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Original question:

DAC81416EVM: DAC61416

DAC81416EVM: DAC61416

Lalita Vattem
Prodigy 30 points

Part Number: DAC81416EVM

Hi,
As mentioned in my previous question, I verified that all the connections and the REFCOMP pin has a 330pF cap as mentioned in the schematic. At the C104 (REF cap) node voltage, I see a of ~2.5V after enabling the internal reference. Now, all the connections have been verified and they seem right to me. 

The strange problem is, my DAC is not turning on. It just shows a current of 0.002 A for 15V and -15 V as Vcc and Vss and does not show anything at the channel outputs. The SPI signals are visible on the oscilloscope. When I try to probe CLK pin with my oscilloscope and also hold the back side of my PCB with a finger (which is directly beneath the diode-capacitor circuit that converts 5 V to 3.3 V to provide as Vio to the DAC), my DAC is switching on, shows a current draw of 0.007 mA and also updated the channels. If I let go of my hand from the PCB or remove the oscilloscope cable from CLK pin, it again stops working. This issue certainly does not feel like a SPI timing issue, rather, a design flaw in my circuit. Please let me know how this can be corrected. I am happy to answer any more questions that you might have regarding this.


  • 0
    TI__Guru 53573 points

    Hi Lalita,

    This could be an assembly issue of the PCB, or a timing issue with the SPI.  Have you confirm that your SPI timing and edge relationships match the datasheet requirements? The SCLK phase and polarity are commonly incorrectly configured, and sometimes adding capacitance is enough to slow the edge to allow some functional communication.  

    Do you have a schematic to share? have you tested this on more than one PCB? Do you see any cold solder joints?

  • 0 in reply to Paul_Frost
    Prodigy 30 points

    Hi,
    Here is a schematic of the DAC. according to these screenshots of the SPI signals, do you think there is a timing issue? I did a bit of reading and think Mode 1 is the right configuration here with CPOL = 0 and CPHA = 1. It works with the evaluation board too. I have shared the DAC schematic here. I do not see any cold solder joints and have only tested this on two PCBs and these problems have been persistent with my hardware.

         


  • 0 in reply to Lalita Vattem
    TI__Expert 3530 points

    Hi Lalita,

     
    Are you using active or passive probe for SPI signal monitoring on oscilloscope? What is the spec of the probe like BW, input resistance and capacitance?
    Do you have any option to connect resistor in the path of SPI lines? 

     
    Thanks,

    Sanjay
     

  • 0 in reply to Sanjay
    Prodigy 30 points

    I am using an analog Discovery 2 digital oscilloscope.

    For a brief period,  I used a regular oscilloscope (you can see the details you asked for, on the oscilloscope) which also required me to probe the CLK line to make it work. The SPI MISO waveform was also a bit weird. I have attached a picture of it here. It somehow is not going down to 0V and has this strange waveform.




    My PCB looks like this and I think it is difficult to add a resistor to the SPI lines.

  • 0 in reply to Lalita Vattem
    TI__Expert 3530 points

    Hi Lalita,

     
    This is what might be happing -
     
    "SPI lines is ringing a lot due to improper layout and hence not so good ground return paths, this might be causing timing violations. You are adding extra parasitic capacitor when you are touching the PCB or probing the SPI digital lines with oscilloscope probe, and this will help to reduce the ringing"


    So it would be good if you can provide the typical spec of the probe (not the oscilloscope) like BW, input resistance and capacitance?
     
    And is it possible to bend/cut the SPI line connector pins and solder resistors in the path?

     

    It would be great if we can connect over a webex meeting and i can check it live and help to debug the issue.

     

    Thanks,

    Sanjay 

  • 0 in reply to Sanjay
    Prodigy 30 points

    Hi,

    The probe has a BW: 6MHz, input resistance: 1 MOhm, Capacitance: 95pF

  • +1 in reply to Lalita Vattem
    TI__Mastermind 34058 points

    Hi Lalita,

    Are you available for a live debug session with Sanjay? If so, Sanjay can contact you via the email associated with your ti.com account and set up a meeting. 

    Best,

    Katlynne Jones

  • 0 in reply to Katlynne Jones
    Prodigy 30 points

    Thank you Sanjay, for the debug session. It helped me find out the issue with my PCB. 
    Kindly share the reference links that you mentioned in the session with me.

    BR,
    Lalita