Jesse, Thanks for your support. I've got a pretty thick skin, and pretty much agree with Olin anyway. But I thought I'd ask the list as a) I might get a solution b) it's an example of getting an additional feature "for free" c) It's an example of what can bite you when you try 2. above. FWIW, I did the product design, borrowing heavily on previous designs by others - including this feature. I think we've used this method for about 10 years now without anyone noticing a problem. It's not of grave concern as the problem only arises if the sensor is disconnected (which only happens if the cable is damaged or the sensor unplugged,) but it's of enough concern to want to get it put right. There's probably about 20k of these in the wild so far (2 years production) and we would expect another 4-5years life at increased production levels. Unfortunately we've just done a pcb update and there are no other major problems to fix that we know about. Richard P 2009/7/10 Jesse Lackey : > I don't see any need for you to flame the poster. =A0He did not ask for > your design review opinions. =A0What you consider "design error" others > may consider cleverly getting a feature for free. =A0The poster may not > have even designed the circuit, and is instead trying to get more life > out of something 10 years old. =A0You have no context to make your > assertion that he has to either rework or redesign: is this a first > proto or are there 5000 boards already produced? =A0What is the failure > rate of the last 10,000 sold in the last 10 years? =A0Maybe this trick has > worked great. =A0You don't know. > > Now I agree that this is a risky design, as is anything that counts on > an unspecified parameter that (in my limited knowledge of analog IC > circuitry) probably varies considerably with temperature and production r= un. > > There may well be no reasonable replacement and he will have to > redesign. =A0I consider it sensible to try get more life out of this (or > any existing, field-proven) design, if possible, knowing the risks of > using parts in unspecified ways, than just junking it on the principle > that anything not designed to all specs should never leave the shop. > That is a business decision that every engineer and company gets to make > for themselves. > > I would like PIClist to remain reasonably friendly for everyone to ask > questions, particularly somewhat unusual things like the OP did, without > getting flamed for doing a design (or trying to fix someone else's > design) that you dismiss out of hand as (apparently) so poor that they > need to be commanded to go fix, as though they are your junior employee. > > Tone matters. =A0Even in engineering. > > J > > > > > > > Olin Lathrop wrote: >> Richard Prosser wrote: >>> I'm looking for a cheap quad opamp with the same pinout as the LM324. >>> The killer is that I want one with a high input bias current. >> >> That's really bad design. =A0You're not going to find a opamp that speci= fies a >> minimum bias current. =A0If you need minimum current, put a resistor the= re. >> >>> I can add additional pullup resistors but that would mean a pcb change >>> and also (probably) retesting. >> >> That's the way it goes. =A0That's the price of a design error getting th= is far >> thru the process. =A0Either rework the existing boards or make new ones. >> Maybe you can add the resistor elsewhere, but kludging this with a opamp >> that appears to work from that production lot at your temperature is just >> going to make things worse. >> >>> So I'm looking for a cheap quad opamp ... with a minimum input >>> bias current >> >> Not gonna happen. =A0Go fix it the right way. >> >> >> ******************************************************************** >> Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products >> (978) 742-9014. =A0Gold level PIC consultants since 2000. > -- > 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