OPA855 datasheet(21/30 Pages) TI1

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VBIAS

3.8 V

5 V

OPA855

499

VOCM = 1.3 V

5 V

3.25 V

5 V

OPA859

ADS54J64

Low-pass

filter

OPA855

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SBOS622A – JULY 2018 – REVISED OCTOBER 2018

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10.2 Typical Application

Figure 55 shows the OPA855 configured as a transimpedance amplifier (U1) in a wide-bandwidth, optical front-

end system. A second amplifier, the OPA859, 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 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.

Figure 55. OPA855 as a TIA in an Optical Front-End System

10.2.1 Design Requirements

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

•

ADC analog differential input range: 1.1 VPP

10.2.2 Detailed Design Procedure

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

1. The total input capacitance (CIN). This total includes the photodiode capacitance, the input capacitance of the

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

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

3. The transimpedance gain (RF).

Figure 55 shows the OPA855 configured as a TIA, with the avalanche photodiode (APD) reverse biased so that

the APD cathode is tied to a large positive bias voltage. In this configuration, the APD sources current into the op

amp feedback loop so that the output swings in a negative direction relative to the input common-mode voltage.

To maximize the output swing in the negative direction, the OPA855 common-mode voltage is set close to the

positive limit; only 1.2 V from the positive supply rail. The feedback resistance (RF) and the input capacitance

(CIN) form a zero in the noise gain that results in instability if left unchecked. To counteract the effect of the zero,

a pole is inserted into the noise gain transfer function by adding the feedback capacitor (CF).

The Transimpedance Considerations for High-Speed Amplifiers Application Report discusses theories and

equations that show how to compensate a transimpedance amplifier for a particular transimpedance gain and

input capacitance. The bandwidth and compensation equations from the application report are available in an

Excel™ calculator. What You Need To Know About Transimpedance Amplifiers – Part 1 provides a link to the

calculator.

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