HD26LS32 Datasheet, PDF (10/15 Pages) Hitachi Semiconductor – Quadruple Differential Line Receivers With 3 State Outputs

HD26LS32
This is because, since the termination resistance is normally matched to the transmission line characteristic impedance,
the value falls to several tens of hundreds of ohms, and the differential input pins are shorted by this termination
resistance. That is, the differential input voltage V_ID comes within the range
V_ID: –0.2 V < V_IA – V_IB < 0.2 V
and the output becomes undetermined.
To prevent this, resistance R₁ is inserted in series with the transmission line as shown in figure 6, minimizing the effect
of the termination resistance. Resistance R₂ is added to increase the current flowing between the termination resistance
and R₁, enabling the value of R₁ to be kept small.
Inserting resistances R₁ and R₂ in this way provides for the differential input voltage V_ID to become 200 mV or higher,
but the following points must be noted.
• Smallest possible R₁ value
If this value is large, the receiver input sensitivity will fall.
• Largest possible R₂ value
If this value is small, the load on the driver will be large.
Figure 7 shows experimental differential input voltages for variations in R₁ and R₂.
Undetermined
"H"
R_T
R_T
Figure 5 Examples of Transmission Line Disconnection
Driver
V_CC
R₁ ^R2
Receiver
R_T
R_{1 R2}
Figure 6 Method of Enhancing Fail-Safe Function
0.6
0.5
100 ^kΩ
300 ^kΩ
R2 ^{= ∞}
V_CC = 5 V
Ta = 25°C
V_CC
R₁ ^R2
0.4
100 Ω
V_ID
0.3
R_{1 R2}
0.2
0.1
0
5
10
15
R1 (kΩ)
Figure 7 R₁, R₂ vs. Differential Input Voltage
Rev.2.00, Jul.16.2004, page 10 of 14