HD26LS31 Datasheet, PDF (9/15 Pages) Hitachi Semiconductor – Quadruple Differential Line Drivers With 3 State Outputs

HD26LS31
An example of termination resistance connection when the HD26LS31 is used as a balanced differential
type driver is shown.
Q₂
Q₁
V_CC
Q₃
R₁
9Ω
Output
Q₄
Figure 2 Output Stage Circuit Configuration
When termination resistance RT is connected between the two transmission lines, as shown in figure 7 the
current path situation is that current IOH on the side outputting a high level (in this case, the Y output)
flows to the side outputting a low level (in this case, the Z output) via RT, with the result that the low level
rise is large.
If termination resistance RT is dropped to GND on both transmit lines, as shown in figure 5 the current
path situation is that the current that flows into the side outputting a low level (in this case, the Z output) is
only the input bias current from the receiver. As this input bias current is small compared with the signal
current, it has almost no effect on the differential input signal at the receiver end.
Figure 6 shows the output voltage characteristics when termination resistance RT is varied.
Also, when used in a party line system, etc., the low level rises further due to the receiver input bias
current, so that it is probably advisable to drop the termination resistance to GND.
However, the fact that it is possible to make the value of RT equal to the characteristic impedance of the
transmission line offers the advantage of being able to hold the power dissipation on the side outputting a
high level to a lower level than in the above case.
Consequently, the appropriate use must be decided according to the actual operating conditions
(transmission line characteristics, transmission distance, whether a party line is used, etc.).
Figure 8 shows the output voltage characteristics when termination resistance R_T is varied.
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