AN-903| Application Note

A Comparison of Differential Termination Techniques

A Comparison of Differential Termination Techniques
Introduction
Transmission line termination should be an important consideration to the designer who must transmit electrical signals from any point A to any point B. Proper line termination becomes increasingly important as designs migrate towards higher data transfer rates over longer lengths of transmission media. However, the subject of transmission line termination can be somewhat confusing since there are so many ways in which a signal can be terminated. Therefore, the advantages and disadvantages of each termination option are not always obvious. The purpose of this application note is to remove some of the confusion which may surround signal termination. This discussion, however, will focus attention upon signal termination only as it applies to differential data transmission over twisted pair cable. Common differential signal termination techniques will be presented and the advantages and disadvantages of each will be discussed. Each discussion will also include a sample waveform generated by a setup consisting o
f a function generator whose signals are transmitted across a twisted pair cable by a differential line driver and sensed at the far end by a differential line receiver. This application note will specifically address the following differential termination options: Unterminated Series/Backmatch Parallel AC

National Semiconductor Application Note 903 Joe Vo August 1993

01189801

FIGURE 1. Unterminated Configuration Since, TIA/EIA-422-A (RS-422) standard defines the DS26LS32A's minimum input resistance to be 4 k, the driver's worst case load, as seen in Figure 1, is then 4 k. In the unterminated configuration, the DS26LS31 driver is only required to source a minimal amount of current in order to drive a signal to the receiver. This minimal DC current sourcing requirement in turn minimizes the driver's on chip power dissipation. In addition, the 4 k driver output load results in a higher driver output swing (than if the driver was loaded with 100) which in turn increases DC noise margin. This increase in noise margin further diminishes the possibility that system noise will improperly switch the receiver. To be sure that there is no confusion, noise margin is defined as the difference between the minimum driver output swing and the maximum receiver sensitivity. On the other hand, if a receiver was used which complies to TIA/EIA-485 (RS-485), the resulting noise margin would be even gr
eater. This is because the minimum input resistance of an RS-485 receiver must be greater than 12 k as compared to 4 k for an RS-422 receiver. The absence of a termination element at the DS26LS32A's inputs also guarantees that the receiver output is in a known logic state when the transmission line is in the idle or open line state (receiver dependent). This condition is commonly referred to as open input receiver failsafe. This receiver failsafe (Note 1) bias is guaranteed by internal pull up and pull down resistors on the positive and negative receiver inputs, respectively. These pull up and pull down resistors bias the input differential voltage (VID) to a value greater than 200 mV when the line is, for example, idle (un-driven). This bias is significant in that it represents the minimum guaranteed VID required to switch the receiver output into a logic high state.
Note: A complete discussion of receiver failsafe can be found in Application Note 847 (AN-847).

Power (Failsafe) Alternate Failsafe Bi-Directional For the purposes of discussion, popular TIA/EIA-422 drivers and receivers, such as the DS26LS31 and DS26LS32A, will be used to further clarify differential termination.

Unterminated
The selection of one termination option over another is oftentimes dictated by the performance requirements of the application. The selection criteria may also hinge upon other factors such as cost. From this cost perspective the option of not terminating the signal is clearly the most cost effective solution. Consider Figure 1, where a DS26LS31 differential driver and a DS26LS32A differential receiver have been connected (using a twisted pair cable) together without a termination element. Because there is no signal termination element, the DS26LS31 driver's worst case load is the DS26LS32A receiver's minimum input resistance.

There are, however, some disadvantages with an unterminated cable. The most significant effect of unterminated data transmission is the introduction of signal reflections onto the transmission line. Basic transmission line theory states that a signal propagating down a transmission line will be reflected back towards the source if the outbound signal encounters a mismatch in line impedance at the far end. In the case of Figure 1, the mismatch occurs between the characteristic impedance of the twisted pair (typically 100) and the 4 k input resistance of the DS26LS32A. The result is a signal reflection back towards the driver. This reflection then encounters another impedance mismatch at the driver outputs which in turn generates additional reflections back toward the receiver, and so on. The net result is a number of reflections propagating back and forth between the driver and receiver. These reflections can be observed in Figure 2.

AN-903

2002 National Semiconductor Corporation

AN011898

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