A method for relaxing tolerance requirements in RC filters

In an adjustable assembly, the problem is to reduce overall adjustments from many high-precision adjustments to a few high-precision and larger number of low-precision adjustments. The present application is the high-volume manufacture of RC active filters. The Lopresti method for filter tuning is used. The author presents a method for mathematically selecting the minimum number of resistors that need to be reserved for fine tuning. In thin-film hybrid ICs serpentine trimmed resistors are useful for area savings. In this case, the resistor track is created in a rectangular block of film material by a laser. But these must be trimmed twice to protect against aging degradation. All resistors are first trimmed to 90% of their nominal design values. The substrate is then baked to stabilize the resistors. The bake operation will usually raise all resistors by a value from 0 to 2%. The circuit is then fine tuned using laser trimming to raise all resistors to the optimum value calculated by the Lopresti tuning algorithm