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LTC489CSW Datasheet(PDF) 8 Page - Linear Technology |
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LTC489CSW Datasheet(HTML) 8 Page - Linear Technology |
8 / 12 page LTC488/LTC489 8 4889fb data rate. With lower quality PVC cables, the dielectric loss factor can be 1000 times worse. PVC twisted-pairs have terrible losses at high data rates (> 100kbps), and greatly reduce the maximum cable length. At low data rates how- ever, they are acceptable and much more economical. Cable Termination The proper termination of the cable is very important. If the cable is not terminated with its characteristic imped- ance, distorted waveforms will result. In severe cases, distorted (false) data and nulls will occur. A quick look at the output of the driver will tell how well the cable is terminated. It is 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 (47Ω), 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 (470Ω), 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. 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 120Ω 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 capaci- tor 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 120Ω 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/(120Ω)(C). APPLICATIONS INFORMATION Figure 8. Termination Effects Figure 9. AC-Coupled Termination 4889 F08 DX PROBE HERE Rt = 120Ω Rt = 47Ω Rt = 470Ω Rt RX RECEIVER DRIVER 488/9 F09 C = LINE LENGTH (FT)(16.3pF) 120Ω C RX RECEIVER |
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