9 Application and Implementation

Note

Information in the following applications sections is not part of the TI component specification, and TI

does not warrant its accuracy or completeness. TI’s customers are responsible for determining

suitability of components for their purposes, as well as validating and testing their design

implementation to confirm system functionality.

9.1 Application Information

The OPA855-Q1 offers very high-bandwidth, high slew-rate, low noise, and better than –60 dBc of distortion

performance at frequencies of up to 100 MHz. These features make this device an excellent low-noise amplifier

in high-speed data acquisition systems.

9.2 Typical Application

Figure 9-1 shows the OPA855-Q1 configured as a transimpedance amplifier (U1) in a wide-bandwidth, optical

front-end system. A second amplifier, the OPA859-Q1, configured as a unity-gain buffer (U2) sets a dc offset

voltage to the THS4520. The THS4520 is used to convert the single-ended transimpedance output of the

OPA855-Q1 into a differential output signal. The THS4520 drives the input of the ADS54J64, 14-bit, 1-GSPS

analog-to-digital converter (ADC) that digitizes the analog signal.

+

±

3.8 V

5 V

OPA855

+

±

499

499

499

±

+

499

5 V

+

±

3.25 V

5 V

OPA859

ADS54J64

Low-pass

filter

U1

U2

Figure 9-1. OPA855-Q1 as a TIA in an Optical Front-End System

9.2.1 Design Requirements

The objective is to design a low noise, wideband optical front-end system using the OPA855-Q1 as a

transimpedance amplifier. The design requirements are:

• Amplifier supply voltage: 5 V

• TIA common-mode voltage: 3.8 V

• THS4520 gain: 1 V/V

• ADC input common-mode voltage: 1.3 V

9.2.2 Detailed Design Procedure

The closed-loop bandwidth of a transimpedance amplifier is a function of the following:

amplifier (common-mode and differential capacitance) and any stray capacitance from the PCB.

2. The op amp gain bandwidth product (GBWP).

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OPA855-Q1

SBOSA57 – FEBRUARY 2021

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