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THS4513 Basic question

Other Parts Discussed in Thread: THS4513, THS4541

Hi,

I am studying THS4513 to support my customer.

I would like to ask you a basic question about Gain setting resister as below.

There is a "Table1 Gain Component Values" on page23, datasheet.

I do not understand how to calcurate RG, RIT.

Could you tell me the equation or formula?

And about RF, 348 ohm is used. Why is this value adopted?

For example, " Kohm"(1K, 2K, etc) value is not appropriate?

Thank you very much for your support in advance.

 

Best Regards,

  • Hello Takumi-san,

    The THS4541 datasheet has a very good explanation of resistor value calculations.  It is a difficult calculation because it is quadratic. 

    The RF value of 348 Ohms is a compromise between using a small value (like 50 Ohms) that minimizes the impact of unwanted parasitic capacitance and a large value (like 2k Ohms) that puts minimal load on the amplifier output stage. 


    Most high speed amplifiers cannot use large values of feedback resistance because undesirable capacitance on the PC board will cause the amplifier to be unstable. 


    Using values that are too small like 50 Ohms will degrade harmonic distortion and voltage swing capability because the amplifier output will have to drive this load in addition to any external load. 

    If you select an RF between 300  and 400 Ohms you will be fine.  For if you want to use the amplifier at a higher gain (like 12dB) you could use a resistor of 500 to 600 Ohms.  This way your Rg will not be too small and the higher gain of the amplifier will provide extra margin of stability. 

    Regards,

    Loren

  • Hi Loren-san,

    Thank you very much for your kind explanation!

    I understood and I will study THS4541 datasheet soon.

    Best Regards,