I don't see any need for you to flame the poster.  He did not ask for 
your design review opinions.  What you consider "design error" others 
may consider cleverly getting a feature for free.  The poster may not 
have even designed the circuit, and is instead trying to get more life 
out of something 10 years old.  You 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?  What is the failure 
rate of the last 10,000 sold in the last 10 years?  Maybe this trick has 
worked great.  You 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 run.

There may well be no reasonable replacement and he will have to 
redesign.  I 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.  Even 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.  You're not going to find a opamp that specifies a
> minimum bias current.  If you need minimum current, put a resistor there.
> 
>> I can add additional pullup resistors but that would mean a pcb change
>> and also (probably) retesting.
> 
> That's the way it goes.  That's the price of a design error getting this far
> thru the process.  Either 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.  Go fix it the right way.
> 
> 
> ********************************************************************
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> (978) 742-9014.  Gold level PIC consultants since 2000.
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