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ADuM1251WARZ Datasheet(PDF) 11 Page - Analog Devices |
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ADuM1251WARZ Datasheet(HTML) 11 Page - Analog Devices |
11 / 12 page Data Sheet ADuM1250/ADuM1251 Rev. F | Page 11 of 12 MAGNETIC FIELD IMMUNITY The ADuM1250/ADuM1251 are extremely immune to external magnetic fields. The limitation on the magnetic field immunity of the ADuM1250/ADuM1251 is set by the condition in which induced voltage in the receiving coil of the transformer is suffi- ciently large to either falsely set or reset the decoder. The following analysis defines the conditions under which this may occur. The 3 V operating condition of the ADuM1250/ADuM1251 is exam- ined because it represents the most susceptible mode of operation. The pulses at the transformer output have an amplitude greater than 1.0 V. The decoder has a sensing threshold at approximately 0.5 V, thus establishing a 0.5 V margin in which induced voltages can be tolerated. The voltage induced across the receiving coil is given by V = (−dβ/dt) ∑ πrn2; n = 1, 2, … , N where: β is the magnetic flux density (gauss). rn is the radius of the nth turn in the receiving coil (cm). N is the total number of turns in the receiving coil. Given the geometry of the receiving coil in the ADuM1250/ ADuM1251 and an imposed requirement that the induced voltage be, at most, 50% of the 0.5 V margin at the decoder, a maximum allowable magnetic field is calculated as shown in Figure 10. MAGNETIC FIELD FREQUENCY (Hz) 100 0.001 1M 10 0.01 1k 10k 10M 0.1 1 100M 100k Figure 10. Maximum Allowable External Magnetic Flux Density For example, at a magnetic field frequency of 1 MHz, the maximum allowable magnetic field of 0.2 kgauss induces a voltage of 0.25 V at the receiving coil. This voltage is approxi- mately 50% of the sensing threshold and does not cause a faulty output transition. Similarly, if such an event occurs during a transmitted pulse (and is of the worst-case polarity), it reduces the received pulse from >1.0 V to 0.75 V—still well above the 0.5 V sensing threshold of the decoder. The preceding magnetic flux density values correspond to specific current magnitudes at given distances away from the ADuM1250/ADuM1251 transformers. Figure 11 expresses these allowable current magnitudes as a function of frequency for selected distances. As shown in Figure 11, the ADuM1250/ ADuM1251 are extremely immune and can be affected only by extremely large currents operated at high frequency very close to the component. For the 1 MHz example, a 0.5 kA current placed 5 mm away from the ADuM1250/ADuM1251 is required to affect the operation of the component. MAGNETIC FIELD FREQUENCY (Hz) 1000 100 10 1 0.1 0.01 1k 10k 100M 100k 1M 10M DISTANCE = 5mm DISTANCE = 1m DISTANCE = 100mm Figure 11. Maximum Allowable Current for Various Current-to-ADuM1250/ADuM1251 Spacings Note that at combinations of strong magnetic field and high frequency, any loops formed by PCB traces can induce error voltages sufficiently large to trigger the thresholds of succeeding circuitry. Exercise care in the layout of such traces to avoid this possibility. |
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