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DAC38J84 questions

Iouri Markin
Intellectual 590 points
Other Parts Discussed in Thread: DAC38J84

Hello,

I am using DAC38J84 and few questions came up:

1. I am setting up NCO @ 100MHZ setting max gain reg 0x0C = 7FF , ( I am reffering to channel A) QMC is enabled output of the DAC terminated to 50R to GND and transformer. When I connect scope I see my signal around 100mV pk-pk @ 100MHZ. But according to the data sheet typical output current is 20mA I would expect signal to be around 1V. If there are any settings what I should have a look or can you please provide some advise how to increase amplitude of the output signal?

2. My digital section is not ready yet to drive JESD of the DAC, does DAC38J84 has an ability to generate some kind of test signal which can be probe at the output? I can control NCO and that's pretty match what I can see at the DAC output. Please advise.

Thank you,

  • 0
    TI__Guru 66114 points

    Iouri,

    1. The level depends on your final load termination and transformer ratio. You can calculate the theoretical through methods described in the app note below:

    You may also access the TINA simulation files through the link provided on the first page. Click on TINA files.

    2. The constant input with NCO is about the only test pattern there is. You can always program the constant code via SPI to generate a ramp like signal. Keep in mind that the output has to be able to pass DC otherwise the DC value will not settle after every step. 

    -Kang

  • 0 in reply to Kang Hsia
    Intellectual 590 points

    Hi Kang,

    Thank you for your replay. Just to clarify: if I dont have input signal ie my DAC output is driven by NCO, what output current level I can expect?
    I understand that if I have input signal from 0 to 65535 than output will be full swing 0 - 20 mA.


    Thank you!

  • 0 in reply to Iouri Markin
    TI__Guru 66114 points

    Hello,

    Differentially, you would still expect 20mA swing assuming that you have full-scale code such as 0xffff and 0x0000 in offset binary mode. The NCO has 0dB and 6dB of gain depending on the setting. The detail is in the datasheet. For CW with 6dB of gain, you would get 20mA swing.

    The amplitude decreases when constant input code decreases.


    -Kang