Electronic – ADS8671 ADC: 1 MSPS, but 15 kHz antialising filter


Looking at the datasheet for the TI ADS8671, it says this device can do 1 MSPS (one million samples per second, right?)

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I would think this would allow sampling of theoretically up to 500 kHz, in practice a little less for the antialising filter.

But on page 25, it shows the frequency response of the integrated antialising filter well below that:

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So for 1 MSPS I get a flat passband only up to 15 kHz? Can that be right?

Further confusing to me is this datasheet describes two ADCs, one with 1 MSPS and another 500 kSPS, yet describes only one filter. And it says the "filter is typically set to 15 kHz". Does "typically set" imply the filter is programmable? I see no such capability in the datasheet.

What am I missing?

Best Answer

I believe this answer on the DSP StackExchange is why. As Andy mentions, 20dB for an AA filter isn't all that great, if the Nyquist frequency isn't much higher than twice your sampling frequency. But, by pushing your Nyquist frequency far higher than your AA filter's cutoff, the magnitude of any aliased signals will be reduced markedly, while reducing the cost of the chip as a whole.

I've wondered this too and never thought to ask about it, so I (pending criticism of this response) learned something here :)

As far as your question about "typically set", I will hazard a guess (though I can't ascertain what the authors meant): the frequency response of the filter appears to change somewhat with different Vref's, and as such the -3dB frequency shifts slightly, "typically" at 15kHz.