4416M-R2R-503 datasheet(50/62 Pages) BOURNS

50 / 62 page

Specifications are subject to change without notice.

326

The Dual Terminator (or Thevenin equiva-

lent) Network is commonly used for TTL

dual-line termination and pulse squaring or

ECL line terminations. In ECL line termina-

tor, R2 functions as an emitter pull-down

resistor and is normally tied to the most

negative supply voltage to provide proper

line currents. R1 is normally tied to ground

and functions as the termination resistor

and in parallel with R2 provides the charac-

teristic impedance of the transmission line.

This results in a zero reflection coefficient

of this line to eliminate reflections.

The Dual Terminator circuit is available in both

SIP and DIP configurations, as shown below.

Example:

Network 4608X-104-221/331 where R1 values are 220W and R2 values are 330

Ω.

Rp = Parallel Resistance of Remaining Circuit (See diagram below.)

RE = Equivalent Series - Parallel Resistance Seen by Unguarded Meter

RER1 = (P8-P2) =

R1 (R2 + Rp)

= 220 (330 + 110) = 146.67

Ω

R1 + (R2 + Rp)

220 + (330 + 110)

2% Tolerance

≈ ±1.96Ω*

RER2 = (P1-P2) =

R1 (R2 + Rp)

= 3300 (220 + 110) = 165

Ω

R1 + (R2 + RP)

330 + (220 + 110)

2% Tolerance

≈ ±1.65Ω*

Rp = 550 = 110

Ω

Utilization of these formulas will enable you to determine the equivalent unguarded

resistance to be expected from any values of R1 and R2 for a Dual Terminator Network.

*2% tolerance

≈ (RER1)2 X .02 = 1.96Ω

SIP

DIP

Testing of Dual Terminators

Since the Dual Terminator circuit has many

resistors in parallel, a direct pin-to-pin

measurement for the values of R1 and R2

can be made using an ohmmeter with

guard capabilities.

The function of the guard pin is to apply and

equal voltage across the adjacent

(parallel) resistance path. When applied,

current flow is eliminated allowing an accu-

rate measurement of the resistor under test.

Using the 8-pin SIP network shown, the

testing method would be as follows:

Test R1 Values

To test the first resistor, connect the ohm-

meter measurement leads between pin 8

and 2. Connect the guard lead to pin 1.

R1 is now guarded and an accurate

measurement can be made.

To test the second R1 resistor, connect the

measurement leads between pin 8 and pin

3. Connect the guard to pin 1 and make

the resistance measurement.

Continue this testing scheme for the

remainder of the R1 resistors, always

guarding pin 1.

Test R2 Values

To test the first R2 resistor, connect the

ohmmeter measurement leads between pin

1 and pin 2. Connect the guard lead to pin

8. The first R2 resistor is now guarded and

an accurate measurement can be made.

To test the second R2 resistor, connect the

ohmmeter measurement leads between pin

1 and pin 3. Connect the guard lead to pin

8 and make the resistance measurement.

Continue this testing scheme for the

remainder of the R2 resistors, always

guarding pin 8.

An example of the type of ohmmeter to be

utilized that incorporates a guarded mea-

surement capability. It must be noted that

guarded measurements using ohmmeters

are satisfactory measurements up to a ratio

of about 10:1 between R1 and R2. Above

a 10:1 ratio, accuracy is degraded and

measurements can be incorrect because of

inadequate guarding capability of the

equipment.

Unguarded Resistance Measurements

In the case where no guarded ohmmeter is

available, the individual resistors can be

evaluated by comparing the unguarded

resistance measurement to the theoretical

value of the equivalent series-parallel

circuit and determining the percent of error

of each resistor.

Dual Terminator Resistor Network

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