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BUF802: Pole at the input of IN_AUX

Part Number: BUF802
Other Parts Discussed in Thread: OPA828, OPA140, TINA-TI

In the reference designs for the BUF802 in CL mode, there is a RC filter between the output of the precision amplifier and the IN_AUX pin of the BUF802. In the datasheet it is 2kΩ / 100 pF with a corner frequency of 800 kHz. Since it is not called out in the detailed design notes, what is the significance of this pole in the feedback path and how was the corner frequency chosen with regard to other parameters in the design (perhaps in relation to the f_HF pole)?

  • Hi James, 

    This RC filter is primarily to filter the noise from any bandwidth of the precision amplifier in excess of the useful bandwidth that is directed through the aux pin path, a bandwidth mostly set by the feedback capacitor. 

    Section and of the datasheet discuss the equations that you can use to set Cf, but I always run parametric sweeps on my TINA-TI model to determine a combination of values that gives me the flattest frequency crossover and reasonable noise, for a given precision amplifier. You might want to experiment with larger capacitors if you are very concerned about grinding down the noise, but this TINA-TI file gives examples with values that work pretty well for the OPA140 and OPA828 precision amplifiers.

    Best regards,