ADA4355ABCZ datasheet(34/45 Pages) AD

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ADA4355

Data Sheet

Rev. A | Page 34 of 45

Dynamic Range Extension

One of the primary benefits of selectable gains on the ADA4355

is the ability to extend the dynamic range of the OTDR

measurement. A length of optical fiber approximately 160 km

long, with 0.2 dB/km of attenuation, was once again tested.

The ADA4355 transimpedance gain was maintained at 200 kΩ,

as was the laser launch power and pulse width at 15 dBm and

20 µs, respectively. Averaging was held to 3 minutes, or 65,000

averages, and a moving average filter was used. For this case,

the APD bias voltage was increased, thereby raising the APD

current gain to approximately 30×.

The measurement result is shown in Figure 93 and reveals that

the system becomes saturated at an 8 µA input current. Because

of this saturation, the first 20 km of fiber cannot be analyzed.

Note that the y-axis of Figure 93 is expressed in terms of

current so that the 8 µA saturation current can easily be seen.

0.1

0.01

0.001

100

1000

10000

100000

DISTANCE (km)

100

120

140

160

180

Figure 93.OTDR Measurement with 200 kΩ Gain

Switching the ADA4355 gain from 200 kΩ to 20 kΩ allows

a maximum signal level that is 10× larger than when using

200 kΩ, which means the saturation current is now 80 µA and

that much of the region below 20 km can now be analyzed.

Figure 94 shows the results from a single frame taken with 20 kΩ

gain and using all the same settings as the 200 kΩ measurement.

The single frame measurement takes 2 ms to complete only and

only the first 20 km are of interest.

DISTANCE (km)

0.1

0.01

0.001

100

1000

10000

100000

100

120

140

160

180

Figure 94. OTDR Measurement with 20 kΩ Gain

Combining the measurements from Figure 93 and Figure 94

yield the results shown in Figure 95. The portion >20 km comes

from Figure 93, and the portion ≤20 km comes from Figure 94.

The combined dynamic range is 34.5 dB, which is an extension

of almost 4 dB at a time cost of only 2 ms.

DISTANCE (km)

0.1

0.01

0.001

100

1000

10000

100000

100

120

140

160

180

Figure 95. OTDR Combined Measurement, 200 kΩ Gain and 20 kΩ Gain

If additional dynamic range is required, the process can be

repeated by using 2 kΩ gain, which raises the saturation current

to 800 µA, and then stitching all three individual results together.

ADA4355ABCZ Datasheet(PDF) 34 Page - Analog Devices

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