Choosing a high-speed silicon PIN detector can yield measurable gains in SNR and timing for near‑IR applications; designers often see improved detection thresholds and sub‑nanosecond timing when amplifier bandwidth and device capacitance are optimized. This write‑up on the SFH2400FA Photodiode delivers exact electrical and optical specs, interpretation of key metrics, recommended test methods, integration tips, and a compact selection checklist so engineers can evaluate suitability quickly.
The SFH2400FA family is a silicon PIN photodiode in a compact SMD three‑pin package designed for fast near‑IR detection. Typical applications include near‑IR sensing, ambient light rejection, short‑range optical links, encoder/read‑head systems, and industrial opto‑sensing. Designers favor PIN devices for the balance of speed, responsivity around 870–900 nm, and a small active area that simplifies optics and reduces junction capacitance for faster response.
| Parameter | SFH2400FA (High-Speed) | Generic 5mm PIN | Engineer's Impact |
|---|---|---|---|
| Rise/Fall Time | 5 ns | 20 - 50 ns | 4x faster pulse detection |
| Capacitance (@5V) | 11 pF | 25 - 40 pF | Lower TIA noise floor |
| Spectral Range | 750 – 1100 nm | 400 – 1100 nm | Inherent daylight filtering |
The SFH2400FA's peak sensitivity at 900 nm makes it perfectly matched for high-power NIR LEDs used in security barriers. By minimizing the active area to 1mm², the device achieves lower noise equivalent power (NEP), allowing for longer detection ranges without increasing transmitter power.
"When laying out the SFH2400FA, common pitfalls include neglecting the guard trace around the high-impedance node. To achieve the datasheet's 5ns rise time, I recommend a four-layer PCB stackup with a dedicated ground plane directly beneath the TIA feedback resistor to minimize parasitic capacitance. If you see 'ringing' in your pulse response, check if your bias decoupling capacitor (typically 0.1µF X7R) is placed further than 2mm from the photodiode cathode."
Responsivity R (A/W) converts incident optical power to photocurrent via Iph = R · Popt. For example, with R = 0.65 A/W at 900 nm, a 1 µW input produces Iph = 0.65 µA; a 10 µW input yields 6.5 µA. Quantum efficiency relates to responsivity by η = (R · hc)/(q·λ); matching detector peak wavelength to source emission maximizes detected current and simplifies amplifier gain budgeting for a target SNR.
A minimal bench setup includes a stabilized broadband or monochromatic source with known spectral output, a calibrated optical power meter, a low‑noise transimpedance amplifier, oscilloscope or lock‑in amplifier, and temperature control. Document bias voltage, integration time, and aperture.
The SFH2400FA Photodiode excels for near‑IR responsivity and fast timing when paired with an amplifier and layout optimized for low capacitance and adequate bandwidth. The most important metrics to verify are responsivity at the operating wavelength, dark current at intended bias, rise/fall time, and junction capacitance.
Use a calibrated monochromatic source or narrow‑band LED at the target wavelength, measure optical power with a calibrated power meter at the detector plane, and record photocurrent under the intended bias. Calculate R = Iph/Popt.
Select amplifier bandwidth roughly 3–5× the signal bandwidth. tr ≈ 0.35/BW. Ensure the feedback resistor doesn't saturate the output at peak illumination.
Measure leakage current with the device completely shielded from light. If it exceeds 10nA at 5V bias, check for PCB surface contamination or flux residue, which are common culprits in SMD assemblies.