ADA4355ABCZ datasheet(33/45 Pages) AD

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Data Sheet

ADA4355

Rev. A | Page 33 of 45

OTDR PERFORMANCE

The adjustable transimpedance gain and LPF bandwidth of the

ADA4355 enable the device to deliver excellent performance over

a wide range of OTDR applications. The combination of a 200 kΩ

transimpedance gain and a 1 MHz LPF cutoff frequency enables

the ADA4355 to reach the low noise levels needed to achieve

the dynamic range required for long haul OTDR applications.

Conversely, the combination of a 2 kΩ gain and a 100 MHz

LPF cutoff frequency provides the higher bandwidth needed for

the narrow pulse widths that are required for the closely spaced

event detection necessary for data center applications.

In addition, an extension in dynamic range can be realized by

combining multiple gain measurements into a single result.

This technique is discussed in the Dynamic Range Extension

section.

High Dynamic Range

Figure 92 shows the OTDR test setup used to measure 160 km

of optical fiber. The transimpedance gain was set to 200 kΩ, and

the LPF was set to 1 MHz. The laser pulse width was 20 µs with

a peak pulsed power of 15 dBm and an avalanche photodiode

(APD) current gain of 3. The measurement time was limited to

3 minutes, allowing for 65,000 averages, which lowered the noise

level to approximately 14 pA rms. Using the SNR = 1

calculation, the OTDR dynamic range achieved is 28 dB.

Additional digital filtering can be used to further lower the

noise for improved dynamic range. Applying a simple moving

average filter with a window size of 1000 samples improves the

dynamic range to 31 dB. Figure 91 shows a comparison of the

OTDR results for before digital filtering and after digital

filtering.

100

120

140

160

180

DISTANCE (km)

–35

–30

–25

–20

–15

–10

–5

BEFORE DIGITAL FILTERING

AFTER DIGITAL FILTERING

Figure 91. OTDR Measurement on 160 km Optical Fiber

1km

40km

APD AND BIAS

LASER AND

DRIVER

FPGA

OTDR TRACE

TO DISPLAY

ADA4355

40km

= SC-UPC CONNECTOR

CIRCULATOR

FIBER UNDER TEST

ADC

Figure 92. OTDR Test Set-Up

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