from all the power supply rails? If they are current limited, it could be causing a reset in the device.

    Otherwise I would still expect this to be a wiring issue.  Can you draw a figure of the wiring you are using? This might also help sanity check the design.

    Thanks,

    Paul

  • 0 in reply to Paul_Frost
    Prodigy 50 points

    Here is a drawing of all of the wiring I did. The DAC is shown with the male headers up. In case it is hard to read the MOSI is conected on the bottom in the location where the MISO is connected on top.

    About 64 mA of current is drawn from the -16.5 and about 16 is from the +16.5 supply. 72 uA is being drawn from the 5V supply. 2.7 mA is being drawn from the teensy's 3.3 V pin

  • 0 in reply to Braden Huffman
    TI__Guru 53623 points

    Can you detail the position of all jumpers and switches as well? Are the grounds all tied together? I see that you differentiate between AGND and DGND.

  • 0 in reply to Paul_Frost
    Prodigy 50 points

    All the grounds are currently tied together, I just labeled them differently in the drawing. Here are the jumpers and switches.

  • 0 in reply to Braden Huffman
    TI__Guru 53623 points

    Hi Braden,

    Looks like the jumpers are correct.  I usually approach debugging boards as three steps:

    1. Confirm all wiring - is there something you overlooked in the wiring? As this is a complex breadboard setup, it like very possible that something is mis-configured.

    2. Check the supplies - is there something wrong or mis-connected in the supplies?

    3. Confirm my digital - using a scope or digital analyzer, is the SPI frame exactly what I am expecting?

    I think we just have a case of something wrong in the setup.  Most boards are pretty robust and are hard to damage - but is still a possibility!

    I think your next step is to look at the digital with a scope.

    Thanks,

    Paul

  • 0 in reply to Paul_Frost
    Prodigy 50 points

    So it doesn't appear that the SPI is outputting as I would expect it to. The MOSI is always high, and the CLK is always low

  • 0 in reply to Braden Huffman
    TI__Guru 53623 points

    Hi Braden,

    I suspect that this is the main problem you need to solve.

  • 0 in reply to Paul_Frost
    Prodigy 50 points

    I'm a student doing research and haven't dealt much with SPI, but I talked to the engineer who is over our shop and he said that it is most likely caused by a lack of MISO feedback from the board to the micro controller. Would this signify a broken board?

  • 0 in reply to Braden Huffman
    TI__Guru 53623 points

    The DAC8734 does not require it's MISO to be monitored.  Most systems with SPI do not require any duplex communication, as opposed to something like I2C.  You SPI source should still output a CLK and MOSI data even with the DAC8734 disconnected.  You should start debugging that.

    Thanks,

    Paul