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Using DAC8560 with OPAMP

Mohanmurali M
Prodigy 220 points

Other Parts Discussed in Thread: DAC8560, LM358

Hi,

We are following the reference design "Bipolar +/-10V Analog Output from a Unipolar Voltage

Output DAC" from TI for using DAC8560 with OPAMP. In our circuit the following changes we have done

1. We have used LM358 OPAMP 

2. We are using OPAMP in Non-inverting amplifier mode since we need unipolar output in the range of 0 - 8 V , DC.

3.In the schematic OPAMP circuit is shown in summing amplifier mode but in the board resistor R14 is not populated. 

During the testing we observed that when we connect this circuit to DC drive (input : 0 - 8 V DC, Output : 0-180 V DC) the 5 amps fuse in the DC drive gets burned. I herewith attached our schematic diagram for your reference.

Please provide a suitable solution.

 

  • 0
    TI__Mastermind 46290 points
    Mohanmurali,

    It would be easiest for us to look into this issue if you could provide the schematic for this design. Additionally, more information about exactly what the circumstance that lead up to damage are would be extremely useful - if you can acquire an oscilloscope capture of the DAC output and amplifier output during the damage event that would be perfect.
  • 0 in reply to Kevin Duke
    Prodigy 220 points

    Dear Kevin,

    Thanks for your response. 

    As you requested I herewith attached the schematic of our design. Regarding the circumstances please find below more details.

    1. We have designed a board which will provide 0-8V DC to a DC Drive (purchased from another vendor). In our board we have used  DAC8560 and LM358 OPAMP to provide 0-8V DC.

    2. We connected our board to DC Drive. When we configure the DAC via micro controller for 0V  we have not face any issue, But when we configure DAC for more than 0V the OPAMP and DAC in our board gets burned and one potentiometer in the DC Drive gets burned. Also 5A line fuse in DC Drive gets burned sometimes.

    3. In our schematic OPAMP circuit is mention as summing amplifier but we are using OPAMP as non-inverting amplifier by removing resistor R14.

    4. The burning situation is happening very quickly hence it is very difficult to capture waveform in oscilloscope.   

    drive system_Schematic_22-09-2015.pdf

       

  • 0 in reply to Mohanmurali M
    Prodigy 220 points
    Dear Kevin,

    As per your request yesterday we have provided necessary details. Please suggest a suitable solution.
  • 0 in reply to Mohanmurali M
    TI__Mastermind 46290 points

    Mohanmurali,

    I don't see any issues with your schematic, except that I cannot find the second channel of the LM358 so I'm not sure how the other channel is being used in the design to understand if that's maybe the root cause of some of the heat. If you're not using the other channel you should simply configure it as a voltage follower and ground the input to keep the amplifier in a known state.

    Have you done any tests on the design without the amplifier output connected to the DC Drive board? Do you see the same problem when testing the DAC / Amplifier without it connected? I would suggest you try to isolate each component: see what happens with just your board and no DC drive board, see what happens if the op-amp is removed and you simply exercise the DAC by itself, and see what happens if the DAC is removed and you simply apply an input voltage to the amplifier. This should give some clues for where the issue is. 

  • 0 in reply to Kevin Duke
    Prodigy 220 points
    Hi Kevin,

    Regarding second channel of LM358, we have not used the second channel. As per your suggestion, we will configure it as a voltage follower and update you the results.

    Regarding carrying out testing without connecting amplifier output to DC drive, already we have done different tests and find below our observations.

    1. We have not connected OPAMP and controlled DAC via uC and it is working fine. We are getting expected result at DAC output.

    2. OPAMP and DAC are connected. In this situation also it is working fine and we are getting expected result at OPAMP output.

    3. We have not connected DAC and provided input to OPAMP through potentiometer and it is working fine.

    Also we observed two more situations and find below the details.

    1. We have not connected DAC, uC and provided input to OPAMP through potentiometer and connected our board to DC drive. In this situation there is no issue and we are getting output from DC driver based on the variable voltage from potentiometer.

    2. We are facing issue when DAC, uC and OPAMP are present in our board and when it is connected to DC drive and driving DAC more than 0
    voltage.

    Any ground isolation to be done? Please provide suitable suggestions.

    Thanks,
    Mohanmurali
  • 0 in reply to Mohanmurali M
    TI__Mastermind 46290 points

    Mohanmurali,

    Based on the information you've provided, it doesn't sound like there is any problem with the PCB housing the DAC, uC, and amplifier. I don't have enough information about the "DC drive" board / solution to understand how a problem is being induced when your design is being connected to it.

    Ground isolation sounds like it could be a possibility, but again I'm not sure how your board and the DC drive are being powered. Do you have any reason to suspect a ground issue?