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DAC80504EVM: DAC80504EVM - Unipolar to bipolar signal conversion

Dejana Vukovic
Intellectual 845 points

Part Number: DAC80504EVM

Hello everyone,

I am using evaluation board DAC80504EVM in order to generate ultrasound signals which to drive three ultrasonic transducers. I would like to generate bipolar signals. However this DAC supports only unipolar signal generation. Therefore, I have found this tutorial http://www.ti.com/lit/an/slaa869/slaa869.pdf for unipolar to bipolar conversion.

I am interested to know if the values for resistors as well as the operational amplifier in the given example going to work with this particular DAC?

I would very much appreciate it if somebody could suggest me the alteration of circuit, if there is the need for it.

Link to board: http://www.ti.com/tool/DAC80504EVM?jktype=tools_software

Link to ultrasonic transducer: http://www.farnell.com/datasheets/1760000.pdf?_ga=2.260115981.1881583463.1580664031-526884284.1562916714

I will appreciate any comment, suggestion or help.

Thanks in advance!

Kind regards,

Dejana

  • 0
    TI__Mastermind 19086 points

    Hi Dejana,

    You can use the circuit mentioned in the attached link for generating bipolar output signals from Unipolar DAC.

    What is the frequency of signal you are generating?

    For the output Op amp key specifications you should look for as follows:

    Op amp with low offset voltage, low bioas current and low offset voltage drift should be used., Having said that good choice for your application depends on what kind of signal you are generating. The verified design link is below.

    https://www.ti.com/tool/TIPD125

    Bipolar +/-10V from a unipolar DAC for industrial voltage drivers. You may need to tweak resistor values for your application.

    Regards,

    AK

  • 0 in reply to Akhilesh K
    Intellectual 845 points

    Hi Akhilesh,

    Thank you for your quick response.

    I want to generate chirp signals of frequency 300 kHz and duration 5 ms. Do you now have a better suggestion regarding opamp?

    Could you suggest me how should I decide the value for resistors? If I need to provide you with more information, let me know.

    Kind regards,

    Dejana

  • 0 in reply to Dejana Vukovic
    TI__Mastermind 19086 points

    Hi,

    I need following info.

    What is the unipolar output range from DAC ? Whats the bandwidth of chirp signal?

    Requirements on bipolar outputs  (lets say +/-5V, +/-10V)

    With Regards,

    AK

  • 0 in reply to Akhilesh K
    Intellectual 845 points

    Hi Akhilesh,

    Once again, I appreciate your fast response.

    According to the http://www.ti.com/lit/ug/slau735a/slau735a.pdf user guide, maximal range for DAC output voltage in case when gain value is 2 is 5V (0-5V).

    Bandwidth of chirp signal is 30kHz.

    Transmitted ultrasound signal will be received with an hydrophone(maximal output voltage 5Vpp) and then converted with ADC. Bipolar input voltage of ADC is +/-5V, but considering that transmitted ultrasound will go through different mediums and therefore is going to be highly attenuated, maybe it would be good for bipolar outputs of DAC to be +/-10V. What do you think?

    Thank you for your big help.

    Kind regards,

    Dejana Vukovic

  • 0 in reply to Dejana Vukovic
    TI__Mastermind 19086 points

    Hi,

    DAC output voltage of +/-10V will be sufficient for your application.

    Lets take DAC output as 0-5V and bipolar output will be +/-10V. You can refer to the TIPD design which I shared and do the design accordingly.

    Let me know if you need any help in getting those resistor values calculation (straight forward equations from the design I shared)

    Regards,

    AK

  • 0 in reply to Akhilesh K
    Intellectual 845 points

    Hi Akhilesh,

    I have been looking into the TIPD design guide that you have suggested me. It is said that for the DAC8560 the choice of Rg2 = 8.25kOhm will result in peak current draw from reference source to be around 333uA, which is under 20mA limit for this DAC.

    If I am not mistaken, for DAC8054 the previously mentioned limit is 5mA(333uA<5mA). Therefore, shouldn't Rg2 = 8.25kOhm also be appropriate for this DAC?

     Additionally, how should I choose Ccomp, Cload and Riso?

    Thank you once again.

    Kind regards,

    Dejana

  • 0 in reply to Dejana Vukovic
    TI__Mastermind 46290 points

    Dejana,

    Akhilesh is out of office on personal time bank until next week, so I will step in for the time being.

    Selecting the resistor values for the gain stage of this circuit is somewhat trivial outside of the two comments mentioned in the document concerning the reference drive capability and noise goals for the overall design. The DAC80504 will be able to drive the 8.25kOhm value just fine, and if you are happy with the noise performance (or just do not have stringent noise goals) then I think you are good to go.

    Concerning CCOMP, CLOAD, and RISO - there was a sort of gimmick element to this reference design where we called it a "universal cable drive solution", meaning it could drive nearly any cable and/or cable length and the parasitic capacitance associated with that cabling would not create a stability issue.

    CLOAD was chosen to be larger than any conventional cable capacitance and therefore dominate the stability response. From there, RISO and CCOMP were chosen to provide stability compensation to the amplifier.

    The reference design wasn't intended to focus on stability compensation. You can find more about the techniques used to stabilize amplifiers in the video series below:

    https://training.ti.com/node/1138805

  • 0 in reply to Kevin Duke
    Intellectual 845 points

    Hi Kevin,

    Thank you very much for your help!

    Kind regards,

    Dejana