OPA855 datasheet(24/41 Pages) TI2

24 / 41 page

OPA855 offers 8 GHz of gain bandwidth, high slew-rate, and low noise which makes this device an excellent for

a wide range of photodetectors. Figure 9-5 shows the OPA855 configured as a transimpedance amplifier (TIA) in

a wide-bandwidth, optical front-end system. Various types of optical sensors can be used as an optical input to

the amplifier: photodiode (PD), avalanche photodiode (APD), photomultiplier tube (PMT), and multipixel photon

counter (MPPC) or known as solid-state photomultiplier (SiPM). Optical detection applications have commonly

used APDs, but ultra-low light source detection has been a challenge in past options. With technologies such

as PMTs and MPPCs, the high intrinsic gain, while maintaining a fast output, requires a low noise, high-speed

interface. The OPA855 can accommodate for these optical challenges and can work equally as well in these

applications.

–

VBIAS

VOUT

Geiger-Mode

APD

Cathode

Anode

GND

Anode

PMT

PIN/

APD

SiPM/MPPC

Photoelectron

Dynode

Figure 9-5. Transimpedance Amplifier With APD or SiPM/MPPC or PMT Inputs

Transimpedance applications require low voltage and current noise for an excellent system performance. As a

result of the high input impedance structure, the OPA855 has a great balance between low input-referred voltage

noise and current noise that is consistent over frequency. Overall, the amplifier noise must have minimal impact

to the total noise of the application. Examine the total input referred noise to the optical sensor.

Noise sources in optical sensors vary especially when introducing gain and photon paralleling. Optical power,

gain, and applied reverse bias are the main characteristics that can affect signal to noise ratio. Standard

photodiodes contribute the lowest noise at the highest quantum efficiency. Internal to photodiodes, noises

sources include shot and thermal. Shot noise is a random occurrence of photodetection which arises in periods

of both light and dark. Dark current is noise that occurs in the absence of an optical source which can be

included with shot noise. And, thermal noise originates from the shunt resistance internal to the diode. At the

lower signal levels, shot noise can dominate. Figure 9-6 shows an example of the noise sources present in

a transimpedance amplifier circuit. The total TIA noise is the root sum square of each component within the

system: photodiode noise, amplifier current noise, amplifier voltage noise, and feedback resistor noise.

iN_Amp

VN_Amp

VOUT

VN_R

iN_D

VBIAS

–

Figure 9-6. Photodiode and TIA Noise Model

OPA855

SBOS622D – JULY 2018 – REVISED MAY 2025

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