Today I gave up trying to fix the first board I populated, and built a second one. The good news is that it's working as expected.
With 27.5 uW incident on each quadrant, I measure about 4.5 V, which is in line with the transimpedance of 200k, a responsivity of about 0.4 A/W and ad additional gain of two coming from the differential driver.
I also measured the noise with a SR785 (it wasn't connected to a GPIB interface and I couldn't find any, so all I have are the following numbers and the attached screenshots).
| Frequency [kHz] |
Noise [uV/rHz] |
Equivalent beam motion [m/rHz] |
| 1 |
1.3 |
5.7e-14 |
| 5 |
2.2 |
9.6e-14 |
| 10 |
3.3 |
14e-14 |
| 30 |
2.4 |
10e-14 |
| 60 |
1.2 |
5.2e-14 |
At low frequency we are dominated by 60 Hz harmonics (probably coming from the laser). At high frequency there are some large peaks of unknown origin. I can't acquire digitally the signals to compute the difference, so I don't know if the noise we see is, for example, laser intensity noise. As soon as the cymac is up and running, I'll run some more tests.
 
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