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ADC input filter
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OM222O:
are really small value capacitors (E.g 10pF) even able to provide any filtering? I feel like even parasitic in the circuit would be higher than 10pF ... I'm using 2 heavily stitched ground planes on both sides.
T3sl4co1l:
You have all the variables needed to solve this -- sample rate (low kHz), and impedance (the series resistor).  C = 1 / (2*pi*F*R). :)

Tiny caps won't do anything in the critical band (presumably the 10kHz~10MHz range, above which the internal EMI filtering (if applicable) should take over), at least not without greatly increasing the resistor values, but then the input bias current offset spec will ruin the offset voltage.  So, you need big values.

Diff cap being bigger is motivated by having more CMRR than diff noise tolerance, plus CM/DM mode conversion as discussed.  Diff noise is also diff signal, so it's not a hard rule, it's application dependent.  Maybe you'd be better off with an instrumentation amp and active filtering in some applications; that would be fine, too.

Tim
coppice:

--- Quote from: wraper on February 10, 2019, 02:04:35 am ---
--- Quote from: coppice on February 10, 2019, 01:35:30 am ---So, I don't know why they recommend a 10 times ratio in the capacitances. A 1000 times ratio is usually a better solution.

--- End quote ---
They recommended at least 10 times. For example, I use 10x ratio in one of my circuits. It's because there may be a HUGE environmental noise starting at few a kHz, yet I need at least 500Hz of bandwidth, and input signal full range is about 10mV. Something like 100pf for bypass is simply not sufficient. And I cannot go too high for cap between inputs as well. So ratio ended up somewhat on low side.

--- End quote ---
Why would that make 100pF insufficient? Its only there to suppress RF, not anything close to the band. The big differential capacitor does that.
iMo:
https://e2e.ti.com/blogs_/archives/b/precisionhub/archive/2015/11/06/three-guidelines-for-designing-anti-aliasing-filters
wraper:

--- Quote from: coppice on February 10, 2019, 09:36:28 am ---
--- Quote from: wraper on February 10, 2019, 02:04:35 am ---
--- Quote from: coppice on February 10, 2019, 01:35:30 am ---So, I don't know why they recommend a 10 times ratio in the capacitances. A 1000 times ratio is usually a better solution.

--- End quote ---
They recommended at least 10 times. For example, I use 10x ratio in one of my circuits. It's because there may be a HUGE environmental noise starting at few a kHz, yet I need at least 500Hz of bandwidth, and input signal full range is about 10mV. Something like 100pf for bypass is simply not sufficient. And I cannot go too high for cap between inputs as well. So ratio ended up somewhat on low side.

--- End quote ---
Why would that make 100pF insufficient? Its only there to suppress RF, not anything close to the band. The big differential capacitor does that.

--- End quote ---
With 100pF they start to do anything way above the frequency of CM noise in-amp can cope with, not to say sufficiently suppress it. The typical scenario is that body of a load cell may be electrically connected to a body of 1kW PWM driven BLDC motor (from crap driver without output filter or anything). Most likely nothing would be grounded. FWIW design of previous supplier failed miserably in that scenario.
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