ADA4356ABCZ datasheet(34/61 Pages) AD

34 / 61 page

Data Sheet

ADA4356

analog.com

Rev. 0

34 of 61

THEORY OF OPERATION

The ADA4356 integrates a field effect transistor (FET) input TIA with three switchable gains (4.54kΩ, 11kΩ, and

133kΩ). 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 40mA to be sustained with no damage to the TIA. The positive node of the TIA is biased to

1.65V, as shown in Figure 2.

Figure 79 shows the overall system transfer function. Because the photodiode provides unipolar current (sink or

source), the overall transfer function has 0.825V offset to maximize the input range of the ADC. When the input

current is 0µA, the ADC differential input is 0.825V. As the input current increases, the TIA output decreases toward

GND. When the input current reaches 1.65V/TZ, the TIA output is at GND, limiting the ADC differential input voltage

to −0.825V. The positive full-scale input current is 1.65V/TZ, and there is room to measure negative input current

down to −0.175V/TZ.

Figure 79. Overall Transfer Function

Figure 80. Typical Application Block Diagram with a Single VCC Supply, FPGA Control, and Data Process

LINEAR INPUT RANGE

1.65V

+1V

ADC

INPUT

0.825V

–0.825V

–1V

INPUT

CURRENT

1.65V

TZ =

0.175V

GSEL0,

GSEL1

VCC

VLDEN

VLD

VEA/VED

+3.3V

FSEL

LDO

PHOTODIODE

INPUT

IIN

PGTIA +

FDA

OTDR DATA

FPGA

MICROPROCESSOR

LVDS LANES

SPI SIGNALS

ADC

SPI

GND

CLKN

ADA4356

CLKP

BANDWIDTH

LPF

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