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WEBENCH® Tools/THS4541: Some question about a Single-ended to differential circuit with THS4541

xin xie
Prodigy 60 points
Part Number: THS4541

Tool/software: WEBENCH® Design Tools

Dear TI:

I want to achieve a Single-ended to differential circuit, the amplitude of the input signal is about -40dB， the bandwidth is about250MHz, so I choose THS4541 as the fully differential amplifier. the schematic diagram and PCB layout  are as flow:

the power-supply is 3.3V, the Vcom is 1.65V by setting the resistance R7 and R9.

Then I test the circuit. the signal generator sent 0dB sine wave. one of the output pin is connected with spectrum analyzer.  when the power is off, the amplitude of the peak shown on the spectrum analyzer is -40dB, the frequency is same with the signal sent by the signal generator. when the power is on. the amplitude of the peak decay a lot, about -70dB.

so,what happened about the circuit?

Is there some wrong in the parameter setting?

please help me 

  • 0
    TI__Genius 15975 points

    Hi Xin,

    First of all I would suggest to remove the C5 capacitor (0.1uF) across the outputs and see if the issue is resolved. I think you are band-limiting the output signal with this capacitor.

    Also, the Vcom seems to be close to 0.953V instead of 1.65V. This can be calculated by the equation: Vcom = (1k/(1k+2.464k))x3.3V.

    Best Regards,

    Rohit

  • 0 in reply to Rohit Bhat
    Prodigy 60 points

    dear Rohit:

    thanks for your reply.

    the values of R7 and R9 are not shown in the schematic. In practic, R9=0, R7=2K. so the Vcom is 1.65V