ADA4355ABCZ datasheet(26/45 Pages) AD

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ADA4355

Data Sheet

Rev. A | Page 26 of 45

THEORY OF OPERATION

The ADA4355 integrates a field effect transistor (FET), input

TIA with three switchable gains (2 kΩ, 20 kΩ, and 200 kΩ).

The gain switches are designed to minimize error sources that

result in slow settling time and slow overload recovery. The

internal overload current protection allows the input current to

exceed the full-scale current while still providing fast overload

recovery. Additionally, the overload current protection enables

analog input current levels up to 40 mA to be sustained with no

damage to the TIA. The positive node of the TIA is biased to

1.65 V as shown in Figure 2.

Figure 77 shows the overall system transfer function. Because the

photodetector provides current in only one direction (sink or

source), the overall transfer function has 0.825 V offset to

maximize the input range of the ADC. When the input current

is 0 µA, the ADC differential input is 0.825 V. As the input

current increases, the TIA output decreases toward GND.

When the input current reaches 1.65 V/TZ, the TIA output is at

GND, limiting the ADC differential input voltage to −0.825 V.

The positive full-scale input current is 1.65 V/TZ, and there is

room to measure negative input current down to −0.175 V/TZ.

LINEAR INPUT RANGE

1.65V

+1V

ADC

INPUT

0.825V

–0.825V

–1V

INPUT

CURRENT

1.65V

TZ =

0.175V

Figure 77. Overall Transfer Function

GSEL0,

GSEL1

VCC

VLDEN

VLD

VEA/VED

+3.3V

FSEL

LDO

PHOTODIODE

INPUT

IIN

TIA

OTDR DATA

FPGA

MICROPROCESSOR

LVDS LANES

SPI SIGNALS

ADC

SPI

GND CLKN

ADA4355

CLKP

BANDWIDTH

LPF

Figure 78. Typical Application Block Diagram with a Single VCC Supply, FPGA Control, and Data Process

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