ADA4351-2ACPZ-R7 datasheet(27/36 Pages) AD

27 / 36 page

Data Sheet

ADA4351-2

THEORY OF OPERATION

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Rev. 0 | 27 of 36

Current Noise

When using large transimpedance gains, 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 × RF). Similar to the leakage

and bias current errors in the TIA, the current noise becomes more

dominant at higher TIA gains. Also, if the photodiode is reverse

biased, users must include its dark current noise contribution, which

can, in some cases, become the dominant current noise term.

In CMOS amplifiers, the input bias current noise increases over

frequency and, therefore, can have a major impact on the total

integrated noise. The ADA4351-2 was 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 1 MΩ, the Johnson noise of the resistor itself

at the output is 127 nV/√Hz (at room temperature), while the 110

fA/√Hz current noise spectral density of the amplifier times the TIA

gain at 10 kHz is a comparable 110 nV/√Hz . Both of these noise

sources are usually dominated by the voltage noise contribution in

most applications (see the Noise Contributions for Transimpedance

Amplifiers section).

ADA4351-2ACPZ-R7 Datasheet(PDF) 27 Page - Analog Devices