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anti-alias-filter for fully differential ADC driver

There is a difficulty when having differential source signal to drive differential ADC.
The ADC driver must have very high bandwidth. So any anti-alias-filter has to be in front of it.
But the filter has to be differential also!
There is a filter circuit for the LTC6363 (data sheet page 35). But as it has several resistors in each gain path with special values.
So it is NOT possible to get all the resistors in one network. With single resistors the ratio and TC tracking is relatively bad,
which in turn will severly degrade CMRR and the gain stabiltity.

As this is a standard task for many applications, I wonder why there is (almost) NO differential ADC driver that has the capability to add a (AAF-) filter-C to it without compromising specs.
(The only INA - AD8428 has fixed gain of 2000 - way too high!).

Thread Notes

  • Hi,

    There are a variety of topology where you can implement an AAF to the ADC driver. The one in the LTC6363 datasheet happens to be a third order LPF to make a steeper roll-off (60 dB) from the cutoff. There are tools online that could help you design your filters. The simplest one could be an active single-pole Low Pass filter which can be done by connecting a capacitor in parallel with the feedback resistor. Or a passive single-pole low pass filter after the output of the ADC Driver. You could always use a resistor array for the feedback and gain setting resistors. In this way you can have a better performance than single discrete ones.

    Regards.

  • Hi JinoL,

    >> ...You could always use a resistor array for the feedback and gain setting resistors. In this way you can have a better performance than single discrete ones.

    I know. But this is for DC only. The problem is that when paralleling with 2 C's, the commom mode rejection will degrade significantly - even when manual trimmed. And when the filtering is on the output of ADC-driver, then it will be unsetteled between conversions.  Exacerbating is, that I want high(est) accuracy...
    https://dict.leo.org/forum/viewWrongentry.php?idThread=380536&idForum=7&lang=de&lp=endePS:
    Why doesn't anyone use a LCR-Filter (in front of ADC-driver) ?
    Does Inductors have too low accuracy / stability / linearity?

    kind regards

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