4608X-102-102 Datasheet, PDF (50/62 Pages) Bourns Electronic Solutions – Resistor Networks Product Selection Guide

Dual Terminator Resistor Network
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.
R2
R2
R2
R2
R2
R2
R1
R1
R1
R1
R1
R1
1
8
SIP
16
9
R1 R1 R1 R1 R1 R1 R1
R2
R2
R2
R2
R2
R2 R2
R1 R1 R1 R1 R1 R1 R1
2 R2
R2
R2
R2 R2
R2
1
8
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.
326
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.
Example:
Network 4608X-104-221/331 where R1 values are 220W and R2 values are 330Ω.
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) =
R1 + (R2 + Rp) 220 + (330 + 110)
146.67Ω
2% Tolerance ≈ ±1.96Ω*
RER2 = (P1-P2) =
R1 (R2 + Rp) = 3300 (220 + 110)
R1 + (R2 + RP) 330 + (220 + 110)
= 165Ω
2% Tolerance ≈ ±1.65Ω*
Rp = 550 = 110Ω
5
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
R1
Specifications are subject to change without notice.