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dc1746af

DEMO MANUAL DC1746A

OPERATING PRINCIPLES

EMI mitigation techniques used include the following:

1. Four layer PCB, allowing for isolated side to logic side

‘bridge’ capacitor. The bridge capacitor is formed be-

tween an inner layer of floating copper which overlaps

the logic side and isolated side ground planes. This

structure creates two series capacitors, each with ap-

proximately 0.008" of insulation, supporting the full

dielectric withstand rating of 2500VRMS. The bridge

capacitor provides a low impedance return path for

injected currents due to parasitic capacitances of the

LTM2881’s signal and power isolating elements.

2. Discrete bridge capacitors (C3, C4) mounted between

GND2 and GND. The discrete capacitors provide ad-

ditional attenuation at frequencies below 400MHz.

Capacitors are safety rated type Y2, manufactured by

Murata, part # GA342QR7GF471KW01L.

3. Board/ground plane size has been minimized. This

reduces the dipole antenna formed between the logic

side and isolated side ground planes.

4. Top signal routing and ground floods have been opti-

mized to reduce signal loops, minimizing differential

mode radiation.

5. Common mode filtering is integrated into the input

pin header and output DB9 connector. Filtering helps

to reduce emissions caused by conducted noise and

minimizes the effects of cabling to common mode

emissions.

6. A combination of low ESL and high ESR decoupling is

used. A low ESL ceramic capacitor is located close to

the module minimizing high frequency noise conduction.

High ESR tantalum capacitors are included to minimize

board resonances and prevent voltage spikes due to hot

plugging of the input supply voltage.

EMI performance is shown in Figure 1, measured using

a gigahertz transverse electromagnetic (GTEM) cell and

method detailed in IEC 61000-4-20, “Testing and Mea-

surement Techniques—Emission and Immunity Testing

in Transverse Electromagnetic Waveguides”.

FREQUENCY (MHz)

0

–30

–20

0

10

20

600 700 800 900

60

DC1746A F01

–10

100 200 300 400 500

1000

30

40

50

DETECTOR = QuasiPeak

RBW = 120kHz

VBW = 300kHz

SWEEP TIME = 17s

# OF POINTS = 501

DC1746A-B

CISPR 22 CLASS B LIMIT

Figure 1. DC1746A Radiated Emissions