9.2.1.2 Detailed Design Procedure

The OPA859 meets the growing demand for wideband, low-noise photodiode amplifiers. 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 9-1 shows the OPA859 configured as a transimpedance amplifier (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 common-mode

voltage of the OPA859 is set close to the positive limit; only 1.5 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 OPA859 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 = 10 kΩ and 20 kΩ. Increasing CPD decreases the closed-loop

bandwidth. To maximize bandwidth, make sure to reduce any stray parasitic capacitance from the PCB. The

OPA859 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 pF and 2 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.

The OPA859 configured as a unity-gain buffer drives a dc offset voltage of 2.95 V into the lower half of the

THS4520. To maximize the dynamic range of the ADC, the two OPA859 amplifiers drive a differential common-

mode of 3.5 V and 2.95 V into the THS4520. The dc offset voltage of the buffer amplifier can be derived using

Equation 1.

VBUF_DC=VTIA_CM− 12×VADC_DIFF_IN

RFRG

(1)

where

• VTIA_CM is the common-mode voltage of the TIA (3.5 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.

OPA859

SBOS852A – SEPTEMBER 2018 – REVISED MAY 2025

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