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CLC012AJE Datasheet(PDF) 10 Page - National Semiconductor (TI) |
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CLC012AJE Datasheet(HTML) 10 Page - National Semiconductor (TI) |
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10 / 18 page  Output Interfacing (Continued) TERMINATING PHYSICALLY SEPARATED OUTPUTS When the two outputs must be routed to physically separate locations, the circuit in Figure 6 may be applied. Alterna- tively, if load termination is desired, the circuit in Figure 7 may be used. The resistive divider network provides 75 Ω ter- mination and establishes proper ECL levels. This circuit con- sumes slightly more power than the previous circuits. Design Guidelines SELECTING THE AUTOMATIC EQUALIZER CAPACITOR The AEC capacitor sets the loop time constant τ for the equalizer’s adaptive loop response time. The following for- mula is used to set the loop time constant: τ =R • C AEC • 10 −6 R is a conversion factor that is set by internal equalizer pa- rameters and cable length. For Belden 8281 coaxial cable, the R values are ( τ = µs, C AEC in pF): Cable Length R Value (Ohms) 100 meters 15000 200 meters 20000 300 meters 32000 For example, a C AEC value of 100 pF results in an adaptive loop time constant of 2 µs at 200 meters of cable. CONNECTION AND OPERATION OF LOS AND MUTE Loss of Signal (LOS) is a CMOS output that indicates the presence of equalized data from the filter. This LOS output can be connected to MUTE to suspend changes in the data outputs DO and DO, if no valid signal exists. This simple configuration prevents random output transitions due to noise. For sparse transition patterns it is recommended that a capacitor be connected to LOS as shown in Figure 1. Add a capacitor to pin 5 to slow the response time of Loss of Signal when LOS is connected to MUTE. The capacitor re- duces sensitivity to pathological patterns. Pathological pat- terns are defined as sparse data sequences with few transi- tions. OUTPUT EYE MONITOR OEM CONNECTIONS The OEM is a high-speed, buffered output for monitoring the equalized eye pattern prior to the output comparator. Its out- put is designed to drive an AC-coupled 50 Ω coaxial cable with a series 50 Ω backmatch resistor. The cable should be terminated with 50 Ω at the oscilloscope. Figure 1 shows a schematic with a typical connection. MINIMUM DATA TRANSITIONS The CLC012 specifies a minimum transition rate. For the CLC012 this sets the minimum data rate for transmitting data through any cable medium. The CLC012 minimum average transition density is found in the Electrical Characteristics section of the datasheet. POWER SUPPLY OPERATION AND THERMAL CONSIDERATIONS The CLC012 operates from either +5V or −5.2V single sup- plies. Refer to Figure 1 when operating the part from +5V. When operating with a −5.2V supply, the V EE pins should be bypassed to ground. The evaluation board and associated literature provide for operation from either supply. Maximum power dissipation occurs at minimum cable length. Under that condition, I CC =58mA. Total power dissipated: P T = (58 mA)(5V) = 290 mW Power in the load: P L = (0.7V)(11 mA) + (37.5)(11 mA) 2 =12mW Maximum power dissipated on the die: P DMAX =PT–PL = 278 mW Junction Temperature = ( θ JA)(278 mW) + TA =T A + 26˚C Layout and Measurement The printed circuit board layout for the CLC012 requires proper high-speed layout to achieve the performance speci- fications found in the datasheet. The following list contains a few rules to follow: 1. Use a ground plane. 2. Decouple power pins with 0.1 µF capacitors placed ≤ 0.1” (3mm) from the power pins. 3. Design transmission strip lines from the CLC012’s input and output pins to the board connectors. 4. Route outputs away from inputs. 5. Keep ground plane ≥ 0.025” (0.06mm) away from the in- put and output pads. Figure 8 shows a block level measurement diagram, while Figure 15 on depicts a detailed schematic. A pseudo-random pattern generator with low output jitter was used to provide a NRZI pattern to create the eye diagrams shown in the Typi- cal Performance Characteristics section. Since most pattern generators have a 50 Ω output imped- ance, a translation can be accomplished using a CLC005 Cable Driver as an impedance transformer. A wide band- width oscilloscope is needed to observe the high data rate eye pattern. When monitoring a single output that is termi- nated at both the equalizer output and the oscilloscope, the effective output load is 37.4 Ω. Consequently, the signal swing is half that observed for a single-ended 75 Ω termination. DS100145-27 FIGURE 7. Alternative Load Terminated Output Interface www.national.com 10 |
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