ADN2820

Rev. 0 | Page 9 of 12

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:

where:

OFFSET = voltage applied to the OFFSET pad

calculated from the following equation:

–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

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

the typical –3 dB low frequency transimpedance cutoff can be

estimated by the equation

required to be a high frequency capacitor type. A ceramic

capacitor is recommended.

1k

10k

100k

1M

10M

100M

0.1nF

1nF

1pF

10pF

10nF

0.1

µF1µF

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

1 pF to 1 µF Capacitance