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THS4541: Reccomended input and feedback resistance

Tomoaki Yoshida
Expert 2200 points
Part Number: THS4541


Hi,

I have something to confirm.

In the reference circuit of the datasheet, the input resistance is about 100 Ω, but in our system it needs to be around several kΩ.

And we assume that the gain is about 3 times.

I think there are no concerns other than noise.

If there is a recommended resistance value, please tell me the reason.

Best regards,

Tomoaki Yoshida

  • 0
    TI__Genius 12631 points
    Hello Tomoaki-san,

    We use small value resistors for a variety of reasons:

    1. Since the amplifiers are low noise we do not want to compromise the overall noise performance by adding large resistors that would dominate.

    2. High-speed amplifiers tend to have input bias currents in the uA range so using large resistors would cause the output offset to increase.

    3. With high bias currents the current noise also increases which would consequently increase the overall output noise since the current noise gets multiplied by the feedback resistance.

    4. Large feedback resistors will create a zero in the noise gain when it reacts with the amplifiers input capacitance. This zero could result in instability or reduced phase margin so to compensate for that a pole is introduced by adding a feedback capacitor in parallel with the feedback resistor. The end result will be a reduction in overall bandwidth. This effect can be simulated in TINA or any SPICE simulator.

    -Samir