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ADM211EARS-REEL Datasheet(PDF) 14 Page - Analog Devices |
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ADM211EARS-REEL Datasheet(HTML) 14 Page - Analog Devices |
14 / 20 page ADM206E/ADM207E/ADM208E/ADM211E/ADM213E Rev. E | Page 14 of 20 Table 10. Test Severity Levels (IEC 1000-4-3) Level Field Strength (V/m) 1 1 2 3 3 10 EMISSIONS/INTERFERENCE EN 55022, CISPR 22 defines the permitted limits of radiated and conducted interference from information technology (IT) equipment. The objective of the standard is to minimize the level of emissions, both conducted and radiated. For ease of measurement and analysis, conducted emissions are assumed to predominate below 30 MHz, and radiated emissions are assumed to predominate above 30 MHz. CONDUCTED EMISSIONS This is a measure of noise that is conducted onto the line power supply. Switching transients from the charge pump that are 20 V in magnitude and that contain significant energy can lead to conducted emissions. Another source of conducted emissions is the overlap in switch-on times in the charge pump voltage converter. In the voltage doubler shown in Figure 31, if S2 has not fully turned off before S4 turns on, a transient current glitch occurs between VCC and GND that results in conducted emis- sions. Therefore, it is important that the switches in the charge pump guarantee break-before-make switching under all condi- tions so instantaneous short-circuit conditions do not occur. The ADM2xxE have been designed to minimize the switching transients and ensure break-before-make switching, thereby minimizing conducted emissions. This results in emission levels well below specified limits. Other than the recom- mended 0.1 μF capacitor, no additional filtering/decoupling is required. Conducted emissions are measured by monitoring the line power supply. The equipment used consists of a line impedance stabilizing network (LISN) that essentially presents a fixed impedance at RF and a spectrum analyzer. The spectrum analyzer scans for emissions up to 30 MHz. A plot for the ADM211E is shown in Figure 33. + + VCC GND S1 S2 C1 S3 S4 C3 V+ = 2VCC VCC INTERNAL OSCILLATOR Figure 31. Charge Pump Voltage Doubler ø1 ø2 SWITCHING GLITCHES Figure 32. Switching Glitches LOG FREQUENCY (MHz) 80 70 0 0.33 30 0.6 60 50 10 40 30 20 6 3 118 LIMIT Figure 33. Conducted Emissions Plot RADIATED EMISSIONS Radiated emissions are measured at frequencies in excess of 30 MHz. RS-232 outputs designed for operation at high baud rates while driving cables can radiate high frequency EM energy. The previously described causes of conducted emissions can also cause radiated emissions. Fast RS-232 output tran- sitions can radiate interference, especially when lightly loaded and driving unshielded cables. Charge pump devices are also prone to radiating noise due to the high frequency oscillator and the high voltages being switched by the charge pump. The move toward smaller capacitors in order to conserve board space has resulted in higher frequency oscillators being em- ployed in the charge pump design, resulting in higher levels of conducted and radiated emissions. The RS-232 outputs on the ADM2xxE products feature a con- trolled slew rate in order to minimize the level of radiated emissions, yet they are fast enough to support data rates of up to 230 kbps. |
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