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AN1003
Termination Techniques for ECL / LVECL / PECL / LVPECL Devices
HIGH-PER.ORMANCE PRODUCTS
Different Termination Schemes
In this application note we will talk about the different ECL / PECL output termination schemes and AC coupling of the ECL / PECL inputs and outputs. Any signal path on a printed circuit board may be considered as a form of transmission line. If the line propagation delay is short with respect to the rise time of the signal then the reflections are masked out and are not seen as an overshoot or ringing; thus when the edge speed increases with faster forms of logic, the line lengths should be shorter in order to retain signal integrity. When high-speed signals are transmitted over long lines, terminations should be used to minimize reflections and signal distortion. These reflections cause ringing on the signal line, which, if severe, will affect system noise immunity. The reflections appear as overshoot and undershoot on the output waveform. In ECL systems, every output must be terminated matching the characteristic impedance of the transmission lines. Some of the most popular values of the transmission line impedance are 50 to 75 for multilayer etched boards, 100 for multi-wire boards, and 100 to 120 for wire-wrap boards. Standard prepackaged termination resistors are available with values of 50, 68, 75 and 100. In this application note, we will discuss four types terminations mentioned below: 1. Parallel Termination 2. Thevenin equivalent parallel termination 3. Series Termination 4. "Y" Termination
Parallel Termination
In this method, the ECL / PECL outputs are terminated with a termination resistor, RP, to a termination supply voltage of VTT = Vcc-2.0V. The value of Rp must be equal to the impedance of the transmission line, Zo. See Figure 1. If there is a mismatch, line reflections will be present with an increase in both noise and propagation delay. The placement of the termination resistors is important and they should be placed as close to their destination as possible. In this parallel terminated lines, the line termination supplies the output pull-down resistors; consequently, no pull-down resistors are required at the outputs of the driving gate. The advantage of using this method is that the average power consumption is reduced but on the flip side, it requires an additional power supply. Average current consumption and power dissipation may be of interest when using this termination scheme, Table 1 shows average current and power dissipation for different transmission lines.
Zo
RP
RP
Figure 1
VCC - 2.0 V
PDavg (mW) Zo 50 75 100 150 Rp 50 75 100 150 Iavg (mA) IC Output 14 9.3 7.3 5 14 9.5 7.3 4.9 Rp 13 9.1 7.1 5
Table 1
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AN1003
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Thevenin Equivalent Parallel Termination
This method does not require an additional power supply. Basically it requires one pull-up resistor between the output and VCC and a pull-down resistor between the output and Vee. The values of these two resistors, RP1& RP2, will change depending on the supply voltage range of the device. The scheme is very easy to implement and it is cost effective compared to parallel termination scheme; however, the power consumption on the external resistors are a little higher. The placement of these termination resistors must be very close to its destination for optimum performance. Equations 1 and 2, will allow us to calculate the values of these termination resistors based on the value of transmission line impedance. Refer to figure 2 for the schematic and table 3 for RP1& RP2 values for different supply voltage range and transmission line impedance. Table 2 depicts average current consumption and power dissipation for different transmission line impedance values using Thevenin equivalent parallel termination scheme.
RP1 = VEE * RT VEE - VTT (1) RP2 = VEE * RT (2) VTT
RP1 = 1.8*Zo() 90 135 180 270 RP2 = 2.25*Zo() 113 169 225 338 PD on RP1 and RP2(mW) 109 72.7 54.5 36.3
Zo() 50 75 100 150
Iavg (mA) 28.2 18.8 14.1 9.4
Table 2 Note: Numbers in Table 2 assumes VCC = 4.5V.
Supply Voltage +3.3V or -3.3V Zo () 50 75 100 150 RP1 127 190 254 380 RP2 83 124 165 248 Supply Voltage +5.0V or -5.0V RP1 83 125 167 250 RP2 125 188 250 375
Table 3
Conditions: * * * * RT = Zo, impedance of the transmision line VTT = VCC - 2.0V VEE = - 3.0V to - 5.5V, VCC = 0V or VCC = +3.0V to + 5.5V, VEE = 0V
Thevenin Equivalent Termination Scheme
Typical ECL/PECL Device RP1 Transmission Line Zo VCC RP1 Typical ECL/PECL Device
RP2
RP2 VEE
Figure 2
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HIGH-PER.ORMANCE PRODUCTS
Series Termination
Using series damping or series termination technique may control overshoot and ringing on longer transmission lines. Series damping is accomplished by inserting a small resistor Rs in series with the output of the gate as shown in figure 3. In this case the value of RT is such that it can drive 5-15mA of current. It is mandatory that RS added to the output impedance must be equal to Zo. Signal transmitted from point A is reflectedat point B. But due to the presence of RS (RS+ROUT=Zo), this signal is not seen at B. The advantage of this method is that the power does not increase as much as in the parallel termination method even when multiple lines are connected to A. Series Termination scheme does not require an extra power supply for termination. It is cost effective, easy to implement, absorbs any reflections returning to source, and is ideal for Back-plane designs, or any designs with impedance discontinuities. The driven inputs must be near the end of the line to avoid receiving a 2-step signal. Table 4 depicts average current consumption and power dissipation for different transmission line impedance values using series termination scheme.
Pdavg (mW) Zo () 50 75 100 150 RP () 269 399 530 791 Iavg (mA) 9.8 7.9 4.9 3.2 IC Output 12.9 8.6 6.5 4.2 RT () 25.8 16.8 12.7 8.1
Table 4
Some recommended values for RT at different supply voltages are:
* *
RT = 150 for Vcc = 3.3V RT = 270 for Vcc = 5.0V
Rs = 43 for Zo = 50 and Vcc = 3.0V to 5.5V
Typical ECL/PECL Device Series Termination: Pull-down resistor to VEE
Typical ECL/PECL Device RS
Transmission Line
A
B
Zo = 50
RS RT RT
Figure 3
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HIGH-PER.ORMANCE PRODUCTS
"Y" Scheme Termination
Another alternative would be the "Y" scheme termination as shown in figures 4 and 5. The advantage of using "Y" scheme termination is that it does not require an extra power supply for termination, it is cost effective and easy to implement. This scheme is applicable only to sources with differential outputs.
"Y" Termination Scheme for 5.0 V Power Supply
Typical ECL/PECL Device
Transmission Line
Typical ECL/PECL Device
Zo
R1 = Z o R1 = Zo
0.1F Figure 4
R2 = 2Zo
VCC
"Y" Termination Scheme for 3.3 V Power Supply
Typical ECL/PECL Device
Transmission Line
Typical ECL/PECL Device
Zo
R1 = Zo R1 = Zo
Figure 5
0.1F
R2 = Zo
VCC
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HIGH-PER.ORMANCE PRODUCTS
Capacitive Coupling Method
Some design applications require the ECL or PECL signals to be AC coupled due to nonstandard ECL signal levels coming from a source. In addition, capacitive coupling is a preferred way of interfacing either a signal from a 3.3V system to a 5.0V device or even interfacing ECL signals with PECL devices. Examples below show some of the suggested solutions for the AC coupled ECL or PECL type signals. When the destination device does not provide a VBB output pin for proper biasing when its inputs are AC coupled, the following scheme shown in figure 6 can be used. * Values of R1 and R2 are determined based on the supply voltage range. * RPulldown =150 for VCC = 3.0V and 270 for VCC = 5.0V * R1 = 68.5 for VCC = 5.0V and 85 for VCC = 3.3V * R2 = 185 for VCC = 5.0V and 122 for VCC = 3.3V When the destination device does provide a VBB output for proper biasing of its inputs in capacitive coupling mode, the solution in figure 7 should be used for proper signal interface.
VCC
R1 R1
Typical ECL/PECL Device
C = 0.1F
Typical ECL/PECL Device
Transmission Line
Zo
C = 0.1F
Pulldown
R2
R2
Figure 6
R
VEE
Typical ECL/PECL Device
C = 0.1F
Typical ECL/PECL Device
Transmission Line
Zo
C = 0.1F
R = 50 R = 50
R
Pulldown
VBB
C = 0.1F
Figure 7
VCC
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HIGH-PER.ORMANCE PRODUCTS
Pseudo Random Data in AC Coupled Environment In applications where serial data does not have continuous transition from High to Low for a long period of time, the inputs of the receiver may reach the same potential level or null-state; in turn, this would make the system unstable. To prevent this problem, a high value resistor should be connected between the input of the receiver (usually the true input) and VCC. Refer to figure 8 for further illustration.
VCC Typical ECL/PECL Device
R1 = 20K to 100K
C = 0.1F
Typical ECL/PECL Device
Transmission Line
Zo
R = 50 R = 50
R
Pulldown
C = 0.1F
VBB
C = 0.1F
Figure 8
VCC
Contact Information
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Semtech Corporation High-Performance Products Division
Marketing Group 1111 Comstock Street Santa Clara, CA 95054 Phone: (408) 566-8776 FAX: (408) 727-8994
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