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THS4524 Drive Capability

Terry Merschat
Intellectual 975 points
Other Parts Discussed in Thread: THS4524, THS4521

Customer wants to know if the THS4524 can drive 12" of 50 ohm trace.  To me it seems that the 50 ohm characteristic impedance is not the issue, but rather that the load is matched to the trace.  Is that correct?

  • 0
    TI__Expert 4115 points

    Terry,

    Generally when setting up characteristic impedance traces on a board the reason is to control the impedance and provide termination to reduce reflections (low VSWR). The best scenario is double termination, where the source and load are both terminated in the characteristic impedance of the trasmission line. I assume in this case they want to use 2x 50 ohm traces, one for each output. For double termination, you place 50 ohms in series with each output of the THS4521 and a 50 ohm load at the end of the transmission lines. This leads to a 6dB reduction in signal amplitude due to the resistor dividers. While there is always the question when driving a reactive load about its effect on the stability of the op amp, with 50 ohms in series with each output (close to them), it is not a problem.  If they want to avoid the 6dB loss and provide single termination, they should put the termination at the output of the THS4521 to avoid stability issues.

    There are graphs of output voltage versus load in the data sheet (page 14 for 3.3V supply and page 19 for 5V supply); 50 double termination is equivalent to 200 ohms differnential load, so each output will be able to drive to about 0.5V from the rails with 5V supply and about 0.3V with 3.3V supply.

    So while we have not tested this configuration, it should be ok. The THS4521 has enough output drive and with output termination resistors there are no stabilty issues.

    Regard,

    Jim Karki

    High Speed Amps