OPA855 datasheet(22/35 Pages) TI

22 / 35 page

Figure 9-1 shows the OPA855-Q1 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-Q1 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.

The equations and calculators in the referenced application report and blog posts are used to model

the bandwidth (f–3dB) and noise (IRN) performance of the OPA855-Q1 configured as a TIA. The resultant

performance is shown in Figure 9-2 and Figure 9-3. The left-side Y-axis shows the closed-loop bandwidth

performance, whereas the right side of the graph shows the integrated input-referred noise. The noise bandwidth

to calculate IRN for a fixed RF and CPD is set equal to the f–3dB frequency. Figure 9-2 shows the amplifier

performance as a function of photodiode capacitance (CPD) for RF = 6 kΩ and 12 kΩ. Increasing CPD decreases

the closed-loop bandwidth. To maximize bandwidth, make sure to reduce any stray parasitic capacitance from

the PCB. The OPA855-Q1 is designed with 0.8 pF of total input capacitance to minimize the effect of stray

capacitance on system performance. Figure 9-3 shows the amplifier performance as a function of RF for CPD

= 1.5 pF and 2.5 pF. Increasing RF results in lower bandwidth. To maximize the signal-to-noise ratio (SNR) in

an optical front-end system, maximize the gain in the TIA stage. Increasing RF by a factor of X increases the

signal level by X, but only increases the resistor noise contribution by √X, thereby improving SNR. Since the

OPA855-Q1 is a bipolar input amplifier, increasing the feedback resistance increases the voltage offset due to

the bias current and also increases the total output noise due to increased noise contributions from the amplifiers

current noise.

The OPA859-Q1 configured as a unity-gain buffer drives a DC offset voltage of 3.25 V into the lower half of

the THS4520. To maximize the dynamic range of the ADC, the OPA855-Q1 and OPA859-Q1 drive a differential

common-mode of 3.8 V and 3.25 V respectively into the THS4520. The dc offset voltage of the buffer amplifier

can be derived using Equation 1.

ADC _ DIFF _ IN

BUF _ DC

TIA _ CM

(1)

where

• VTIA_CM is the common-mode voltage of the TIA (3.8 V)

• VADC_DIFF_IN is the differential input voltage range of the ADC (1.1 VPP)

• RF and RG are the feedback resistance (499 Ω) and gain resistance (499 Ω) of the THS4520 differential

amplifier

The low-pass filter between the THS4520 and the ADC54J64 minimizes high-frequency noise and maximizes

SNR. The ADC54J64 has an internal buffer that isolates the output of the THS4520 from the ADC sampling-

capacitor input, so a traditional charge bucket filter is not required.

OPA855-Q1

SBOSA57A – FEBRUARY 2021 – REVISED MARCH 2021

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