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DAC37J84: DAC37J84 Gain Issues

Richard Elmquist
Mastermind 45250 points
Part Number: DAC37J84
Other Parts Discussed in Thread: TRF37T05,

My customer is seeing the following issues:

The DAC 37J84 was originally set to provide a full scale current of 30.16 mA by choosing a bias resistor value of 1.91 K.

The common mode voltage was 0.480 V when measured across one of the 50 Ohm resistors. Two 50 Ohm resistors were placed in parallel (one in front of the filter and another after the filter which is between the DAC and the IQ modulator (TRF37T05)).

I wanted to set the DAC full scale current to 20 mA. So I changed the bias resistor value to 2.94 K (results in19.54 mA full scale current). When I measured the common mode voltage, it was close to zero, and the power measured at the IQ Modulator output port was -65 dBm (before resistor change, it was around -20 dBm). Then, the 2.94 K was replaced by 1.91 K. it resulted in the values that were obtained originally. Then 1.94 K was replaced by 2.1 K and it yielded the same results as those of 2.94 K.

I would like to know why it works for only one resistor value that was originally selected for biasing the full scale current

I have a partial schematic and the results of the filter test if you want me to send them to you. Please let me know if you need them.

Thanks for your help with this!

Richard Elmquist

  • 0
    TI__Guru* 91947 points

    DAC with modulator.pdf1665.TSW38J84EVM-SCH_B.pdfRichard,

    See if these two documents help with this.

    Regards,

    Jim

  • 0 in reply to jim s
    TI__Mastermind 45250 points

    Jim,

    Thanks for your quick response!

    I will let you know if the customer has any further questions once he looks at the references.

    Thanks again.

    Richard Elmquist

  • 0 in reply to Richard Elmquist
    TI__Mastermind 45250 points

    Jim,

    The customer stated that that the reference design material did not help.

    Can you look at this a little deeper?

    Here is his questions again with some refinements:

    The DAC37J84 was biased to produce a full scale current of 30 mA and the maximum PP input voltage to the IQ Modulator was set to 1.5 V. I want to scale down the full scale current to 20 Mas and the maximum PP input voltage to the IQ Modulator to 1V by choosing a bias resistor value of 2.94 K. The calculation is shown in the data sheet.

    When the bias resistor value was changed from 1.92 K (this was what we had before to produce 30 mA Full Scale Current) to 2.94 K, it brought common mode voltage to 0.081 V from 0.480 V and the RF power at the IQ Modulator output port to -65 dBm from -20 dBm. When the resistor value was changed again to 2.1 K from 2.94 K, the change in resistor value yielded same result as those of 2.94 K. When the original resistor was put back, it worked as before we changed the resistor values (CMV – 0.480 V, RF Power - -20 dBm.). My question is, why it works only for that value and does not work for any other value?

    I can send you the customer's schematic etc. if that would be helpful.

    Thanks for your help with this!

    Richard Elmquist

  • 0 in reply to Richard Elmquist
    TI__Guru* 91947 points

    Richard,

    Tell them that the coarse gain adjustment is another option to lower the swing without changing the rbias resistor. Can you send us their schematic? What common mode are they trying to achieve? Not sure why they are seeing no change in the CM when switching to a 2.1K resistor. Did they Ohm out the part to make sure it was 2.1K?

    Regards,

    Jim 

  • 0 in reply to jim s
    TI__Mastermind 45250 points

    Jim,

    I will pass your comments to the customer and send you the schematic etc. that they sent me to your email.

    Thanks for your help with this!

    Richard Elmquist