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PCB layout guidelines for ultra low-noise analog front end
ricko_uk:
Thank you all for our feedback, much appreciated!! :)
Rerouter:
The extremes are less well documented. It mainly gets into calculating error contrubutions.
E.g. if measuring ppm you have to account for thermal gradients on the board generating small voltage differences because one pin is slightly hotter than another. And this gradient shape can change depending on the input voltage to the amplifier as its output power may change. Or the rail that is contrubuting that power may shift
Power supply ripple rejection is a good way to soak up margins. Also some op amps are far better than others in terms of what application your solving. I find mapping out the noise graphs into frequency bins for each device gives a good overveiw for what frequencies you end up the most sensitive to.
Plan your return paths. Treat anything critical like a differential signal. If a minutes work can remove an uncertainty of 0.1ppm a few hundred times over the length of a project. It starts adding up
Keep decoupling loops seperated from other grounds. All device power connections go to the capacitor. Then the plane or trace which feeds it
Choosing where you common your grounds can matter. For me it usually ends up at the measurement connector ground rather than the supply ground. With good considerstions for just how much current is flowing along that ground.
ricko_uk:
Thank you all :)
Rerouter:
To fill in some gaps.
Op amp noise
https://www.analog.com/media/en/training-seminars/tutorials/MT-049.pdf
https://www.analog.com/media/en/training-seminars/tutorials/MT-047.pdf
http://www.ti.com/lit/an/slva043b/slva043b.pdf
Seebeck Effect (Thermal)
http://ww1.microchip.com/downloads/en/appnotes/01258b.pdf
https://electronics360.globalspec.com/article/9981/pcb-layout-for-precision-operational-amplifiers
Johnson-Nyquist (Passive components Noise)
https://www.analog.com/en/analog-dialogue/raqs/raq-issue-25.html
http://www.resistorguide.com/resistor-noise/
Dielectric Absorbsion (Capacitors)
https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20140008867.pdf
https://nepp.nasa.gov/files/25847/2013-Taverovsky-paper-NEPPweb-MLCCsVabs-n264.pdf
EMI Reduction
http://www.ti.com/lit/an/szza009/szza009.pdf
General Guide
https://www.analog.com/media/en/training-seminars/design-handbooks/Basic-Linear-Design/Chapter12.pdf
radioactive:
Another thought... leave some pads for some Tantalum caps if you didn't already. Ceramic caps can be problematic if there are issues with vibration / stresses on the pcb when dealing with low voltage measurements. "microphonics".
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