vladimir.bossilkov@LIGO.ORG - posted 10:13, Friday 10 January 2020 - last comment - 13:00, Friday 10 January 2020(54403)
Issue in Pcal Boards
Since my last aLog on this topic, I have disconnected the temperature sensors and manually run currents across where it was to look for cross coupling, and found that there was none. This leaves me to understand that there is a lot of unknown weirdness on the Pcal board. I did tests where I input current to the Pcal board where the photodiode would be, and found temperature dependances that I would expect to be from just the Resitance of resitor R5 on the boad (board document link is further down this post). This lead me to look into what might be going wrong between the Transimpednace amplifier and the Photodiode - Hence motivating this post.
Reading carefully through the spec sheet I noticed that for the Transimpedance amplifer (THS4631) [https://www.ti.com/lit/ds/symlink/ths4631.pdf [Table 1, on page 10]] , the capacitance values of our photo-diode don't really line up with this table, and what our feedback capacitor value is.
The stability equation is as follows:
I refer to the board diagram in D1300210-v5.
Where C_F = Required feedback capacitance (C5 on our board, currently 2.7 pf)
R_F = Feedback resistance (R2 on our board, currently 20000 Ohm)
f_GBWP = Unity Gain bandwidth of our Transimpedance Amplifier (first thing listed in spec sheet: 325 MHz)
C_i = the Capacitance of the input (in this case our photo-diode)
If our photo-diode has a capacitance of 300 pf (when it has a 5 V voltage across it) - this would evaluate to C_F = 2.7 pf. (You'll have to chase up the spec sheet of the photodiode upon the internet, there is no convenient direct link. The model is C30665GH, and I'm referring to the 2018 version of the spec sheet)
*However*, we run our photo-diode in photovoltaic mode where it has better noise characteristics. In this case it actually has a capacitance of 1000 pf - meaning we need C_F = 4.96 pf
This should be the last piece in the puzzle with regard to things not understood in our board, and why its doing weird things with rising temperatures. *This currently affects all Pcal instruments (all IFOs)*
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If you are reading this in Calibration, and not sure what it means for the IFO and not some hand wavy "temperature dependence study" sence:
Pcal is only ever calibrated at DC frequency (0 Hz).
Having the wrong feedback capacitance introduces frequency dependant response change. Simply: With increasing frequency, the response change will have some non unity magnitude, and is unstable after some frequency.
To understand that change, the system needs to be modelled in a SPICE package.