STOP and THINK don't waste time

We had a case recently where two methods of analysis didn't agree.  This is normally a time to stop and think.  However, "customer pressure" caused us to lurch forward with a wild-guess fix.  Only I blew the whistle, yes I can be a bummer at work.  I get paid way too much to deal with this sort of thing.  When different measurement methods conflict, there is information to be had!  STOP and THINK.

Circuit analysis can be done in a few different ways.  In most normal situations, a bad circuit is bad, no matter how you analyze it.  For example, we have both AC and Transient analysis in a circuit simulator.  The AC analysis uses linearized elements based on the circuit and its operating condition.  This analysis normally is used to determine stability, gain and bandwidth of a circuit.  The transient analysis is easier for non experts to understand.  Its a time-domain simulation of the circuit which is equivalent to having an oscilloscope probe available to monitor every point in time on the circuit.  The way the waveforms "wiggle" is based on the inputs to the system as well as the properties of the circuit such as gain and bandwidth.

If you have a case where the AC analysis says your circuit is really fast and functional, and the transient analysis shows a "flat line" or no response, then you have an issue.  The correct thing to do in this situation is to determine WHY the two analysis conflict.  Fast vs. dead?  Both can't be true.  One analysis says the circuit is fast, the other says it doesnt work. Never in this situation is it worthwhile to "fix" the dead circuit or "force it" to work.  Don't bother until you understand why the two simulations give different results.  Any other activity is a waste of time, unless you hate your job and just want to burn time.  I discussed this with a circuit expert at Maxim today with violent agreement.

 A real-world example of this is op-amp slew.  This is a nonlinear phenomena when a circuit does not have enough current to charge and discharge the junctions in the circuit fast enough.  A linearized "AC" analysis will show health.  However, a transient analysis would show a response in slow motion.  What this means, is that for small signals the circuit is fast, but for larger "wiggle" the circuit is starved for current.  In this case, you conclude you "didn't use enough current" in the main design.  Once you allow the circuit to have the current it requires, the two response should match.  Your circuit was broken.  When a linear analysis does not match a transient analysis, 9/10 times it has to do with something nonlinear in the system.  The rules of superposition no longer apply.  Search for the nonlinearity and you will find your problem. 

In conclusion, when different methods of analysis don't agree, you need to STOP and THINK about why they don't.  That is enough information in itself.  Don't assume a fix for something you don't understand, or you will fix it again.. and again.... and again... and again...