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Lowpass filter for LTC6363

I'm trying to add a 1 pole RC filter with a 20kHz cutoff upstream of the LTC6363 as an anti-alias filter. My configuration is like this:

This is for audio frequency input signals and I'll be doing a 16x or 32x decimation factor to provide additional anti-alias performance.

According to LTSPICE I can add a 30nF capacitor as shown:

...and this gives me the magnitude response I'm after, but it adds a ton of noise @ 380kHz which I assume will alias into my input signal. If there's a more optimal solution please advise! 

  • Hi Dave,

    What happens if you put 30nF from the + input of U1 to ground? This should balance the frequency response of both sides of the amplifier and will hopefully give you better results. Also, make sure to use low voltage coefficient capacitors to minimize distortion. Film capacitors or ceramic C0G capacitors work well.

  • Thanks for the suggestion. Here's the frequency response from LTSpice with the single capacitor as I showed it earlier:

    It is the same for both Vout+ and Vout-, but it will mean asymmetrical board layout which I don't imagine will be good for common mode rejection.  If I add the capacitor you suggested I get this:

    ie no roll-off until close to 1Mhz.

  • I'm also able to do this:

    This yields better noise performance and symmetry in the board layout, but the frequency response only matches up until 200kHz or so.

    I'm not sure why Vout+ and Vout- responses don't match, is it because the input is single-ended?

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