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ADM3071EARZ Datasheet(PDF) 16 Page - Analog Devices |
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ADM3071EARZ Datasheet(HTML) 16 Page - Analog Devices |
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ADM3070E/ADM3071E/ADM3072E/ADM3073E/ADM3074E/ADM3075E/ADM3076E/ADM3077E/ADM3078E Rev. E | Page 16 of 20 HOT-SWAP CAPABILITY (ALL EXCEPT ADM3071E/ADM3074E/ADM3077E) Hot-Swap Inputs When a circuit board is inserted into a hot (or powered) back- plane, differential disturbances to the data bus can lead to data errors. During this period, processor logic output drivers are high impedance and are unable to drive the DE and RE inputs of the RS-485 transceivers to a defined logic level. Leakage currents up to ±10 μA from the high impedance state of the processor logic drivers can cause standard CMOS enable inputs of a tran- sceiver to drift to an incorrect logic level. Additionally, parasitic circuit board capacitance can cause coupling of VCC or GND to the enable inputs. Without the hot-swap capability, these factors can improperly enable the driver or receiver of the transceiver. When VCC rises, an internal pull-down circuit holds DE low and RE high. After the initial power-up sequence, the pull-down circuit becomes transparent, resetting the hot-swap tolerable input. LINE LENGTH vs. DATA RATE The RS-485/RS-422 standard covers line lengths up to 4000 feet. For line lengths greater than 4000 feet, Figure 37 illustrates an example line repeater. ±15 kV ESD PROTECTION Two coupling methods are used for ESD testing: contact discharge and air-gap discharge. Contact discharge calls for a direct connection to the unit being tested. Air-gap discharge uses a higher test voltage but does not make direct contact with the test unit. With air-gap discharge, the discharge gun is moved toward the unit under test, developing an arc across the air gap, thus the term air-gap discharge. This method is influenced by humidity, temperature, barometric pressure, distance, and rate of closure of the discharge gun. The contact discharge method, while less realistic, is more repeatable and is gaining acceptance and preference over the air-gap method. Although very little energy is contained within an ESD pulse, the extremely fast rise time, coupled with high voltages, can cause failures in unprotected semiconductors. Catastrophic destruc- tion can occur immediately as a result of arcing or heating. Even if catastrophic failure does not occur immediately, the device can suffer from parametric degradation that can result in degraded performance. The cumulative effects of continuous exposure can eventually lead to complete failure. Input/output lines are particularly vulnerable to ESD damage. Simply touching or connecting an input/output cable can result in a static discharge that damages or completely destroys the interface product connected to the input/output port. It is extremely important, therefore, to have high levels of ESD protection on the input/output lines. The ESD discharge can induce latch-up in the device under test, so it is important that ESD testing on the input/output pins be carried out while device power is applied. This type of testing is more representative of a real-world input/output discharge, which occurs when equipment is operating normally. The transmitter outputs and receiver inputs of the ADM307xE family are characterized for protection to a ±15 kV limit using the human body model. HUMAN BODY MODEL Figure 33 shows the human body model and the current waveform it generates when discharged into low impedance. This model consists of a 100 pF capacitor charged to the ESD voltage of interest, which is then discharged into the test device through a 1.5 kΩ resistor. 100% 90% 36.8% 10% tRL tDL TIME t R2 C1 R1 HIGH VOLTAGE GENERATOR DEVICE UNDER TEST ESD TEST METHOD HUMAN BODY MODEL ESD ASSOC. STD 55.1 R2 1.5kΩ C1 100pF Figure 33. Human Body Model and Current Waveform 256 TRANSCEIVERS ON THE BUS The standard RS-485 receiver input impedance is 12 kΩ (1 unit load), and the standard driver can drive up to 32 unit loads. The ADM307xE family of transceivers has a ⅛ unit load receiver input impedance (96 kΩ), allowing up to 256 transceivers to be connected in parallel on one communication line. Any combi- nation of these devices and other RS-485 transceivers with a total of 32 unit loads or fewer can be connected to the line. REDUCED EMI AND REFLECTIONS The ADM3070E/ADM3071E/ADM3072E feature reduced slew rate drivers that minimize EMI and reduce reflections caused by improperly terminated cables, allowing for error- free data transmission at rates up to 250 kbps. The ADM3073E/ ADM3074E/ADM3075E offer higher driver output slew rate limits, allowing for transmit speeds of up to 500 kbps. |
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