Rob, What's keeping the non-inverting input (of the TL072) from drifting off into a rail? I didn't hear anything in your description that sounded like a DC path. Dave Rob Robson wrote: >The op amp non-inverting input is coupled to the output of an LM1972 digital >attenuator via a 1uf electrolytic. This part of the circuit is identical to >one that has been in use flawlessly for years. The inverting input is tied >directly to the output, which feeds a 5532 summing amp via a 0.1uF ceramic >cap. This part is different from the old circuit in that the >now-problematic op amp also fed a 5532 buffer amp with a Z in of 2.7k via a >10uF electrolytic. I realize my LF cut-off through the summing amp is >higher now due to the smaller coupling cap, but this is desirable in this >application. All traces are short. The input of the digital pot (also via >a 1uF electrolytic) does go off-board to another board within the unit, but >this was also the case in the working legacy system. Would an input current >limiting resistor really make a difference on a JFET input op-amp with its >characteristically high Z-in? Would this unity-gain buffer be more stable >if there was a nonzero resistance in the feedback loop? > >I'll try to increase only the negative rail as you've suggested and see what >happens. The time it takes for the fault to appear (approx 22 hours) is >slowing down the troubleshooting process, but I have several units to play >with. > >RR > > >----- Original Message ----- >From: "Dwayne Reid" >To: "Microcontroller discussion list - Public." >Sent: Wednesday, January 25, 2006 11:17 AM >Subject: Re: [EE]: op amp latch-up problem > > > > >>At 10:44 PM 1/24/2006, Rob Robson wrote: >> >> >>>I have an LM1972 uPOT on +/-5V rails feeding a TL072 on +/-12V rails >>>wired as a unity gain buffer as per the LM1972 datasheet. I've used >>>this circuit for years with a linear power supply without any >>>problems, but when the circuit was ported to a system with a >>>(noisier) switching power supply, the TL072 would latch up after >>>only a few hours of operation . First problem: I had put the TL072 >>>on +/-5V rails without consulting its spec sheet (too low, as it >>>turns out), so I substituted the more forgiving LF353. The latch-up >>>condition would still occur after several hours of operation. >>> >>> >>Which latch-up condition do you refer to? There are two: SCR type >>latch-up where the chip draws significant current *or* >>phase-inversion latch-up where the output of an op-amp swings hard to >>one rail because one or both inputs went out of the common-mode voltage >>range. >> >>I'm assuming a phase-inversion type latch-up but it would be useful >>to have that confirmed. >> >>Like Bob, I've been using the TL07x family for a couple of decades, >>in at least a hundred different products (probably way more than 100) >>without incident. >> >>The TL07x family does include the positive supply rail within the >>common-mode voltage range. Can you try adding an external negative >>supply such that the negative rail is at least 3V lower than the >>lowest expected signal? I'd suggest starting off at -8V or >>-9V. Leave the + supply rail at 5V. Then let the thing run and see >>if the problem occurs. >> >>Are the TL072 inputs exposed directly to the outside world? What >>signal range are you expecting? >> >>dwayne >> >>-- >>Dwayne Reid >>Trinity Electronics Systems Ltd Edmonton, AB, CANADA >>(780) 489-3199 voice (780) 487-6397 fax >> >>Celebrating 21 years of Engineering Innovation (1984 - 2005) >> .-. .-. .-. .-. .-. .-. .-. .-. .-. .- >> `-' `-' `-' `-' `-' `-' `-' `-' `-' >>Do NOT send unsolicited commercial email to this email address. >>This message neither grants consent to receive unsolicited >>commercial email nor is intended to solicit commercial email. >> >>-- >>http://www.piclist.com PIC/SX FAQ & list archive >>View/change your membership options at >>http://mailman.mit.edu/mailman/listinfo/piclist >> >> >> > > > > -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist