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OPA2677: Driving a variable load without (or lowest) effect on the gain

Mojtaba Najafi
Prodigy 120 points
Part Number: OPA2677


Tool/software:

Hello,

I am using the OPA2677 to drive multiple twisted pair lines in parallel, resulting in a variable load. The input is a differential signal from a DAC with an output impedance of approximately 19Ω. I require a fixed gain between 1 and 2, with a bandwidth greater than 100MHz, and the circuit must operate with a single 12V supply.

It appears that the input impedance is matched using a balun, and both the gain and bandwidth meet the requirements. However, I have observed that the gain drops by approximately 2.5dB depending on the load value. Is there a way to reduce this dependency?

Best regards,

TX-LineDriver.TSC

  • 0
    Prodigy 120 points

    Or should I use differential VGA insted?

  • 0 in reply to Mojtaba Najafi
    TI__Mastermind 18025 points

    Hi Mojtaba,

    The difference in gain you are seeing is due to the voltage divider occurring with the isolation resistor and the load you are adjusting. Is the load likely to be changing in the design to where adjusting the signal gain for this attenuation would not work? I would also like to add for the capacitive load shown in the simulation, we would recommend an isolation resistor around ~20ohms. The graph for this information is on page 7 of the datasheet. This would further attenuate the signals you are seeing at the load as there is an even larger drop across the isolation resistors for the same load.

    Best Regards,

    Ignacio