Datasheet
SCEA019 - January 2001
Texas Instruments GTLP Frequently Asked Questions 7–109
24 Why not use Thevenin voltage dividers for termination?
Thevenin voltage dividers can be used as a termination scheme; resistors are easy to obtain
and can be connected to the readily available 3.3-V power supply. However, this is not the
optimal termination scheme. There are several reasons why voltage regulation is essential on
GTLP backplanes:
• Static dc current through the termination - A Thevenin equivalent of two 50-ohm resistors
to 3.3 V, with the backplane tied to the center connection, yields about 33 mA of constant
dc current while the output driver is off (high state). Typically, terminations are the two
extremes of the backplane, so the total is 66 mA for the backplane. When the device’s
output is on (low output state), current increases to about 112 mA because the lower
termination resistor is bypassed through the 4-ohm GTLP device.
Now, consider a 1.5-V regulated termination scheme. In the off output state, zero current
flows because there is no path to ground from V
TT
(termination voltage). While in the on
state, this produces 80 mA of current. Therefore, as the output is switching states from
low to high and vice versa, the average current flowing through the voltage-regulated
termination is considerably lower than the Thevenin voltage-divider termination.
• Noise considerations - If the Thevenin voltage divider is tapped off the supply voltage of
3.3 V and the backplane termination voltage, V
TT
, is taken from the center connection,
any noise that is riding on the supply has now been coupled to V
TT
. This could prove to
be a problem because, if the noise is large enough, potentially it could interfere with the
switching thresholds of the GTLP inputs. Voltage regulation eliminates this problem with a
reliable voltage source. Ideally, a voltage regulator should be mounted at each end.
• Impedance matching – This probably is the most important reason for voltage regulation,
because matching the loaded impedance of the backplane is essential to reduce or totally
eliminate reflections that occur with improper termination. Using a Thevenin voltage
divider is more difficult because one also has to adjust V
TT
to be 1.5 V. The numbers
used above are for V
TT
of 1.65 V (half of 3.3 V), so the resistors have to be chosen to
produce a 1.5 V V
TT
and terminate the backplane properly. Although it sounds easy, in
practice, it is not.