Here's a transimpedance amplifier board that fits inside a 1" lens-mount. It has low noise LDOs (LT3042 and LT3094) and a SOT-23-6 op-amp footprint (for e.g. OPA657 or OPA847).
The first board has a S5973 photodiode, an OPA657, a 10 kOhm feedback resistor, and a 0.2pF feedback capacitor. The circuit self-oscillates without any feedback capacitor.
The backside of the board has an MMCX-connector for the output signal, and 3 wires for power supplies (max +/-20V).
The noise-floor shows an ~18 dB bump as the 59 MHz -3dB bandwidth is approached - not that great. At low frequencies the noise-floor agrees with the thermal noise of a 10k resistor. Shot-noise from about 10uW optical power, producing a 4 uA photocurrent (photodiode responsivity ~0.4A/W), should be clearly (3 dB or so) above the 10k resistor thermal noise floor.
The noise-bump could be decreased with a larger feedback-capacitor, but this reduces bandwidth. The source capacitance is due to the S5973 photodiode (1.6 pF) and the differential (0.7 pF) and common-mode (4.5 pF) input-capacitance of the OPA657 op-amp. Clearly a lower input-capacitance opamp would be better. Stay tuned for tests with OPA858...
Note to self: can't use the Siglent SA's default detector mode of "Positive-peak" if we want quantitative dBm numbers from the analyzer. Use "sample" detector mode!
5 thoughts on “One Inch Photodetector, v1”
Fantastic idea! I'm also quite a fan of building custom boards for my optics experiments. Have you tried out the OPA858 version yet?
I'm inspired by the SM1 mount - I think I will "copy" you and see if I can build a variant for a quadrant PD 🙂
Since this blog was posted, I put the kicad sources on github: https://github.com/aewallin/One-Inch-Photodetector
I think I built one board with OPA855 or OPA859, but it self-oscillated and I did not have time to debug it properly. Obviously the inductance and capacitance on the op-amp input pins are very critical for the high-speed parts!