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APPLICATION INFORMATION
IMPEDANCE MATCHING AND REFLECTIONS
0.1
0.05
0
0 5 10 15
Voltage - V
0.15
0.2
t - Time - ns
TIME DOMAIN RESPONSE
0.25
20 25
Z
S
= 0 Ω
Z
O
= 100 Ω
Z
T
= 132 Ω
V
I
V at Load
0.1
0.05
0
0 5 10 15
Voltage - V
0.15
0.2
t - Time - ns
TIME DOMAIN RESPONSE
0.25
20 25
Z
S
= 0 Ω
Z
O
= 100 Ω
Z
T
= 90 Ω
V
I
V at Load
SN65LVDS348 , SN65LVDT348
SN65LVDS352 , SN65LVDT352
SLLS523E – FEBRUARY 2002 – REVISED MAY 2004
A termination mismatch can result in reflections that degrade the signal at the load. A low source impedance
causes the signal to alternate polarity at the load (oscillates) as shown in Figure 14 . High source impedance
results in the signal accumulating monotonically to the final value (stair step) as shown in Figure 15 . Both of
these modes result in a delay in valid signal and reduce the opening in the eye pattern. A 10% termination
mismatch results in a 5% reflection (r = Z
L
- Z
O
/Z
L
+ Z
O
), even a 1:3 mismatch absorbs half of the incoming
signal. This shows that termination is important in the more critical cases, however, in a general sense, a rather
large termination mismatch is not as critical when the differential output signal is much greater than the receiver
sensitivity.
Figure 14. Low-Source Impedance Figure 15. High-Source Impedance
For example a 200-mV drive signal into a 100- Ω lossless transmission media with a termination resistor of 90 Ω
to 132 Ω results in ~227 mV to 189 mV into the receiver. This would typically be more than enough signal into a
receiver with a sensitivity of ±50 mV assuming no other disturbance or attenuation on the line. The other factors,
which reduce the signal margin, do affect this and therefore it is important to match the impedance as closely as
possible to allow more noise immunity at the receiver.
14
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