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DAC904 transformer configuaration

Other Parts Discussed in Thread: DAC904, OPA690, OPA847

HI

My customer would like to use the DAC904  with the following configuration :

"IOUT should be set to 20 mA.
For the generation of a single-ended signal we want to use a 50 ohm ADT2-1T (2:1 Transformer).
The output voltage is in one case be + / - 1V, otherwise 0 .. 2V.
I would IOUT and IOUT_inv (and not in a case, the center tap to GND, in the other case), each with 50 ohms to AGND wired
Then would by a 2:1 Transformer the output voltages achieved?"

regards,

kamal

 

  • Hi Kamal,

    I don't quite understand the question, but I'll try to answer below. If you have schematics of what they want, it might be easier.

    First of all, they can take a look at the following app note for guidance:

    http://www.ti.com/lit/an/slaa399/slaa399.pdf

    If a 2:1 transformer is used with 50 ohms to ground on each leg and a 50 ohm load at the output of the transformer then the signal swing at the 50 ohm load will be 0.707 Vpp (1 Vpp differentially at the DAC output). This doesn't meet their +/- 1 Vpp requirement. So perhaps I don't understand the circuit they are describing.

    Regards,
    Matt Guibord 

  • FROM MY CUSTOMER:

    I interprete Your answer as follows:

    To get +/- 1Vpp we need 70 Ohm instead 50 ohm to GND on each leg of the transformer.

    Then we have a voltage of 1.4V at the DAC output.

    Is there no conflict with the output compliance Range which is in the data sheet 1.25V (smaller als 1.4V)?

  • Hi could you please verify the comment from my customer below?

    I interprete Your answer as follows:

    To get +/- 1Vpp we need 70 Ohm instead 50 ohm to GND on each leg of the transformer.

    Then we have a voltage of 1.4V at the DAC output.

    Is there no conflict with the output compliance Range which is in the data sheet 1.25V (smaller als 1.4V)?

  • Hi Kamal,

    To get 2 Vpp, you need an equivalent differential impedance of 100 ohms at the DAC output. This would be the case if you used a 4:1 transformer, with 100 ohms to ground on each leg, and 50 ohms at the output of the transformer. However, this would only give you 2 Vpp at the DAC output not at the transformer output. The voltage translation through the transformer would put you at 1 Vpp. The only way to make this larger would be to double all of the impedances (200 ohms to ground on each leg, 100 ohms at output of transformer), but then you would have 4 Vpp at the DAC output, which would have each single-ended output swinging +/- 1 Vpp. This would be right at the compliance voltage limit...

    A better way might be to interface the DAC with an amplifier to convert the signal from differential to single-ended and provide some gain.

    Regards,
    Matt Guibord 

  • Hi Matt,

    My customer write the following comment:

    2 Vpp can not be achieved with transformers.
    I now want to increase the op amp with a DAC output voltage.
    Do you think  the OPA847IDBVTG4 is suitable?

     

    regards

    kamal

  • Hi Kamal,

    Sorry the late reply. That amplifier is only suitable for gains greater than or equal to 12. This is likely more than is needed... You would need to talk to the amp team for recommendations.

    Regards,
    Matt Guibord 

  • Hi my customer would like to change tteh part since he need only a gain of 2.

    Comment from Customer:

    In our application we have a gain of two and therefore I will replace the OPA847 by an OPA690.

    What do You think about this?

  • Hi Kamal,

    That seems like a better choice.

    Regards,
    Matt Guibord