®
RCD16-47B
RCD NETWORK
FOR BUS TERMINATION
Application Specific Discretes
A.S.D.
TM
MAIN APPLICATIONS
In any electronic equipment where a suitable bus
terminationis requiredto avoid signal reflectionsand
distortions :
PC and workstation computer
Data-line analyzers
DESCRIPTION
With the increasing speed of data transmission,
line reflections provide signal distorsions and the
overshoots or undershootsproduced on the signal
edges can cause the malfunction of the whole
system.
To avoid these negative effects from leading to
problems, a suitable termination is required.
Dedicated to bus termination, the RCD16-47B
provides by far the best method to minimise stray
emissions from PCB tracks.
FEATURES
NETWORK OF 16R-C-D LINETERMINATIONS
RESISTANCE : R = 47
Ω
, TOLERANCE +/- 10%
CAPACITANCE: C = 33 pF, TOLERANCE +/- 10%
SCHOTTKY DIODE : (D)
BENEFITS
Provides impedance matching, thus increasing
noise immunity and minimizing distortion.
Lowers EMI / RFI radiation.
No DC power dissipation.
Eliminates negative voltages : no current will
change the bias of the protected device.
Uses the best of all termination schemes.
SO20
SSOP20
FUNCTIONAL DIAGRAM
1
2
20
19
3
4
5
6
18
17
16
15
7
8
14
13
9
10
12
11
COMPLIESWITH THE FOLLOWING STANDARDS:
MIL STD 883C - Method 3015-6
C= 100 pF
R = 1500
Ω
- V
PP
= 2 kV
- 3 positive strikes and 3 negativestrikes (F= 1Hz)
October 1998 - Ed : 2A
R = 47
Ω,
tolerance
+/− 10 %
C = 33 pF, tolerance +/- 10 %
D = Schottky diode
1/7
RCD16-47B
APPLICATION NOTE : BUS TERMINATION
With the increasing speed of data transmission (PC, TV, ...), engineersare naturally confrontedwith effects
that were of less significance with slower circuits. Among these are the effects described in transmission
line theory : line reflections provide signal distortions and the overshoots or undershoots produced on the
signal edges can finally cause the malfunction of the whole system.
1. Reflection at a non terminated line
The figure below shows the circuit of a transmission system in which a memory device (input impedance of
100 k
Ω
) is connected at the end of a line with line impedance Zo.
10
Ω
100 kΩ
If this line is not properly terminated, a certain amount of the energy is reflected back, inducing a reflection
phenomena that can distort the signal. This can result in improper operation of the system.
The simulation shown on the above figure illustrates the signal distortion producedby line reflection at the
end of the line which is not well terminated.
Even if the signal at the start of the line has the correct form (upper curve), considerable distortion arises at
the end of the line (lower curve). On the positive edge, the overshoot can exceed the maximum operating
voltage of the used circuit technology which will then be destroyed. Also, the following negativeundershoot
may reach a level low enough to change the value of the logic state. If it affects of an address line, a wrong
memory cell will be addressed, and in the case of a data line, the data can be corrupted.
This phenomena also occurs on the falling edge.
To avoid these negative effects from leading to problems in a system, a suitable termination is
required.
2/7
RCD16-47B
2. The RCD termination
The traditional solution to properly match each line of the bus consists of the use of several discrete
resistances, capacitors and small schottky diodes. For a 16-bit bus, this requires 48 discrete components.
SGS-THOMSON offers an innovating solution with a monolithic structure using
ASD
TM
technology(*).
The
ASD
TM
technologyenables to integrate monolithically 16 of these RCD ”basic cells” onto a single chip
device. The
RCD16-47B
reduces component cost and assembly cost, saves board space and improves
reliability.
The simulation illustrates the signal distortion produced by line reflection at the end of the line when such a
termination is used.
The resistor provides the path termination for PCB track, thus resulting in low reflection phenomena.
The capacitor of 33 pF blocks DC currents while acting as a short circuit during signal transitions, and
holds the bus at the last logic level. It reduces power consumption and avoids excessive current.
The small Schottky diode clamps the negative remaining undershoots which can result from impedance
mismatch. This damps negative voltages and prevents the logic signal from rising above the logic level
’0’ threshold after a falling edge.
The RCD termination provides optimal solution compared to all other termination techniques.
(*) ASD™ = Apllicaion Specific Discretes.
3/7
RCD16-47B
ABSOLUTE MAXIMUM RATINGS
(0°C
≤
T
amb
≤
70°C)
Symbol
P
I
F
V
RRM
V
PP
T
stg
T
j
Parameter
Total power dissipation per package
Continuous forward current per Schottky diode
Repetitive peak reverse voltage
Maximum electrostatic discharge
MIL STD 883C - METHOD 3015-6
Storage temperature range
Maximum junction temperature
Value
500
50
7.5
2
- 55 to + 150
150
Unit
mW
mA
V
kV
°
C
°C
THERMAL RESISTANCE
Symbol
R
th(j-a)
Parameter
Junction to ambient
Package
SO20
SSOP20
Value
100
140
Unit
°C/W
ELECTRICAL CHARACTERISTICS
Symbol
R
C
C
t
I
R
V
F
Parameter and test conditions
Connection resistance (note1)
Total capacitance
Leakage current
Forward voltage
T
amb
= 25°C
45
1
10
0.5
1
Typ.
Max.
0.25
Unit
Ω
pF
µ
A
V
F = 1 MHz, V
R
= 0V, V
RMS
= 30 mV
V
R
= V
RRM
I
F
= 1 mA
I
F
= 16 mA
T
j
= 25
°C
T
j
= 70
°C
T
j
= 25
°C
T
j
= 25
°C
Note 1 : Rc is the resistance between pin 1 and pin 11 or between pin 10 and pin 20
4/7
RCD16-47B
PSPICE MODEL per RCD CELL
L
3 nH
SO20
1.75 nH SSOP20
R1 3Ω
R3 9Ω
R2 47Ω
Cvar
C1 33pF
Cvar
Dschot
Dschot parameters
Cvar parameters
t
SO20 Package
Z magnitude versus frequency
Ω
Phase versus frequency
°
SSOP 20 Package
Z magnitude versus frequency
Ω
Phase versus frequency
°
5/7
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