4

I have designed a product that uses a T.I. ADS1230 20-bit Delta-Sigma ADC.

I've assembled 20 prototype boards. Two of those boards are having completely insane oscillations (+30/-30 raw values) with inputs connected to a stable VREF/2. I've changed all the analog components to make sure that none is faulty, but the problem remains. The other 18 boards works as expected.

I'm trying to find an explanation of the problem but so far I got nothing. After reading some forums of T.I. one of the engineers commented that flux residue that has not been properly cleaned after PCB assembly can impact sensitive circuitry.

Does anyone can confirm that?

P.S.: I always wash all the PCBs after assembly using isopropyl alcohol and a ESD brush to remove all soldering/flux residue.

RHaguiuda
  • 1,141
  • 1
  • 13
  • 26
  • It's certainly one possibility. If so, cleaning the boards again may help. Also ... Delta-Sigma ... DC ... add a tweak to vary the DC VREF/2 by a few mv. It used to be that some D-S converters were prone to limit cycle oscillations dependent on the precise input value. You may be able to make these stable with a few mv change ... and some of the others unstable likewise. –  Sep 03 '15 at 20:22
  • http://mathews-engineering.com/ArticlesByTomMathews/CleanPCBs_2.pdf – Matt Young Sep 03 '15 at 20:24
  • Re: limit cycles : see http://electronics.stackexchange.com/questions/26450/what-does-limit-cycle-mean-in-delta-sigma-modulator and https://www.eecs.qmul.ac.uk/~josh/documents/ReissSandler-IEEETCAS2008.pdf or http://www.eecs.qmul.ac.uk/~josh/documents/PerezReiss-AES122.pdf –  Sep 03 '15 at 20:28
  • 1
    This is just general debug advice if you can't figure anything else out. Take some good boards and some bad boards, and swap the ADS1230 components from good to bad. Then check to see if good boards become bad and bad boards become good or (as is sometimes the case) it is inconclusive. If the bad boards stay consistently bad, at least you know the problem is on your board. But if the problem tracks with the ADC, then you know it is related to some manufacturing parameter or something. Could even be bad ADC's! – user57037 Sep 03 '15 at 21:32
  • How are the inputs connected to the reference? Have you tried a solder blob between the AINP and AINN pins directly on the chip to be sure it isn't something up-steam? Also it might be useful to log the oscillations and have a look at the frequency spectrum. – Jon Sep 04 '15 at 08:49

0 Answers0