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DAC38RF82EVM: DAC38RF82EVM questions

Kailyn Chen
Genius 9560 points
Part Number: DAC38RF82EVM
Other Parts Discussed in Thread: DAC38J84, TSW14J57EVM

Hello，

One customer met a question as following:

   【May I ask whether DAC38RF82EVM can only generate sine wave in general? I asked technical support before, and said that if we want to generate square wave, we need to dismantle the TCM3-452X-1+ transformer. In my opinion, the function of this transformer is a 2:1 impedance transformer, which is used for impedance matching of 50ω load, differential to single-ended conversion, and DC bias of DAC output. If it is removed, how can it be matched with the detector?

2. Where can I find the programming methods (such as .TSW files， software and programming examples) of this evaluation board?】

Best regards

Kailyn 

  • +1
    TI__Guru* 91947 points

    Kalyn,

    To match the output impedance one would typically use a resistor network as shown in the attached document, that would be placed between the DAC and an amplifier for a DC coupled application. I have also attached a schematic example of a board using a DAC38J84 connected to three amplifiers and one transformer. 

    The software and EVM User's Guide can be downloaded from the DAC38RF82EVM product folder on the TI website. In section 3.2 of the User's Guide, it describes how to save the register settings from the GUI. These .cfg files then can be used to load all DAC registers after power up if the the user does not want to enter all of the values using the GUI tabs. 

    The DAC38RF82EVM works with the TSW14J57EVM pattern generator. The TSW14J57EVM uses the HSDC Pro GUI software which uses the .TSW files for sending custom test data to the DAC. Examples of .TSW files can be found at the location shown in the attachment after HSDC Pro GUI has been installed on a PC.

    Regards,

    Jim

    Jim 5482.TSW3080EVM-SCH_E2_3_30_c.pdfDAC to Op Amp Part 1, Current-sinking DACs slyt342.pdf  

  • 0 in reply to jim s
    TI__Genius 9560 points

    Hi Jim,

    Thank you for your reply.

    The customer wants to use DAC38RF82EVM to generate square waveform, if he removed the transformer to realize it, there are two transformers on the demo board, T5 and T7, which one should remove? 

    In addition, if removed, does it need to add resistor network to match the termination resistor? Thanks

    Best regards

    Kailyn 

  • 0 in reply to Kailyn Chen
    TI__Guru* 91947 points

    Kailyn,

    CHB is not easily configurable for differential output as there is only one SMA available.

    To convert CHA, the user must do the following:

    Remove T7, L10, C398.

    Install L7 and L8 with proper pullup resistor values.  Install R248 and R241 with 0 Ohms.

    Replace C413 and C399 with 0 Ohm resistors.

    The attached document explains how to properly terminate the output when interfacing to an amplifier. 

    Regards,

    Jim 

    DAC38RF8x+LMH5401_interface.docx

  • 0 in reply to jim s
    TI__Genius 9560 points

    Hi  Jim,

    Thank you for your reply.

    I'd like to confirm again, according to the above steps, does it output square waveform? The customer wants to output square waveform.

    jim s said:
    Install L7 and L8 with proper pullup resistor values

    What value do you recommend? 

    Best regards

    Kailyn

  • 0 in reply to Kailyn Chen
    TI__Guru* 91947 points

    Kailyn,

    It depends on what the DAC is driving. What is the impedance of the load? Per the attached example I sent, the pullup value in parallel with the load impedance should be ~ 50 ohms. The full-scale output current the customer plans on using will also effect the pull-up resistor value used (see attached example I sent).  

    You will also need to change the bias voltage connected to these resistors. This will not be easy as it will require the +1.8V_VDDOUTA trace be removed from L7 and L8. One way is to install the new parts so that only one end is soldered to the board pad that is connected to IOUTAP/N and the the other end is connected to an external supply voltage. Per the example I sent, this would be 5V.

    Regards,

    Jim