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DAC7512: Dead DAC

Jack Summers11
Intellectual 420 points

Part Number: DAC7512
Other Parts Discussed in Thread: LP2985

Hi, I am having intermitent trouble with a DAC circuit and would appreciate a little help.
I am trying to generate a voltage signal between +5 and gnd using a DAC7512.  I feed this signal to the non-inverting pin of an op amp and there is nothing else on the output wire.  A week or two ago, I assembled a circuit and it tested out fine.  I went back to do some more testing this morning and I am not getting any signal out of the DAC.  The output voltage from the DAC does not vary from zero.  I am signalling the DAC using a TM4C123 Launchpad dev board that uses 3.3 volt logic.  Inputs to the DAC from the MCU look fine on my scope.  I  am wondering if my +5 volt power may be an issue.  Is there any problem with using 3.3 volt logic and 5 volt VDD?  I generate the +5 volts from +12 volt power supply using a LP2985 LDO with the cap values described in the LDO data sheet.  Since the DAC was fine before and seems dead now, I am wondering if a voltage spike to the DAC might have this effect.  The DAC is the only thing on the +5 volt circuit but I have other things going on up stream of the LDO in my power supply circuit.  The LDO seems to be working fine. Things upstream of the LDO include a buck regulator and a sizable inductor to generate -12 v.  
I am a chemist by training and profession and I am trying to teach myself electronics in my free time.  The upshot is that I may not know some of the basics of analog electronics.

Thanks in advance for your help.
Jack

  • 0
    TI__Guru 56833 points

    Hi Jack,

    I generally start my debugging with the basics:

    1. Check the supplies and ground.  You should verify the voltage on VDD is stable and continuous.  You should also verify that your device is correctly grounded and that the grounds of your power supply are also correctly connected.  

    2. You are using 3.3V logic, and the device is rated for VIH level of 2.4V using a 5V supply, so you should be good on that front.  You should verify that your edge transitions are clean and the relationship of the edges are valid per the "SERIAL WRITE OPERATION" figure on the datasheet.

    3. You should confirm that the data looks correct as configured on your MCU.  Remember that there are a few bits in the command that set the power down mode of the device.  Are you powering it down?

    Is this on a breadboard or a PCB? If it is on a breadboard, then you should verify every connection is good.

    Thanks,

    Paul

  • 0 in reply to Paul_Frost
    Intellectual 420 points

    Hi Paul,

    Thanks for your response. You gave me some good things to think about. 

    To address some of your questions, I hooked the board up to the scope again and found a couple of problems:  First, I was set up to read on the rising edge instead of the falling edge as required (oops).  I had hoped that fixing that would fix my problem but it did not.  I think it is likely that I put the DAC in power-down mode at some point by having the data line high when it was supposed to be low.  I do not see in the data sheet where the DAC requires a specific signal to exit power-down mode and I am assuming that it should exit power-down when a new command is issued.  Please correct me if I am wrong.

    Anyway, I now think my issue is with problem 2:  I do not appear to be getting the necessary voltage out of the CS and SCK pins. The figure below shows a photo of a scope trace with the sensitivity of the three channels set to 1 volt per division.  As you can see, the high voltage on channel 1 (SCK) is only about 2V and that of channel 3 (CS) looks like about 2.2.  The MOSI channel looks ok.  As you said, the data sheet for the DAC says that signals should be 2.4 volts.

    To see whether something in my pcb was affecting signal voltages from the microcontroller, I disconnected my board (pcb connected through headers, not breadboard) and ran another scope trace.  This did not cause an increase in voltage on these pins on the microcontroller board.

    So, at this point, it looks like the problem lies with the microcontroller not the DAC.  I guess that I could fix this with a couple of amplifiers, but that means another pcb, purchasing more hardware, and a few more hours with the  soldering iron.  Any other suggested fixes for this? 

    Also, is there some way to shift this thread over to the microcontrollers forum?

    Thanks,

    Jack

     

  • 0 in reply to Paul_Frost
    Intellectual 420 points

    Hi Paul,

    Thanks for your response. You gave me some good things to think about. 

    To address some of your questions, I hooked the board up to the scope again and found a couple of problems:  First, I was set up to read on the rising edge instead of the falling edge as required (oops).  I had hoped that fixing that would fix my problem but it did not.  I think it is likely that I put the DAC in power-down mode at some point by having the data line high when it was supposed to be low.  I do not see in the data sheet where the DAC requires a specific signal to exit power-down mode and I am assuming that it should exit power-down when a new command is issued.  Please correct me if I am wrong.

    Anyway, I now think my issue is with problem 2:  I do not appear to be getting the necessary voltage out of the CS and SCK pins. The figure below shows a photo of a scope trace with the sensitivity of the three channels set to 1 volt per division.  As you can see, the high voltage on channel 1 (SCK) is only about 2V and that of channel 3 (CS) looks like about 2.2.  The MOSI channel looks ok.  As you said, the data sheet for the DAC says that signals should be 2.4 volts.

    To see whether something in my pcb was affecting signal voltages from the microcontroller, I disconnected my board (pcb connected through headers, not breadboard) and ran another scope trace.  This did not cause an increase in voltage on these pins on the microcontroller board.

    So, at this point, it looks like the problem lies with the microcontroller not the DAC.  I guess that I could fix this with a couple of amplifiers, but that means another pcb, purchasing more hardware, and a few more hours with the  soldering iron.  Any other suggested fixes for this? 

    Also, is there some way to shift this thread over to the microcontrollers forum?

    Thanks,

    Jack