Digital Integrated Circuit Susceptibility

Abstract

The purpose of this paper is to define the susceptibility characteristics of integrated circuits and to present the results of a research program which investigated some of these characteristics. Integrated circuit technology has very rapidly progressed to the point where it commands a major role in the electronic industry. This rapid ascent has resulted in a definite lack of thorough information pertaining to integrated circuit susceptibility. A research program was initiated to determine the amount and validity of the available information and to obtain supplemental laboratory data.A brief introduction to integrated circuits is presented to establish a common reference for discussion and to allow easy interpretation of the test data. The basic operation of typical Diode-Transistor Logic (DTL), Transistor - Transistor Logic (TTL) , Current Mode Logic (CML) . and Metal-OxideSilicon-Field Effect Transistor Logic (MOST) circuits are discussed with emphasis on these factors that tend to increase their susceptibility. The next section of the paper deals with the DTL circuit which has been one of the most popular types of integrated circuits. Several DTL gates and some binary circuits from various manufacturers were evaluated on the basis of interference rejection capability on the input and on the signal common, which are the two most susceptible areas. The interference rejection measurements were conducted with the circuits subjected to several degrees of loading and over a wide temperature range. In most cases, the susceptibility spectrum measurements extend from DC to over 50 megacycles. The input/output voltage and current characteristics were also measured with respect to temperature and degree of loading. Attention is given to some of the improved DTL manufacturing methods such as replacing offset diodes with transistors for greater fan out, and the recent dielectric isolation technique. The time delay of the present family of DTL circuits is limited to approximately 10 nanoseconds due to the parasitic capacitances and the saturated mode of operation. To obtain switching speeds which would give time delays in the 5 nanoseconds region, a diferent type of logic such as CML must be used. CML circuits are a non-saturated design and exhibit susceptibility characteristics that are different from the DTL circuits. To allow easy comparison between the diferent logic types, the laboratory evaluation of the CML circuits was performed in the same manner as the DTL circuits. TTL circuits which utilize an emitter input have susceptibility characteristics that are different from either the DTL or the CML circuits. Again for the sake of comparison, the laboratory evaluation followed that of the two preceding circuit types. All of the DTL, CML, or TTL circuits have thresholds which are generally less that 1 .5 volts, which may be unsatisfactory for certain applications. In areas that require higher thresholds, a MOST type circuit can be utilized. The MOST integrated circuits are much aimpler than the other circuit types. The MOST characteristics, such as high thresholds and very high Input impedance, allow the circuits to perform very satisfactorily in the direct coupled mode. As would be expected, the susceptibility characteristics of the MOST circuits difer somewhat from the other circuit types. The last section of this paper is the most important one. All of the significant data on each circuit type is compared with the data from the other circuit types. This comparison illustrates several factors such as the affect of loading and temperature on the rejection margins and the dependence of the circuit threshold on the initial design. Other circuit factors that would influence a system operation are also discussed.