TERMINATORS Khodifad Pankaj

eInfochips Training and Research Academy

Sola, Ahmedabad

Abstract

When does a system need terminating resistors? According to theory, there are two cases:

the long reflection case and the short ringing case. When a line is long, meaning the cable length

exceeds one-sixth of the electrical length of a rising edge, the cable needs terminators. Without

terminators, reflections at either end of a long cable render signal transmission impossible.

When a line is short, it may still need terminators if it drives a capacitive load. By analyzing

highly inductive circuits which are capacitively loaded, showing the effect of high-Q ringing. The

ringing phenomenon in short lines has the same practical effect as reflections in long lines.

I. Introduction

Characteristic impedance is the ratio of voltage to current in a transmission line. The instant

a signal propagates the transmission line, the input impedance of the PCB trace looks resistive.

The PCB trace impedance is a value that needs special attention from a designer. It is difficult to

match the load and the source in complex designs. But in theory a perfectly matched transmission

line will be devoid of any reflections from the far end and thereby will not have any reflected

signal causing an addition or subtraction to the original signal at the near end and so on. Reflections

can cause ringing of the signal. If the signal has enough time to settle to its final value after all the

reflections before the next transition the design will not encounter issues. If the time is not adequate

then there might be an error to the value that is found due to the reflections [1].

Reflections on a transmission line can be avoided by using terminations to the transmission

line. The most popular terminations used in the digital logic are either end termination or series

termination. End terminations use a resistor or split a pair of resistors which connect to VCC and

ground respectively with the line as the reference point. Figure 2 shows the end termination

topology on a transmission line near the load with a split resistor termination.

II. End Terminators

When using end terminations, each driving gate connects directly to its transmission line,

with the terminator located at the receiving end (see Figure 1). End-terminated lines have these

properties:

(1) The driving waveform propagates at full intensity all the way down the cable.

(2) All reflections are damped by the terminating resistor.

(3) The received voltage is equal to the transmitted voltage.

A. Rise Time of End Terminators

Figure 1. Calculating the rise time of an end terminator

We may deduce the signal rise time of an end-termination circuit by intuitive reasoning or

by more detailed mathematics. We shall attempt the intuitive approach first and then double check

using detailed math.

Our intuitive approach splits the circuit in Figure 1 into two parts. The left part, or driving

part, is composed of the driving gate, the transmission line, and the terminating resistor. We can

model the Thevenin equivalent driving impedance for this part of the circuit as the impedance of

a long transmission line 4, in parallel with the terminating resistor (also Z0). The net effect, for

short-term events, is a drive impedance of Z0/2ohm[2].

The right part, or receiving part, includes only the receiving gate, modeled in Figure 1 as a

capacitor. This capacitive model is appropriate for most CMOS, TTL, or ECL situations.

Recognizing this circuit as a simple RC filter, we know the RC time constant:

For the 10-90% rise time of an RC filter.

Given an incoming signal with a rise time of Ti, we combine it with the rise time of the

termination circuit Term to find the resulting actual rise time at point B:

B. DC Biasing of End Terminator

The termination circuit in Figure 1 rarely appears in TTL or CMOS circuits because of the

large drive current required in the HI state. When the driving gate in Figure 1 switches its output

to Vcc, it must supply a current of Vcc /RI to the terminating resistor. When the driving gate

switches its output to ground, no output current flows. Assuming we are using typical 65-0

transmission lines, the current required for a 5-V drive signal is 5/65 = 76 mA. Very few drivers

can source that much current.

Compare this drive requirement with the drive capabilities of TTL, which sources much more

current when driving LO than when driving HI, or CMOS which sources equal amounts of current

in both directions.

Figure 2 shows a popular terminating arrangement called the split termination. In this arrangement,

the parallel combination of R1 and R2 equals Zo, the characteristic impedance of transmission line

A. The ratio Ri/R2 controls the relative proportions of HI and LO drive current.

Figure.2 Split termination

III. Source Termination

The source termination scheme connects each driving gate through a series resistor to its

transmission line. The value of the series resistor plus the output impedance of the driving gate

should equal Zo, the characteristic impedance of the transmission line. The reflection coefficient

at the source will then be zero. See Figure 3 [3].

Figure 3. Source Termination

The driving waveform is cut in half by the series-termination resistor before it begins

propagating down the line.

The driving signal propagates at half intensity to the end of the line.

At the far end (an open circuit) the signal reflection coefficient is +1. The reflected signal

is half-intensity. The half-sized reflection plus the original incoming half-sized signal

together bring the signal at the receiving end to a full level.

The reflected signal (half-sized) propagates back along the line toward the source, where

it damps out at the source termination.

After the end reflection returns to its source, the drive current drops to zero where it remains

until the next transition. In fast systems, the next transition starts before the end reflection

returns.

IV. Middle Terminators

Sometimes an engineer ties together a big hair-ball network of gates without thinking about

terminations. The length of a rising edge may be much shorter than the size of the network. This

problem is exacerbated by tri-state drivers, for which there is no well-defined source or destination.

Intuition tells us that each transmitted step will rattle around in the wiring for quite some time

before settling down.

If devices connected to the network require monotonically rising edges, we are in trouble.

There is in general no way to fix such a problem, short of slowing down the rising edges (or

filtering the received signal). If the network is sampled in time, we can arrange for the sampling

time to wait until the net settles after each transition before sampling it. In the sampled case, we

just need to reduce the settling time, not eliminate it.

There are at least four approaches to this problem:

(1) Add a source terminator to every driver.

(2) Add an end terminator at each receiver.

(3) Add a shunt termination in the middle of the network.

(4) Add series resistance between every juncture of branches.

Figure 4. Middle Terminators

Option (1) is well defined, takes little power, provides a little bit of damping, and reduces

the settling time.

Option (2) requires a lot of drive power but works well in star configurations. A star

configuration has a discrete wire leading from each active circuit to a single point where

all wires tie together. Reflections are confined to the segment between the source and the

central connection.

Combining options (1) and (2), while wasting even more power, is a perfect solution to the

star problem. Unfortunately, that configuration attenuates each signal as it passes through

the central star. There are no reflections, but the received signal levels are very small.

We don't know why people use option (3). It just lowers the impedance of the central part

of the network, where it is already too low.

Option (4) attenuates the signal at every juncture. Using the circuit in Figure 4, the signal

attenuates by one-half as it passes through each juncture. This damps out reflections

quickly (the round-trip attenuation is one-fourth) but also cuts down the signal level

severely when the signal goes through many junctures. Constraining the system to perhaps

no more than three series junctures, we can easily arrange receivers sensitive enough to

tolerate that much attenuation.

V. AC Biasing for End Terminators

Capacitors are sometimes incorporated in end-termination circuits to reduce the quiescent

power dissipation. Consider the two circuits in Figure 5.The time constant RIC is chosen very

large compared to the signal clock time.

Figure 5. AC Biasing for End Terminators

If we can guarantee that the drive circuit spends half its time in each state (we call such a

circuit DC-balanced), the average value accumulated on capacitor CI will be halfway between the

HI and LO voltages. Resistor R1 will then have a voltage magnitude of AV/2 continuously

impressed upon it.

VI. Resister Selection

A terminating resistor should reduce or eliminate unwanted reflections on a transmission

line. It can perform this function only when its resistance value matches the characteristic

impedance of the transmission line.

Add the uncertainty in terminating resistance to the uncertainty in transmission line

impedance to calculate the total worst-case terminating mismatch. The resulting total is divided by

2 to find the expected reflection percentage. The transmission line impedance is usually less certain

than the terminating resistor value. Knowing, for example, that the transmission line is likely to

vary by ±10%, most designers would specify terminating resistors with a 1% tolerance.

If signal fidelity is of the utmost importance, consider using both source and end

terminators. This procedure cuts the received signal level in half but reduces reflections

dramatically. Any reflecting signal must bounce off both source and destination ends, squaring the

effective reflection coefficient. The tolerance required for termination matching at either end is

greatly relaxed. This approach is used extensively in microwave circuitry to improve gain flatness

over wide frequency ranges. In digital electronics, this double-termination technique is used only

in conjunction with line receiver components capable of discriminating reduced-size receive

signals.

VII. Conclusion

Specify both a resistance value tolerance and a power rating on terminating resistors.

Parasitic inductance in terminating resistors causes unwanted reflections. Combination RC circuits

can terminate DC-balanced lines with no wasted quiescent power. Source terminators have a

slower rise time and usually smaller residual reflections than end terminators. Do not daisy-chain

receivers on lines having source terminators.

Subtract from the ideal source-termination value the output impedance of your driver. At

low-pulse repetition rates, source terminators dissipate little power. The peak drive power for a

source-terminated line and an end-terminated line (biased at the halfway point) are the same.

VIII. Reference

1. High Speed Digital Design Principles, January 2009 by Sathish Venkataramani, Technical

Marketing Engineer, Intel Corporation

2. High Speed Digital Design by H.W.Johnasan

3. http://www.sabritec.com/technotes/PDF/High_Speed_Digital_Tutorial.pdf