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ADM1491EBRMZ-REEL7 데이터시트(PDF) 10 Page - Analog Devices |
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ADM1491EBRMZ-REEL7 데이터시트(HTML) 10 Page - Analog Devices |
10 / 16 page ADM1490E/ADM1491E Rev. B | Page 10 of 16 THEORY OF OPERATION The ADM1490E/ADM1491E are RS-422/RS-485 transceivers that operate from a single 5 V ± 5% power supply. The ADM1490E/ ADM1491E are intended for balanced data transmission and comply with both TIA/EIA-485-A and TIA/EIA-422-B. Each device contains a differential line driver and a differential line receiver and is suitable for full-duplex data transmission. The input impedance of the ADM1490E/ADM1491E is 12 kΩ, allowing up to 32 transceivers on the differential bus. A thermal shutdown circuit prevents excessive power dissipation caused by bus contention or by output shorting. This feature forces the driver output into a high impedance state if, during fault conditions, a significant temperature increase is detected in the internal driver circuitry. The receiver contains a fail-safe feature that results in a logic high output state if the inputs are unconnected (floating). The ADM1490E/ADM1491E feature very low propagation delay, ensuring maximum baud rate operation. The balanced driver ensures distortion-free transmission. Another important specification is a measure of the skew between the complementary outputs. Excessive skew impairs the noise immunity of the system and increases the amount of electromagnetic interference (EMI). TRUTH TABLES Table 6. Abbreviations in Truth Tables Letter Description H High level I Indeterminate L Low level X Irrelevant Z High impedance (off ) Table 7. Transmitting Inputs Outputs DE DI Z Y H H L H H L H L L X Z Z Table 8. Receiving Inputs Output RE A − B RO L ≥ +0.2 V H L ≤ −0.2 V L L −0.2 V ≤ A − B ≤ +0.2 V I L Inputs open H H X Z ESD TRANSIENT PROTECTION SCHEME The ADM1490E/ADM1491E use protective clamping structures on their inputs and outputs to clamp the voltage to a safe level and dissipate the energy present in ESD (electrostatic). The protection structure achieves ESD protection up to ±8 kV human body model (HBM). ESD Testing Two coupling methods are used for ESD testing: contact dis- charge 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 unit under test. With air discharge, the discharge gun is moved toward the unit under test, developing an arc across the air gap; therefore, the term air discharge. This method is influenced by humidity, temperature, barometric pressure, distance, and rate of closure of the discharge gun. The contact discharge method, though 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 destruction can occur immediately because of arcing or heating. Even if cata- strophic failure does not occur immediately, the device can suffer from parametric degradation, resulting in degraded performance. The cumulative effects of continuous exposure can eventually lead to complete failure. Figure 27. ESD Generator I/O lines are particularly vulnerable to ESD damage. Simply touching or plugging in an I/O cable may result in a static dis- charge that can damage or destroy the interface product connected to the I/O port. It is, therefore, extremely important to have high levels of ESD protection on the I/O lines. The ESD discharge can induce latch-up in the device under test. Therefore, it is important to conduct ESD testing on the I/O pins while power is applied to the device. This type of testing is more representative of a real-world I/O discharge in which the equip- ment is operating normally when the discharge occurs. C1 R2 HIGH VOLTAGE GENERATOR DEVICE UNDER TEST NOTES 1. THE ESD TEST METHOD USED IS THE HUMAN BODY MODEL (±8kV) WITH R2 = 1500 Ω AND C1 = 100pF. |
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