7

FN7216.3

August 2, 2007

waveform is a squarewave, and inductive loads can cause

as much as 1V of overshoot. This does not mean that the

current delivered to the diode has overshoot - just the

voltage seen at the EL6201 output. Measurements show

that the EL6201 output edge rate is about 300psec - a speed

nearly impossible to deliver over practical interconnects to

the diode.

General Considerations

EMI and Grounding

From an EMI point of view, the edge rate of the output

current is much more important than that of the output

voltage. The components are generally small and will be

placed over a ground plane, so antenna effects that launch

voltage-mode EMI are small. Measurement shows that a

practical current edge rate is about 1ns, so interconnect

should be over a ground plane and short to minimize

inductively launched EMI. Most EMI seems to come from the

supply wires connected to the diode/EL6201 board. The

internal resistance and inductance of capacitors prevents

common. There needs to be a lossy series inductance and

secondary bypass on the supply side to control signals from

propagating down the wires. Alternatively, a series supply

resistor can be used, which will also be useful in reducing

EL6201 power dissipation. Figure 22 shows the typical

connection.

The L Series of Figure 22 must be carefully chosen. The

goal is to get a series reactance of around 70

Ω at 300MHz,

so 40nH would suffice. The inductor should be shielded to

reduce EMI and have no saturation effects at the supply

currents drawn by the EL6201. Finally, there should be no

self-resonance at the operating frequency or its harmonics.

Also important is circuit-board layout. At the EL6201’s

operating frequencies, even the ground plane is not low-

impedance, and ground loops should be avoided. Figure 23

shows the output current loops:

For the sourcing current loop, the current flows through the

supply bypass capacitor. The ground end of the bypass thus

should be connected directly to the EL6201 ground pin

(output ground pin of the 8 Ld package). A long ground

return path will cause the bypass capacitor currents to

generate voltage drops in the ground plane of the circuit

will pick this up as an interfering signal. Similarly, the ground

return of the load should be considered as noisy and other

grounded components should not connect to this path.

Slotting the ground plane around the load’s return will

eliminate adjacent grounded components from seeing the

noise.

of the power ground to avoid noise pick-up.

EL6201’s ground very directly lest they pick up high-

frequency noise interference. They also should have

minimal capacitance to ground. Trimmer resistors can be

FIGURE 21. OUTPUT CURRENT WAVEFORM - 1GHz

BANDWIDTH

+5V

L Series: 70

Ω reactance at

300MHz (see text)

0.1µF

Chip

EL6201

GND

0.1µF

Chip

FIGURE 22. RECOMMENDED SUPPLY BYPASSING

SINKING CURRENT LOOP

SOURCING CURRENT LOOP

SUPPLY

BYPASS

LOAD

GND

(8 Ld

PACKAGE)

FIGURE 23. OUTPUT CURRENT LOOPS

EL6201