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ADS1259: Input filter for ADS1259

Barney
Prodigy 230 points
Part Number: ADS1259

For an application where the ADS1259 operates on a single +5V supply and the REFOUT (+2.5V) is tied to the AINN pin.

If a low pass RC filter is placed before the AINP pin with R=50 Ohms and C=10nF (fc = 159,155 Hz), would this basically provide an anti-alias filter at all sample rates (10 Hz to 14,400 Hz) because the ADS1259 is oversampling at 921,600 Hz (fclk/8 where fclk = 7.3728MHz) and implementing digital low pass filtering?

Would a low pass filter cut-off frequency of 460,800 Hz or lower provide a universal anti-alias filter for all sample rates (10Hz to 14,400 Hz)?

What is the lowest low pass filter cut-off frequency (or the highest RC value combination) that can be placed in front of the ADS1259 inputs without using a buffer between the low pass filter and the ADS1259?

Thanks

  • 0
    Prodigy 230 points

    Thanks for the reference FAQ.  This is helpful to the non-electrical engineer looking to eval the ADS1259.

    The FAQ describes a different situation than my question however.  The situation I am looking at has only a R and Ccm on the AINP input.  Are you recommending that I add a Cdiff between my input on the AINP pin and REFOUT on the AINN pin to create a differential filter as described in the FAQ?

    Also, I thought fdiff = 1/(2*pi*R*(2*Cdiff+Ccm) which is different than what is spelled out in the FAQ.

    Given a relatively low resistor needed (say R=50 Ohms) how large can the capacitor be on the input to the ADS1259 (beyond that needed for anti-alias)? 

    Thanks

  • +1 in reply to Barney
    TI__Mastermind 42871 points

    Hi John,

    The methodology described in the FAQ for determining the AA filter cutoff certainly applies to your situation. Moreover, the FAQ notes that if you have a single-ended input (or in your case, pseudo-differential), you would only add the filter to the AINP input but the considerations would be the same for the differential filter.

    There is no real maximum cap value that can be used on the input. However, the filter time constant might be very slow if you choose large cap values, making it difficult to sample quickly. Also, we recommend using C0G-type caps (or similar ceramic caps) for input signals since these offer the best voltage and temp coefficients. These types of caps are typically limited to smaller values (0.1uF or less), which inherently limits the input cap size.

    -Bryan