Trap for (not so) young players - Page 1
Forum section: electronics / beginners

author: EPTech
author_rank: Regular Contributor
timestamp: August 02, 2016, 09:33:10 am
content: Hi There,I am just throwing this in here because I want to avoid others from wasting precious time chasing old goats.I got this sensor circuit on a bread board and on a PCB. It works on the bread board but not on the PCB. The issue lies with a sample and hold circuit which is in fact a switchable floating reference with large time constant, about 10s. The switch is a DG411 CMOS analog switch. When I switch the SH to hold, the thing droops (capacitor voltage drops) rapidly. I tried many things: comparing the circuits again and again, supply decoupling, trying different capacitors, different IC's, nothing worked. During the many tests, I noticed that even in sample mode, it would not track correctly when comparing the SH input and the output of the OPAMP. I swapped the DG411 for another and suddenly the circuit worked. But after soldering the components back on that I toke off to isolate the Sample and Hold circuit, it stopped working again. I gave the board a good clean to check for dry joints and do some more measurements and suddenly it worked again. Then it hit me. Big time palm to the face momentSo here is the trap: the gel type solder flux I used made a decent enough impedance path for current to leak out of the sample and hold capacitor. On the PCB the SMD components are closer together, so leakage will occur more rapidly, especially when it is a badly cleaned, hand soldered prototype. The through hole components on the solder-less bread board do not suffer from the leakage because there is no flux involved.

author: StillTrying
author_rank: Super Contributor
timestamp: August 02, 2016, 02:38:29 pm
content: You don't give the value of your sample cap, nor whether there's a buffer, butguard ringis what you might have to look into.http://www.edn.com/design/analog/4368681/3/Design-femtoampere-circuits-with-low-leakage-part-onehttp://electronics.stackexchange.com/questions/24852/implementing-guard-trace-ring-in-pcb-design

author: Buriedcode
author_rank: Super Contributor
timestamp: August 02, 2016, 06:36:00 pm
content: A few years ago I had a few boards (lithium charger, MCP73843) come back that would not start to charge a battery.Turns out the 'no clean' flux I used (cleaned with isopropyl alcohol), but clearly not enough) was enough to create a few meg resistance between the pads of an 0603 ceramic cap. This effectively meant the oscillator internal to the chip never started so the chip didn't start it's little statemachine. I removed almost all the components off the board, one by one, until I saw some dodgy looking flux under that cap - cleaned replaced the cap with a different flux and *didn't* clean the board afterwards, and it was bang on. Still lost 3 hours of my life scratching my head.So yes, be careful with the flux, and what you clean it with, and how you clean it, you can get residue mixed with your cleaning agent stuck under SMD passives and allow enough leakage to cause headaches.

author: EPTech
author_rank: Regular Contributor
timestamp: August 02, 2016, 07:26:35 pm
content: Quote from: StillTrying on August 02, 2016, 02:38:29 pmThank you for the info. Interesting article but I am not looking to avoid stray capacitance caused by a guard ring. A sample and hold implies that I want to have a capacitor on the input of the opamp. It is a good idea to implement a guard ring around the input stage for sure, so I may do that. Thanks for the info.