Specifying Selected Op Amps and

　　It is not infrequent that commercially available standard IC components do not fit a particular application as they are specified. Often， however， a standard device selected to tighter limits will work. Thereupon， the IC manufacturer may be requested to supply a specially tested device. The usual chain of events for a selected part is as follows： A specification is sent to the manufacturer with a request for quote. It is evaluated at the manufacturer for feasibility， yield， and testing requirements. Then price and delivery are quoted to the customer. （Sometimes this route is shortened by calling the manufacturerÐbut this does not always work.） Some insight into the IC design and IC testing can help both the manufacturer and IC user with special selection. Proper specification helps the manufacturer test as well as reduce IC costs. Ambiguous or impossible specs will usually result in the return of the specification to the customer for clarification and delay the delivery of the required parts. The manufacturer is usually familiar with the product and production spread of devices. Further， test equipment is available for measuring parameters specified by the data sheet. In general， tighting selected data sheet parameters causes no problems. Further， no additional test equipment is needed for these testsÐonly the limits need be changed. Perhaps one of the largest problems is over-specification. Each tightened specification reduces the number of parts available to the specification. For example， tightening several specifications at once could result in a 1% or 0.1% yield; to supply 100 parts at this yield， between 10，000 and 100，000 parts might have to be tested， and that gets expensive.