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Original question:

DAC3283 3rd harmonic issue

DAC3283: DAC3283 output attenuation

user1818781
Prodigy 70 points
Part Number: DAC3283

Hi

I use DAC3283 in my project by set the dacclk @ 491.52MHz, and the data rate is 245.76MHz。

When I set full scale code of the output signal,and the rate at 1MSPS,2MSPS,3MPS,....,20MSPS, and the signal attenuation a lot.

I have read the datasheet the lowpass filter is effect this, and I can not find what's wrong about device, this  makes me confused.

Some help and Thanks.

  • 0
    TI__Guru 66014 points

    Hi,

    The output full-scale current of the DAC3283 is always 20mA full-scale. The output interface from the DAC differential output to the single-ended 50ohm is through the ADT Mini-circuits transformer (Default on the EVM is 4:1 ratio transformer) with a typical high pass cut-off of 20MHz. This means the transformer will not respond to any signal excitation (or not as well) on signals with frequencies less than 20MHz. 

    If you want to have output frequencies below 20MHz, you will need to find a different transformer with a different frequency response, or you may need to interface directly at DC. There are several articles we can refer you if you want to have DC interface.

    -Kang

  • 0 in reply to Kang Hsia
    Prodigy 70 points

    Hi,

    Thanks for your reply. I have used the TC4-1W transformer of the DAC3283 output interface, and it is ac coupled.

    Now I want the output voltage to be dc level, and i use a 25Ω resistor connect to 3.3V, then the current sink through the resistor, and the output ac voltage is 2.8V centered, 0.5Vpp, I think.

    But when I catch the signal from the oscilloscope , the output ampitude is still attenuation when the frequency from 1MHz to 20MHz. I don't what the problem is.

    Thanks.
  • 0 in reply to user1818781
    TI__Guru 66014 points
    Hello,

    Yes, with 3.3V pull-up and 25ohm connected to 3.3V and DAC output (DAC output positive and negative leg, respectively), you should see 3.05V with 20mA full-scale current.

    The full-scale current of 20mA is complementary and shared by the positive and negative output of the DAC. With mid-scale 16-bit code, there are 10mA flowing through the positive and negative leg. With 25ohm drop from 3.3V, you should see 3.3V - 10mA*25 = 3.05V on both legs.

    You can try the following:
    1. send a full-scale negative (i.e. 0x0000 in offset binary code), use a multi-meter to measure the positive and negative leg. The negative leg should have 0mA current, resulting pulling up to 3.3V. The positive leg should have 20mA flowing through it, resulting 3.3V - 20mA*25 = 2.8V.
    2. send a full-scale positive (i.e 0xFFFF in offset binary code), use multi-meter to measure again. You should see the opposite trend.

    There are no other AC coupling path within the DAC to cause such effect.

    If you are able to observe such a trend with a multi-meter, this means your output is truely DC coupled. Then the other high pass effect could be your scope probe or scope setting, where AC coupling setting need to be set as DC coupling.

    -Kang
  • 0 in reply to Kang Hsia
    Prodigy 70 points

    Thanks，Your replies means a lot to me。