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ADS8598H: Selection of anti-aliasing cut-off frequency of LPF

Prodigy 250 points

Replies: 3

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Part Number: ADS8598H

Dear Members,

I am designing with ADS8598H, with following signal property and will sample at high sampling rate around 200 times or more of maximum signal frequency. My signal frequency is max 200Hz, ignoring noise from environment and circuit components. The objective of filter and cut-off frequency - remove the noise frequency as much as possible, make sure circuit/filter do not change the phase and delay parameters with frequencies, change is acceptable but must be constant till two times of signal, i.e. 400 to 500 Hz. The fix value of phase change and constant delay is ok, it will not impact analysis.

I am trying with filterPro and trying to find these value.

Does any thum rule or guideline availble, to select cut-off freq and type of topology to cover above problem?

Regards

Dr. Mahabir Prasad

  • Hello Dr.Mahabir,

    ADS8598H has an internal 3rd-order LPF on each channel and the cutoff frequency is 24kHz for +/-10V input range(16kHz for +/-5V range), so any external filter with higher cutoff frequency than 24kHz will not be useful to filter noise.

    In your case, you are trying to design an external filter with a lower cutoff frequency, it will be helpful to filter more noise in your system. If a passive LPF filter is expected, a large value resistor or capacitor will have to be considered to achieve a lower cutoff frequency, an additional gain error caused by the  large value resistor with 1Mohm input impedance should be considered, also the drift of the large value resistor may be a concern.

    You may be considering a active low-pass filter, a variety of circuit topologies exists for building a active filter and the topology used to build a filter will have an effect on its actual phase response. The following table compares the phase-shift ranges for the various low-pass filter topologies:

    I'm not an expert on filter design, these app note and user guide may be a good start and reference for you:

    Active Low-Pass Filter Design

    FilterPro™ User's Guide

    Best regards,

    Dale

  • In reply to Dale Li:

    Thanks for detailed reply. I am using active RC filter. I have compared all the topology with the same design parameters and found below, which is also confirmed one of application note on TI.

    1. I found three best - Bessel, Linear Phase and Butterworth are best pick to select, Chebyshev did not give good results as per Filter Pro Simulation tool.

    2. The pass band flatness - butterworth, Pass band least phase change - Bessel. Linear Phase is middle of both, it gives middle results. Below are results in order, buttherworth, LP and Bessel for multiple frequencies, the filter cut-off was 10KHz, order of 8. The results, recommands to use bessel or LP. The performance of bessel is marginally better to LP. In similar study in research papers and product manuals, LP is recommanded. Being novice in filters, I will go by their wisdom, till someone give more insite to select bessel. The butterworth is ruled out, phase and delay.

    Linear Phase and Minimum phase filters are same or different?

  • In reply to Mahabir Prasad:

    Hello Dr.Mahabir,

    I just found you had asked one question at the end of your post. I apologized for the late response.

    Linear phase is a property of a filter, where the phase response of the filter is a linear function of frequency. As the name suggests, a filter with minimum phase minimizes the phase shift incurred at each frequency while delivering the desired magnitude response.

    I'm not an expert on these, just let you know my understanding.

    Thanks&regards,

    Dale

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