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ADN2820ACHIPS

10.7 Gbps, 3.3 V, Low Noise, TIA with Average Power Monitor

Analog Devices 

OUTPUT OFFSET ADJUST INPUT

Long reach optical links may suffer from unbalanced 1 and 0

signal shaping due to dispersion and/or optical or avalanche

amplification noise. The ADN2820 enables the user to adjust

the input-referred slice level by adjusting the output offset with

the ADN2820’s outputs dc-coupled.

With the OFFSET pad open (not bonded), the average output

voltage offset [OUT – OUTB] is internally balanced to be less

than ±5 mV. When externally driven by a voltage source, the

ADN2820 average output voltage offset [OUT – OUTB] is

linearly proportional to an applied OFFSET input voltage:

Applied Offset (V) = (OFFSET (V) – ~1.6 V) × OFFSETGAIN (mV/V)

where:

OFFSET = voltage applied to the OFFSET pad

OFFSETGAIN = 120 mV/V

With transimpedance, TZ, the input referred slice adjust can be

calculated from the following equation:

Input Slice Adjust = 1/TZ × (OFFSET (V) – ~1.6 V) × OFFSETGAIN (mV/V))

50

40

30

20

10

0

–10

–20

–30

–40

–50

0

0.3 0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7 3.0

OFFSET CONTROL INPUT (V)

Figure 15. Input Slice Adjust vs. OFFSET Calculation Using Typical

[OUT,OUTB] vs. OFFSET Measurement Data

ADN2820

LOW FREQUENCY TRANSIMPEDANCE CUTOFF

CAPACITOR SELECTION

Digital encoding methods may generate long strings of 1s or 0s,

requiring the transimpedance amplifier pass band to extend to

1 MHz or below. To accommodate this requirement, the

ADN2820 has –3 dB low frequency transimpedance cutoff set

by external capacitor CLF. For CLF, values greater than 1000 pF,

the typical –3 dB low frequency transimpedance cutoff can be

estimated by the equation

f–3dB ~ 2 kHz × (1 µF/CLF)

Because CLF is not part of the 10 Gbps signal chain, it is not

required to be a high frequency capacitor type. A ceramic

capacitor is recommended.

100M

10M

1M

100k

10k

1k

1pF

10pF 0.1nF

1nF

10nF 0.1µF

1µF

EXTERNAL CLF CAPACITANCE VALUE

Figure 16. Low Frequency Transimpedance Cutoff vs. CLF Capacitance Using

Typical Data with a 0.1 µF Ceramic Capacitor and Simulation Results with

1 pF to 1 µF Capacitance

Rev. 0 | Page 9 of 12

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