Analog Voltages Traveling on a PCB - Page 1
Forum section: electronics / projects

author: mclemens1969
author_rank: Contributor
timestamp: April 01, 2017, 04:31:24 pm
content: I am working on a small heater control project. In several places I want to bring analog voltages across a PCB (and across a flat flex cable) over to my microcontroller for ADC measurement. This trip is maybe 3 inches or less. These are very slow moving DC voltages. I was wondering what best practices are for situations like this. Are there PCB layout tips or cable signal ordering tips, which can lead to higher fidelity measurements.Thanks for any insightsMike in Alaska

author: matkar
author_rank: Regular Contributor
timestamp: April 01, 2017, 05:39:08 pm
content: In general avoid routing fast changing (digital) or high current tracks in parallel with your analog tracks.The same applies to flat cable. Place a GND track between analog and digital lines.

author: max_torque
author_rank: Super Contributor
timestamp: April 01, 2017, 07:18:18 pm
content: There are two issues with "long" analogue signals:1) Noise pick up - as you say your signals are "Low frequency" then you can simply put a suitable low pass filter in front of your ADC, and hence reject any high frequency noise. Of course, a suitable Anti-Aliasing filter is always a good idea in front of an ADC anyway2) The impedance of the trace carrying the signal - again, maybe an issue, maybe not. if your signal is of a very low impedance adding more track length may skew the result. For example it would be a bad idea to put say a 5ohm trace in series with something like a 100Ohm PTC thermistor etc. In this case you would need to add a remote buffer close to the actual sensor, to output a suitable signal back to the ADC. Flat flex pcbs generally have a higher specific impedance because they use very thin copper plating.What are your signal sources?

author: mclemens1969
author_rank: Contributor
timestamp: April 02, 2017, 05:50:19 pm
content: Thank you both for your replies.> What are your signal sources?My signal sources are:1) The incoming power supply DC voltage (7V-25V DC). This would be divided down to the ADC range. Should I bring this in close to the uC and then divide it? Or divide it far away and bring the low voltage over?2) The VIOUT output of an Allegro current sensor:http://www.allegromicro.com/~/media/Files/Datasheets/ACS722-Datasheet.ashxThe data sheet lists the output as:Output Capacitance Load CL VIOUT to GND – – 10 nFOutput Resistive Load RL VIOUT to GND 4.7 – – k?Mike

author: max_torque
author_rank: Super Contributor
timestamp: April 02, 2017, 06:24:02 pm
content: Assuming you are going to use a basic, high impedance resistive divider to get the supply voltage down into the ADC range than you are better doing this close to the micro (assuming your supply voltage is fairly clean). Again, assuming that supply voltage doesn't change very fast, then you can use a high impedance divider, and then a small capacitor to give the ADC a nice charge buffer to work with., which will also form a nice low pass filter.The ASC output is harder, but again, depends on the bandwidth of the signal you want to digitise ie how fast do you want to measure the current sensor output. You'll certainly want to keep the signal path away from any switching or high current tracks if at all possible (cross at right angles where necessary, don't run parallel to those tracks!) Depending on the layer count of your PCB this will either be very easy or quite hard......

author: mclemens1969
author_rank: Contributor
timestamp: April 02, 2017, 07:17:01 pm
content: Hello, thanks!I only intend to read the current level once a second at maximum.