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LTC488 Datasheet(PDF) 8 Page - Linear Technology |
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LTC488 Datasheet(HTML) 8 Page - Linear Technology |
8 / 12 page 8 LTC488/LTC489 S APPLICATI I FOR ATIO best to look at a driver connected to the end of the cable, since this eliminates the possibility of getting reflections from two directions. Simply look at the driver output while transmitting square wave data. If the cable is terminated properly, the waveform will look like a square wave (Figure 8). If the cable is loaded excessively (47 Ω), the signal initially sees the surge impedance of the cable and jumps to an initial amplitude. The signal travels down the cable and is reflected back out of phase because of the mistermination. When the reflected signal returns to the driver, the ampli- tude will be lowered. The width of the pedestal is equal to twice the electrical length of the cable (about 1.5ns/foot). If the cable is lightly loaded (470 Ω), the signal reflects in phase and increases the amplitude at the drive output. An input frequency of 30kHz is adequate for tests out to 4000 ft. of cable. 488/9 F08 DX PROBE HERE Rt = 120 Ω Rt = 47 Ω Rt = 470 Ω Rt RX RECEIVER DRIVER Figure 8. Termination Effects AC Cable Termination Cable termination resistors are necessary to prevent un- wanted reflections, but they consume power. The typical differential output voltage of the driver is 2V when the cable is terminated with two 120 Ω resistors, causing 33mA of DC current to flow in the cable when no data is being sent. This DC current is about 60 times greater than the supply current of the LTC488/LTC489. One way to eliminate the unwanted current is by AC coupling the termination resistors as shown in Figure 9. The coupling capacitor must allow high frequency energy to flow to the termination, but block DC and low frequen- cies. The dividing line between high and low frequency depends on the length of the cable. The coupling capacitor must pass frequencies above the point where the line represents an electrical one-tenth wavelength. The value of the coupling capacitor should therefore be set at 16.3pF per foot of cable length for 120 Ω cables. With the coupling capacitors in place, power is consumed only on the signal edges, and not when the driver output is idling at a 1 or 0 state. A 100nF capacitor is adequate for lines up to 4000 feet in length. Be aware that the power savings start to decrease once the data rate surpasses 1/(120 Ω )(C). 488/9 F09 C = LINE LENGTH (FT)(16.3pF) 120 Ω C RX RECEIVER Figure 9. AC Coupled Termination |
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