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LTC2859IDD-TRPBF Datasheet(PDF) 11 Page - Linear Technology |
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LTC2859IDD-TRPBF Datasheet(HTML) 11 Page - Linear Technology |
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11 / 16 page  LTC2859/LTC2861 11 285961fb Receiver and Failsafe With the receiver enabled, when the absolute value of the differential voltage between the A and B pins is greater than 200mV, the state of RO will reflect the polarity of (A-B). The LTC2859/LTC2861 have a failsafe feature that guaran- tees the receiver output to be in a logic HIGH state when the inputs are either shorted, left open, or terminated (externally or internally), but not driven for more than about 3μs. The delay prevents signal zero crossings from being interpreted as shorted inputs and causing RO to go high inadvertently. This failsafe feature is guaranteed to work for inputs spanning the entire common mode range of –7V to +12V. The receiver output is internally driven high (to VCC) or low (to ground) with no external pull-up needed. When the receiver is disabled the RO pin becomes Hi-Z with leakage of less than ±1μA for voltages within the supply range. Receiver Input Resistance The receiver input resistance from A or B to ground is guaranteed to be greater than 96k when the termina- tion is disabled. This is 8X higher than the requirements for RS485 standard and thus this receiver represents a one-eighth unit load. This, in turn, means that 8X the standard number of receivers, or 256 total, can be con- nected to a line without loading it beyond what is called out in the RS485 standard. The input resistance of the receivers is unaffected by enabling/disabling the receiver and by powering/unpowering the part. The equivalent input resistance looking into A and B is shown in Figure 9. The termination resistor cannot be enabled by TE if the device is unpowered or in thermal shutdown mode. Switchable Termination Proper cable termination is very important for good signal fidelity. If the cable is not terminated with its char- acteristic impedance, reflections will result in distorted waveforms. The LTC2859/LTC2861 are the first RS485 transceivers to offer integrated switchable termination resistors on the receiver input pins. This provides the tremendous advan- tage of being able to easily change, through logic control, the proper line termination for optimal performance when configuring transceiver networks. When the TE pin is high, the termination resistor is en- abled and the differential resistance from A to B is 120Ω. Figure 10 shows the I/V characteristics between pins A and B with the termination resistor enabled and disabled. Figure 10. Curve Trace Between A and B with Termination Enabled and Disabled Figure 9. Equivalent Input Resistance into A and B (on the LTC2859, Valid if Driver is Disabled) APPLICATIONS INFORMATION 60Ω 60Ω A TE B 2859/61 F09 >96k >96k |
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