DC1746A-A Analog Devices

DEMO MANUAL DC1746A LTM2881: Isolated RS485/RS422 Module Transceiver + Power DESCRIPTION Demonstration circuit 1746A is an isolated RS485/RS422 output voltage V and communicates all necessary CC2 Module transceiver + power featuring the LTM 2881. signaling across the isolation barrier using LTCs isolator The demo circuit is a 2500V galvanically isolated Module technology. RMS RS485/RS422 transceiver interface. The demo circuit Design les for this circuit board are available at features an EMI optimized circuit con guration and printed DEMO MANUAL DC1746A OPERATING PRINCIPLES EMI mitigation techniques used include the following: 4. Top signal routing and ground oods have been opti- mized to reduce signal loops, minimizing differential 1. Four layer PCB, allowing for isolated side to logic side mode radiation. bridge capacitor. The bridge capacitor is formed be- tween an inner layer of oating copper which overlaps 5. Common mode ltering is integrated into the input the logic side and isolated side ground planes. This pin header and output DB9 connector. Filtering helps structure creates two series capacitors, each with ap- to reduce emissions caused by conducted noise and proximately 0.008 of insulation, supporting the full minimizes the effects of cabling to common mode dielectric withstand rating of 2500V . The bridge emissions. RMS capacitor provides a low impedance return path for 6. A combination of low ESL and high ESR decoupling is injected currents due to parasitic capacitances of the used. A low ESL ceramic capacitor is located close to LTM2881s signal and power isolating elements. the module minimizing high frequency noise conduction. 2. Discrete bridge capacitors (C3, C4) mounted between High ESR tantalum capacitors are included to minimize GND2 and GND. The discrete capacitors provide ad- board resonances and prevent voltage spikes due to hot ditional attenuation at frequencies below 400MHz. plugging of the input supply voltage. Capacitors are safety rated type Y2, manufactured by EMI performance is shown in Figure 1, measured using Murata, part GA342QR7GF471KW01L. a gigahertz transverse electromagnetic (GTEM) cell and 3. Board/ground plane size has been minimized. This method detailed in IEC 61000-4-20, Testing and Mea- reduces the dipole antenna formed between the logic surement TechniquesEmission and Immunity Testing side and isolated side ground planes. in Transverse Electromagnetic Waveguides. 60 50 CISPR 22 CLASS B LIMIT 40 30 DC1746A-B 20 10 0 DETECTOR = QuasiPeak RBW = 120kHz 10 VBW = 300kHz SWEEP TIME = 17s 20 OF POINTS = 501 30 0 100 200 300 400 500 600 700 800 900 1000 FREQUENCY (MHz) DC1746A F01 Figure 1. DC1746A Radiated Emissions dc1746af 2 dBV/m