thickness (d) en r (see Figure 2). When Checked under a

microscope: the thickness of the wires is 0.3 mm. The pitch

is 0.5 mm, while the ;r must be 1 for air. The impedance of

two parallel wires is given by this formula,

Z = (276/r) * log{(2*a)/d}

With the data above filled in this formula the result is:

Z = 144

Ω

Both the measured and the calculated numbers matches

very closely. The impedance of the flex cable is a physical

parameter so when designing a transmission path using this

flex cable, the impedance of the total path must be based on

140

Ω. There is another parameter which is the termination

resistance inside the LMH6525 or LMH6526 which is 100

Ω.

When terminating the 140

Ω transmission path with an im-

pedance of 100

Ω a mismatch will occur causing reflections

on the transmission line. To solve this problem it is possible

to connect directly at the input terminals of the part two

resistors of 20

Ω one on every pin to keep it symmetrical.

Normally this causes signal loss over the total extra series

resistance of 40

Ω when using a voltage source for driving

the transmission line. An advantage of a LVDS source is it’s

current nature. The current of a LVDS output is 3.5 mA and

this current produces across a resistor of 140

Ω a voltage of

490 mV, while this voltage across the 100

Ω internal termina-

tion resistor of the part remains at 350 mV, which is conform

the LVDS standard. With the usage of a series resistance of

40

Ω and the termination resistor of 100Ω the total termina-

tion resistance now matches the line impedance and reflec-

tions will be as low as possible. A helpful tool for calculating

impedances of transmission lines is the: ‘Transmission Line

Rapidesigner’ available from the National Semiconductor

Interface Products Group. Application Note AN-905 details

the use of this handy software tool.

The Read Enable and Enable inputs are slower and much

less critical. The Oscillator Enable input is toggled in combi-

nation with the write pulse so special attention should be

given to this signal to insure it is routed cleanly. It may be

desirable to put a termination resistor close to the LMH6526

for the Enable Oscillator line, to achieve the best turn-on and

turn-off performance of the oscillator.

OUTPUTS

In order to achieve the fastest output rise times the layout of

the output lines should be short and tight (see Figure 3). It is

intended that the Output B trace be routed under the decou-

pling capacitor and that the ground return for the laser be

closely coupled to the output and terminated at the ground

side of the decoupling capacitor.

The capacitance on the output lines should also be reduced

as much as possible. As always the loop area of the laser

current should be minimized and keep in mind that it is

important not to have vias in the current path of the output

lines. Via’s will introduce some inductance which lead to

extra overshoot on the pulse shape.

DECOUPLING CAPACITORS

As mentioned before, the decoupling capacitors are critical

to the performance of the part. The output section above

mentioned that the power-side decoupling capacitor should

be as close as possible to the V

the B output should pass under the decoupling capacitor.

Similarly the analog-side decoupling capacitor should be as

close as possible to the V

shows a layout where the analog (V

pling cap C1 is placed next to pins 6 and 7. (Note the layout

is rotated 90 degrees from the last figure.) The ground

extends into a plane that should connect to the oscillator

amplitude and current setting resistors. C2 is the power-side

decoupling capacitor and it can be seen placed as close to

the V

output trace. This layout has also provided for a second

power decoupling capacitor C3 that connects from V

different GND copper pour. It must be noted that the two

ground planes extending from C2 and C3 must be tied

together. This will be shown in the thermal section below.

Bear in mind that the closeness of the parts to the LMH6525/

6526 may be dictated by manufacturing rework consider-

ations such that the LMH6525/6526 can be de-soldered with

a hot-air rework station without the need to remove the

capacitors. The relevant manufacturing organization can

provide guidelines for this minimum spacing.

20110122

FIGURE 2. Parallel Wires

20110119

FIGURE 3. Laser Connection

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