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Original question:

INA293-Q1: Current Sense Waveform Different - Noise Problems

INA293-Q1: INA293 Common mode filtering

wayne edwards
Prodigy 30 points
Part Number: INA293-Q1
Other Parts Discussed in Thread: INA293

Tool/software:

What is the recommended approach to adding a common-mode filter to the input of an INA293? 

Datasheet section 8.1.2 addresses differential mode filtering, but not common-mode. 

  • 0
    TI__Expert 3245 points

    Hi Wayne,

    Can you explain why you are wanting to put a common-mode filter on the input?

    We don't recommend putting any filter at the inputs if it can be avoided as it hurts the accuracy of the INA293. A common-mode filter can also degrade your CMRR. If you can show me what is going on I can recommend a solution to help.

    Best,

    Holly

  • 0 in reply to Holly R
    Prodigy 30 points

    CMRR is only about 80 dB at 100 kHz and continues to fall with increased frequency. In switching power applications, broadband noise (100 kHz to >10MHz) is likely to be coupled onto the inputs. In extreme cases we have observed degraded performance due to broadband noise coupled onto amplifier inputs.  

    Before we get too far into this, How is CMRR Measured?  Depending on that result, there may be less concern than initially thought. 

  • +1 in reply to wayne edwards
    TI__Expert 3245 points

    Wayne,

    You can refer to this AppNote (Section 4) for general guidelines but I will explain here. https://www.ti.com/lit/ug/slya042/slya042.pdf?ts=1736128184347

    The following figure shows an example of a common-mode filter:

    Applying the differential capacitor limits the affect of unequal RC time constants on either input. It is recommended that C_diff be at least 10x the values of C_CM.

    The common-mode caps do not affect the gain error, only the resistors. The datasheet explains how to calculate the gain error factor that the resistors cause. Section 8.1.2.

    In regards to how the CMRR is measured, we apply a change to the common-mode and then measure the output. In the equation below AD is the differential gain and ACM is the common mode gain.

     

    If you have any further questions let me know.

    Best,

    Holly