The same general principles of isolation of signal leads and shielding of sensitive circuits apply to both analog and digital systems There are, however, distinct differences in the techniques of interference suppression m digital and analog systems

Because of the higher signal levels and the high- frequency signals used m digital devices, standing waves, stray inductances and capacitances, RFI, and line propaga­tion delays may become problems if not carefully taken into consideration Many digital control signals have rise times in the nanosecond range, so there are frequency components in the hundreds of MHz range Connecting cables must be treated as transmission lines to pass this information from circuit to circuit

The points discussed in the following should be considered to provide suitable interconnections between units of high-speed digital systems

1 Cables carrying frequency components above 100 kHz should be terminated properly, і e, both ends having an impedance match between the cable and circuit input or output Ordinary wire has a characteristic impedance of about 150 ohms at frequencies where standing waves become a problem, so this is a good value to choose as a terminating resistor when an approximate first choice is called for If coaxial cable is used, the terminating resistor should match the cable impedance The output of the equipment must be able to supply the current to drive the characteristic impedance of the cable under continuous load conditions, if long-term, constant d-c signal levels are expected [Fig 10.24(a)]

2 Another method of terminating a data cable is shown in Fig 10 24(b) A Zener diode is used to clamp the reflected signal to ground and limit signal excursions above the desired input signal This circuit is useful in the control of standing waves where not enough continuous power is available to supply a low terminating impedance and a series of high-speed single-polarity pulses is to be sent along a line Figure 10 24(cl outlines the same technique except that the grounding diode is returned to a —0 7-volt line to

CABLE TERMINATION WITH TERMINATING RESISTOR

CABLE TERMINATION WITH GROUNDED DIODE

CABLE TERMINATION WITH SEPARATE RETURN LINE

Fig 10 24—Data system terminations

ensure that the reflected wave on the line is clamped to a value approaching zero.

3. The usefulness of a digital approach to instrumenta­tion derives partly from the fact that a signal is either off or on, and all modern digital systems have varying degrees of sensitivity to noise pulses in the circuit “Noise immunity” is usually defined as the minimum voltage difference between the higher O-voltage level and the lower 1 level, this typically ranges between 1 and 10 volts Because of this, millivolt or microvolt interference levels are of no conse quence, so many of the elaborate shielding and guarding techniques described in this chapter are not needed for digital systems Inputs to pulse-sensitive devices, such as some flip-flops, should be protected by shielding or other means against spikes on the input lines, however, the rapid rise time of an induced noise spike may cause spurious triggering even though it is less than the specified d-c noise immunity level [28]

(a) Data-System Power and Ground Connection. In

general, a small computer or other digital system may be connected to both power and ground by following standard electrical code procedures It is important, however, that the various cabinets of the system be connected together by a low-resistance bus, such as No 4 AWG copper wire, and the system be tied to a good earth ground The steel frame of the building in which the computer is housed is a good ground, or, if this is not convenient, a large water pipe is also a good ground. The ground wire should be run along with the signal cables when it is used to interconnect different portions of the system. A good a-c ground for a signal return is important As an example, the d-c resistance of a bond strap 0 002 to 0.003 in. thick, 1 in. wide, and 1 to 5 in. long would be negligible at direct current but would be about 0 1 ohm at 10 MHz and 15 ohms at 1000 MHz. This relatively large change in impedance with frequency indicates there is no substitute for a short grounding
connection with a large cross section and low self­inductance

The quality of the ground depends on the type of system since a self-contained system (one having no remote transducers or A/D converters) is more immune to noise than, for example, a system accepting thermocouple signals directly. For this reason all the interference-reduction ideas mentioned in this chapter should be applied to any system that has many small signal circuits