4116R-R2R-503 datasheet(47/62 Pages) BOURNS

47 / 62 page

Specifications are subject to change without notice.

323

For designers developing high performance systems, exact

termination resistances which account for line loading may be

desirable. This resistance (or equivalent resistance) can be cal-

culated using the formula:

Rterm =

1 + Cd / Co

where Zo is the characteristic impedance of the line, Cd is the

total capacitance associated with the receiving devices (typically

5 pF per input gate) or other loads off the line, and Co is the

intrinsic capacitance of the line.

The series termination technique suppresses reflections at the

driving device should any waveforms be reflected back from the

driven end of the line. Series termination preserves power since

there is no current path to ground or Vcc as in the other meth-

ods. However, this technique results in incident signals that

transition slowly. It is also not appropriate for distributed loads

due to the half-amplitude waveforms which exist at intermediate

points along the line.

RC termination represents a compromise between power con-

sumption and effect on performance. Its principle of operation is

similar to parallel termination, but the capacitor blocks the DC

component of the signal, thus reducing power consumption.

However, the effectiveness of this method depends on the fre-

quency and duty cycle of the application. RC termination also

can be an expensive technique if implemented using discrete

components rather than a network.

Typical Usage

While not every address, data and control line may require

pull-up/pull-down or termination as part of the system’s design,

the table below shows common practice for some popular

devices.

MICRO-

MICROPROCESSORS

CONTROLLERS

68000

68020

80286 80386

68HC11

8051

No. address lines

Termination

No. data lines

Termination

No. control lines

Termination

Total I/O lines

Termination

702

701

NO. OF LINES

BOURNS P/N

PACKAGE

4608H-701-RC/CC

High Profile

4609H-701-RC/CC

Conformal

4610H-701-RC/CC

SIP

NO. OF LINES

BOURNS P/N

PACKAGE

4610H-702-RC/CC

High Profile

Conformal SIP

For all RC terminators, standard R values are 50, 68, 75 and 100

ohms. Standard values for C are 47, 100, 500 and 1000pF. See

data sheet to select custom combinations of R and C.

REFERENCES:

1. Blood, W.R., MECL System Design Handbook, Motorola, Inc., 1983.

2. F100K ECL Data Book, Fairchild Semiconductor Corp., 1986.

3. MECL Device Data, Motorola, Inc., 1987.

RC Terminator Networks 700 Series

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