PCB layout guidelines for ultra low-noise analog front end - Page 1
Forum section: electronics / projects

author: ricko_uk
author_rank: Super Contributor
timestamp: January 30, 2020, 08:39:54 pm
content: Hi,I have experience in general low-noise PCB layout. But I am looking for EXTREME guidelines (documents, videos or any resources) for analog front end layout measuring microvolts (sometimes to nanovolts) signals.I am not talking about just the basic well know guidelines (PCB decoupling, plane separation, GND via fences etc) or even the much less known/used one such as amplifier guard ring traces etc. I am talking about truly extreme PCB design guidelines.Any suggestions and pointers to any resources (PDF, videos, web pages etc)?Many thanks

author: thm_w
author_rank: Super Contributor
timestamp: January 30, 2020, 09:41:19 pm
content: I think low-noise and low level signals are two related but slightly different things.You may want to look at the Keithley voltmeter designs for example:https://xdevs.com/review/k182m/https://www.eevblog.com/forum/testgear/nanovoltmeters-performance/low noise amp:https://www.eevblog.com/forum/metrology/need-lna-for-low-frequency-low-level-preamp/http://www.hoffmann-hochfrequenz.de/downloads/lono.pdfGuard ring traces, shields, board cutouts, teflon standoffs.

author: nctnico
author_rank: Super Contributor
timestamp: January 30, 2020, 09:44:28 pm
content: IMHO these kind of designs come down to designing ground paths (=power supply return paths). A guideline only goes so far.

author: moffy
author_rank: Super Contributor
timestamp: January 31, 2020, 12:53:37 am
content: Quote from: nctnico on January 30, 2020, 09:44:28 pmI agree, plus taking care to minimise voltages produced by temperature differences across components, due to thermocouple effects. Also keep the impedances low throughout.

author: Someone
author_rank: Super Contributor
timestamp: January 31, 2020, 01:03:01 am
content: Quote from: ricko_uk on January 30, 2020, 08:39:54 pmWhats the bandwidth of the measurement? That makes a world of difference to the approach. The basics come back to considering all the possible noise sources and their impact on the final result. Not sure there are any public guides on the matter as it is both domain specific, and commercially valuable information.

author: ricko_uk
author_rank: Super Contributor
timestamp: January 31, 2020, 02:04:02 am
content: Thank you all for our feedback, much appreciated!!

author: Rerouter
author_rank: Super Contributor
timestamp: January 31, 2020, 02:04:18 am
content: 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 shiftPower 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 upKeep decoupling loops seperated from other grounds. All device power connections go to the capacitor. Then the plane or trace which feeds itChoosing 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.

author: ricko_uk
author_rank: Super Contributor
timestamp: February 01, 2020, 07:14:09 pm
content: Thank you all

author: Rerouter
author_rank: Super Contributor
timestamp: February 01, 2020, 11:17:44 pm
content: To fill in some gaps.Op amp noisehttps://www.analog.com/media/en/training-seminars/tutorials/MT-049.pdfhttps://www.analog.com/media/en/training-seminars/tutorials/MT-047.pdfhttp://www.ti.com/lit/an/slva043b/slva043b.pdfSeebeck Effect (Thermal)http://ww1.microchip.com/downloads/en/appnotes/01258b.pdfhttps://electronics360.globalspec.com/article/9981/pcb-layout-for-precision-operational-amplifiersJohnson-Nyquist (Passive components Noise)https://www.analog.com/en/analog-dialogue/raqs/raq-issue-25.htmlhttp://www.resistorguide.com/resistor-noise/Dielectric Absorbsion (Capacitors)https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20140008867.pdfhttps://nepp.nasa.gov/files/25847/2013-Taverovsky-paper-NEPPweb-MLCCsVabs-n264.pdfEMI Reductionhttp://www.ti.com/lit/an/szza009/szza009.pdfGeneral Guidehttps://www.analog.com/media/en/training-seminars/design-handbooks/Basic-Linear-Design/Chapter12.pdf

author: radioactive
author_rank: Regular Contributor
timestamp: February 02, 2020, 01:21:36 am
content: 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".