When second sources aren't

February 19, 2016 // By James Bryant
James Bryant, who was an applications manager for Analog Devices for 30 years, stresses that second-source components – almost by definition – not identical to the primary source and therefore present numerous ways to trip the unwary engineer.

[Q]: We have been manufacturing a system using an Analog Devices' component for many years. Recently our purchasing department bought a consignment of cheaper replacement devices from another company and the whole batch of systems using them failed to work. Every specification on their data sheet is the same as ADI's, and when we test them they are well within those specifications. What is happening?

 

[A] The operation of your system relies on some feature of our product that is not featured on the data sheet and is different in the second source.

This can apply to any component, not just ICs or active devices. In an entirely different context I keep a bottle of hot sauce in my kitchen to adjust the heat of curries and other hot dishes. I recently bought a new bottle - my usual brand was unavailable so I bought another with the same Scoville rating (i.e. the data sheet specification was the same). This second sauce was horrible - the heat was indeed the same, but it had much slower onset (first taste seems bland, but the tongue burns several seconds later, which may cause the cook to judge too soon and add too much) and, while my usual sauce just contributes heat, the second sauce has a bitter acrid taste in addition to its heat which spoils the carefully planned flavor of the dish. Obviously the flavor of electronic components is irrelevant, but other unspecified parameters may be critical.  

While I should like to tell you that all will be well if you remain loyal to Analog Devices - and that is probably true - it is quite important that you ensure that there is sufficiently close liaison between your purchasing department and system designers that when a change of component is proposed, for cost or any other reason, the new component is evaluated to ensure that it is truly