I have looked at the photo of the Xend QPD from elog 9367, as well as the schematic for the board (D990272).
Things that will need swapping out:
- Thick film resistors in the signal path need to be changed to thin film.
- MAX333 needs to be replaced with MAX333A. The 333 has "ON Resistance" of 140-175 ohms, whereas the 333A has "ON Resistance" of 20-35 ohms.
- AD797 needs to be replaced by OPA140. The 797 is a low voltage noise op-amp, but for a diode we want low current noise. AD797 has 2pA/rtHz at 1kHz, whereas the OP140 has 0.8fA/rtHz at 1kHz (see Zach's elog 8125 re: OPA140).
I have ordered from digikey via techmart 10 each of the MAX333A's and the OPA140's. (4 per QPD times 2 QPDs plus 2 spares = 10). Both of these new chips have the same footprint and pinout as the part that they are replacing, so it'll be a fairly easy task.
Next up, I need to make a LISO model for the circuit for one of the quadrants, to see what shape it'll turn out to be. Part of this will include deciding what resistors and capacitors to put in the OPA140 gain stage.
Right now, the AD797s say on the schematic that the gain options are different by a factor of 5, but the actual QPD has a different resistor than is on the schematic, and there is also a capacitor in parallel with each resistor, so I need to just pull those out, and pick some values that make sense to me.
Rana and I have discussed ignoring the 2nd and 3rd gain switching options on each quadrant, as that is getting to be more fine control than we need.
Other things on the board:
- The 50 ohm resistors to ground for the "QPD_rtn" have all shorted. Rana says this is good, so leave it as-is.
- The positive input to the AD797's all have a 100 ohm resistor to ground, rather than just being connected to ground. Why is this??
For now, I will probably just work on the QPD head, and not the whitening board. For now, we can run with 1 stage of whitening, and if we need lower noise, we can revisit the whitening board (including replacing the thick film resistors with thin film).
When thinking about what gains I want on my gain stages, I want to have my full arm power (~700 TRX units) be ~20,000 counts from the ADC. So, I want my single arm power (1 TRX unit) to be ~30 counts from the ADC. This is not such a big number, so this may also require more thinking. |