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INA240: how to design anti-alias filter for INA240+ADS7253

Part Number: INA240
Other Parts Discussed in Thread: ADS7253,

Hi,

Here is a customer design using INA240+ADS7253 as below. The sampling frequency is 10ksps, working common voltage is 48~72V. They found anti-alias filter(R30,R31,C17) influence the sampling performance a lot, sampling vibration appears. Do you have any suggestion on anti-alias filter design? thanks!

-- Yuan

  • Hi Yuan,

    The existing filter bandwidth is too much larger than typically recommended by design literature. You may want to look into this blog, and this video for best practices on setting the proper component values. Further, if you have specific questions regarding the ADC, please make a separate post to Data Converters Forum.

    On the other hand, there are at least two requirements at the front end:

    (1)    The filter bandwidth has to be large enough to accommodate the input signal.

    (2)    The filter should not destabilize the INA240 output.

    Not sure about (1) but I think (2) is likely violated, and R30 and R31 are too small relative to C17, hence the poor measurements due to oscillation. You can try to make them larger, such as >100Ohm to see if the measurement improves.

    Regards, Guang

  • Hi Guang,

    Thank you!  customer has more questions:

    customers current loop is 50us, what's the recommended cut-off frequency for them?

    and  from " Occasionally, a single-pole, low-pass filter may not be enough. Some applications, like vibration sensing, may use less oversampling to analyze signals over a wider bandwidth. This puts the passband of the digital decimation filter closer to fMOD and gives the anti-aliasing filter less room to roll-off. In these instances, you can add a second or third pole with additional RC pairs to achieve an even sharper filter response." do they need a second or third pole with additional RC pairs?

    thanks!

     

    --Yuan

  • Hi Yuan,

    The ADC datasheet recommends a filter cap bigger than 400pF.

    The original PCB design called a 470nF, this is overly conservative. More importantly it might destabilize the INA240 output. Further the bandwidth of 7.7KHz is too low to allow a 50uS control loop to settle.

    I think a single stage filter is sufficient. However the RC values need to be adjusted, a 2x50Ohm+10nF will likely work well with a bandwidth of 160KHz.

    Regards, Guang