What is the proper Rf value for AD8130?

Hello:

I need to use the AD8130 in G>1 configuration, using Rf/Rg to set the gain. What is the proper value of the Rf? The data sheet does not say. The Circuit Note CN-0142 shows an example with Rf=499 ohms, but it is just an example where this particular value is not discussed in full. Is the 499 ohms mandated for some reason, or can I choose a different Rf?

Specifically, I would like to use Rf of about 100 ohms to keep the Johnson noise down. Is it possible? Will AD8130 survive such a low Rf?

Thank you -- Wojtek

Parents
  • 0
    •  Analog Employees 
    on Feb 27, 2013 2:51 AM

    Hi Wojciech,

    Thank you for using AD8130. One thing that you should note is that the feedback resistor (Rf) affects the  effective load (Reff) of your amplifier. For inverting configuration, Reff = RL // Rf, and for non-inverting configuration Reff = RL // (Rf + Rg) (You may look at the AD8657 datasheet for more detailed explanation). Choose Rf such that the condition Isc > Vout(max)/Reff is satisfied. Isc stands for the short circuit current of the amplifier, which can be found in the specifications of the datasheet. In the case of the AD8130, Isc is 55 mA. By satisfying that equation, you are assured that your amplifier is capable of driving your load.

    I also want to add that your Vout,max (Vs - Vheadroom) will vary with respect to your output current. See Figure 130 of the datasheet.

    Let me know if you have\ other questions.

    Best Regards,

    Emman

Reply
  • 0
    •  Analog Employees 
    on Feb 27, 2013 2:51 AM

    Hi Wojciech,

    Thank you for using AD8130. One thing that you should note is that the feedback resistor (Rf) affects the  effective load (Reff) of your amplifier. For inverting configuration, Reff = RL // Rf, and for non-inverting configuration Reff = RL // (Rf + Rg) (You may look at the AD8657 datasheet for more detailed explanation). Choose Rf such that the condition Isc > Vout(max)/Reff is satisfied. Isc stands for the short circuit current of the amplifier, which can be found in the specifications of the datasheet. In the case of the AD8130, Isc is 55 mA. By satisfying that equation, you are assured that your amplifier is capable of driving your load.

    I also want to add that your Vout,max (Vs - Vheadroom) will vary with respect to your output current. See Figure 130 of the datasheet.

    Let me know if you have\ other questions.

    Best Regards,

    Emman

Children
No Data