Data Sheet

ADA4352-2

analog.com

Rev 0

35 of 47

Current Noise

When using large transimpedance gains (that is, 450 kΩ), the current noise of the main amplifier can be a major

source of noise in a system. The current noise flows through the feedback resistor and appears at the output as a

voltage (current noise × RFX). Also, if the photodiode is reverse-biased, its dark current noise contribution can, in

some cases, become the dominant current noise term.

In CMOS amplifiers, the input current noise increases over frequency and, therefore, can have a major impact on

the total integrated noise. The ADA4352-2 is designed to have low current noise, but it increases with frequency. A

110 fA/√Hz flat current noise spectral density from 10 kHz to 100 kHz is used in the Photodiode Circuit Design Wizard

to represent the worst case high frequency noise.

Using a TIA gain of 450 kΩ, the Johnson noise of the resistor itself at the output is 86 nV/√Hz (at room temperature),

while the gained-up 110 fA/√Hz current noise spectral density of the amplifier at 10 kHz is a comparable 49.5

nV/√Hz. These current noise sources are usually insignificant compared to the voltage noise contribution (refer to

the “Noise Contributions for Transimpedance Amplifiers” section in the ADA4351-2 data sheet).