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Current Output DAC Passive termination Query

Suresh Surya51
Prodigy 65 points

Hi,

This query is regarding passive termination for current source DAC. I have gone through the application note SLAA399 – November 2008 (Passive Terminations for Current Output DACs). I have planned to use 1:1 transformer as represented in Figure 4 of the application note.

Can I increase the output power by not mounting the 100ohm parallel termination resistors? As per the calculations, the Output power will be -2dBm if 100 ohm parallel termination is used, 0.46 dBm if no parallel termination is used and -3.98 dBm if 50 ohm parallel termination is used.

Will changing the termination resistors, impact in any other parameters other than output power?

Kindly advise.

Regards,
Suresh Surya.B.S

  • 0
    TI__Genius 12965 points
    Suresh,

    As noted in the app note, you just need to guarantee a couple of things:
    1. Vcm needs to be around AVDD=3.3V.
    2. The compliance voltage requires the voltage swing at each P and N pin to not exceed 2.5V-3.8V. Exceeding these pin compliance voltages will cause distortion and possible damage to the outputs.

    You can use any transformer and load to meet these requirements. The app note used a 50 ohm termination case as in most situations you would be connecting to 50 ohm test equipment or RF devices.

    Ken.
  • 0 in reply to Ken C
    Prodigy 65 points

    Hi

    Thanks for your reply. I am using a current source DAC where the center tap terminal of the primary will be connected to GND which bring the VCM = 0V. Also with this termination resistors, voltage swing at output pins will not go beyond safe values.

    My query was what will be the expected output power when parallel termination RES is not mounted. Refer following figure.

    Thanks in advance

    Surya.

  • 0 in reply to Suresh Surya51
    TI__Genius 12965 points
    The figure did not come through. However the same holds true for the current source outputs as well. There will be a compliance voltage that needs to be met. As long as that is met for the device then you can use any combination of transformer and load.

    Lets assume you are looking at figure 4 from the app note. Typically if you do not have the 100 ohms in parallel you will have only the 50 ohm impedance from the Rload, transformer coupled to the left side of the transformer. This will mean 25 (50 ohms split to each IOUT side) ohms in parallel with 50 ohms on each of the IOUT so the resulting impedance will be 50//25=16.67 ohms as seen by each IOUT pin. With 20mA this will give you a 333.33mVpp. Differentially this will be 666.66mVpp.

    As per the app note, you can then transfer this voltage back to the right side across the 50 ohm load to calculate your power.

    Ken