PDH photodiode

Wideband photodetector based on a Si-PIN photodiode (Hamamatsu S9055-01) and a transimpedance amplifier (TIA). The detector features an additional non-inverting (voltage) amplifier stage AC-coupled to the TIA designed for the purpose of Pound-Drever-Hall (PDH) laser locking.

An alternative version with Excelitas C30617L-100 InGaAs photodiode for applications between 960 nm and 1700 nm can be found here. All specs of the amplifiers hold for this version as well accounting for the significantly increased sensitivity of the photodiode.

• Supply: ±5 V (typ. 40mA)

• Max. output : 2.5 V @high-Z / 1.25 V @50 Ω

• TIA gain: 20 kΩ

• AC gain: 10 - 25 (default: 10)

• Bandwidth: ~160 MHz

• DC sensitivity

  • ~7 V/mW @ 780 nm (S9055-01)
  • ~13 V/mW @ 960 nm (C30617L-100)

• Noise equivalent power (NEP)

  • < 25 pW/Hz^1/2 @ 780 nm (0.1 - 50 MHz: ~2.5 pW/Hz^1/2) for S9055-01
  • < 14 pW/Hz^1/2 @ 780 nm (0.1 - 50 MHz: ~1.4 pW/Hz^1/2) for S9055-01

• Optional: Low-pass filters at DC out (1. order) and AC out (2x 1. order)

Design Files

The design files can be found on the releases page and include the following resources:

• Schematics as a PDF
• Gerber files
• Pick & place position files
• Bill of materials as a CSV file and also as an interactive HTML version

The latest revision of those files can be found here.

Wideband TIAs using LTC6268-10/6269-10

The LTC6268-10/6269-10 is an ultra-low input bias operational amplifier. The datasheet (p. 12-14) gives some useful information for TIA applications. The main ideas are cited here:

• The maximal achievable bandwidth is: $$f_c = \sqrt{\frac{GBW}{2\pi R_f C_{in}}} \le 195\text{ MHz}$$ Where GBW is the gain bandwidth product of the op amp (4 GHz), R_f is the feedback resistance (default: 20kOhm), and C_in is the total capacitance at the inverting input of the op amp. I.e. the input capacitance of the op amp (0.45 pF), the capacitance of the photodiode (at the given reverse voltage, ~0.4pF), and some unknown but not negligible parasitic capacitance of the circuit (e.g. soldering pads). Hence:

  • Use a low capacitance photodiode
  • The fundamental bandwidth limit scales as R_f^-1/2

• Every tiny parasitic feedback capacitance C_f reduces the actual bandwidth to: $$f_c = \frac{1}{2\pi R_f C_{f}}$$ For the example above this is the case for C_f as small as 40fF.

  • The actual bandwidth limit scales as R_f^-1 if C_f is not negligible.
  • Reduce C_f to a minimum.

• The LTC6268-10/6269-10 is a decompensated amplifier. It is only stable for a noise gain above 10. For a TIA the relevant high frequency noise gain is given by the impedance of C_f and C_in. Resulting in the condition: $$\frac{C_{in}%2b C_f}{C_f}\ge 10$$ This means:

  • If the TIA is not stable, try to reduce C_f or increase C_in (accepting a reduction of the bandwidth)
  • If the TIA is stable for a given input capacitance, a lower bound for the bandwidth is given by: $$f_c \ge \frac{9}{2\pi R_f C_{in}}$$

Analog Devices photodiode wizard

The web tool https://tools.analog.com/en/photodiode/ is useful when designing photodetectors based on TIAs. Use the tool to estimate noise performance and frequency response, if it is necessary to modify the circuit.

Related work

T. Preuschoff, Laser Technologies for Applications in Quantum Information Science, Ph.D. thesis, TU Darmstadt, 2023, https://tuprints.ulb.tu-darmstadt.de/23242/

License

This work is released under the CERN OHL v.1.2 See www.ohwr.org/licenses/cern-ohl/v1.2 or the included LICENSE file for more information.