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Original question:

INA186: Application note for noise improvement

INA186: Rf and Cf in IN+/IN-

Far Hung
Expert 7410 points
Part Number: INA186


Hi 

The Rf in the input side, if will affect accuracy or it can improve noise performance?

Do we have the general application note for Cf design or typical design?

  • 0
    Guru 140200 points

    Hi Far,

    section 8.1.3 of datasheet contains very useful information about this input filtering.

    Kai

  • 0
    TI__Intellectual 1235 points

    Hi Far,

    As Kai mentioned, Section 8.1.3 of the INA186 datasheet further describes this input filter. Additionally, we have a TIPL training video: 3.9 Input filter error, that explains how to calculate and analyze the resulting error from input filters. 

    Using an input filter helps filter out load current or Vshunt noise. The input filter does not improve the inherent noise of the device.

    Rf can affect gain error according to Equation 5 provided in datasheet. A differential capacitor (Cf) should always be used if using input resistors. Cf helps stabilize the differential input voltage as internal capacitors are switching. Usually >500 pF is sufficient.

    To determine the Rf and Cf values, you need to know the minimum signal BW required for the system, and/or the load noise frequencies you want to attenuate. I recommend using Rf < 500 Ω and fc = 50 kHz, thus Cf = 1 / (2 * 2 * π * Rf * fc). We'd recommend fc to be slightly higher than the highest BW (45 kHz for INA186A1 variant). We recommend Cf be a C0G (NP0) capacitor type. With this you do not sacrifice any signal BW, but attenuate higher frequency transient noise.

    Cierra