THS789 Datasheet, PDF (28/34 Pages) Texas Instruments – THS789 Quad-Channel Time Measurement Unit (TMU)

THS789
SLOS776A – SEPTEMBER 2012 – REVISED DECEMBER 2015
Driver
50-W Transmission Line
Equal Length
THS789
100 Ω RT
LVDS
Receiver
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Figure 11. Typical Input Connection to the THS789
8.2.2.7 LVDS-Compatible Outputs
Figure 12 shows a typical wiring diagram of an LVDS output. The transmission line lengths must be matched. A
termination resistor may be required if the chosen receiver does not have an internal resistor. Concerning
termination resistors: LVDS was originally conceived with twisted-wire pairs of approximately 100-Ω line-to-line
impedance. The 100-Ω resistor between lines is simple and effective to terminate such a line. For the higher-
speed operation of the THS789 device, use a pair of 50-Ω transmission lines, such as microstrip on the PC
board. The same 100-Ω resistor line-to-line termination works well, because the line signals are equal and
opposite in phase. This results in the center of the 100-Ω resistor having a constant voltage equal to the
common-mode voltage and each side having an apparent 50-Ω termination. An improvement in the termination
can be achieved by splitting the 100 Ω into two 50-Ω resistors and ac-grounding (bypassing) the center to ground
with a 1000-pF (not critical) capacitor. The termination improvement is usually small and increases the room and
parts count. It is the best approach as long as the PCB layout high-frequency performance is not compromised
by the higher parts count. As mentioned previously, the driver is optimized to drive 50-Ω transmission lines and
provides a driving-point impedance approximating 50 Ω to suppress reflections. Figure 8 is a simplified schematic
of the output driver. A standard ECL-like circuit drives the outputs through 25-Ω resistors. The combination of the
resistors and the emitter-follower output impedance approximates 50 Ω. The output emitter-followers are biased
by current sources that are switched to conserve power. A feedback loop varies the voltage on the two RLs to
set and maintain the 1.28-V common-mode voltage of the LVDS-compatible outputs. Another feedback loop
holds the emitters of the current switches to 0.4 V to keep the 4-mA current source from saturation.
The outputs are short-circuit-proof to a 3.3-V power supply. Shorts to ground should be avoided, as the power
dissipation in certain components may exceed safe limits.
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