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 Fri Aug 3 15:53:25 2018, gautam, Update, SUS, Low noise bias path idea Fri Aug 3 16:27:40 2018, rana, Update, SUS, Low noise bias path idea Sun Aug 5 15:43:50 2018, gautam, Update, SUS, Another low noise bias path idea Wed Aug 8 23:06:59 2018, gautam, Update, SUS, Another low noise bias path idea Wed Aug 15 19:18:07 2018, gautam, Update, SUS, Another low noise bias path idea Thu Aug 16 23:06:50 2018, gautam, Update, SUS, Another low noise bias path idea Mon Oct 1 22:20:42 2018, gautam, Update, SUS, Prototyping HV Bias Circuit
Message ID: 14129     Entry time: Fri Aug 3 15:53:25 2018     Reply to this: 14130
 Author: gautam Type: Update Category: SUS Subject: Low noise bias path idea

Summary:

The idea we are going with to push the coil driver noise contribution down is to simply increase the series resistance between the coil driver board output and the OSEM coil. But there are two paths, one for fast actuation and one that provides a DC current for global alignment. I think the simplest way to reduce the noise contribution of the latter, while preserving reasonable actuation range, is to implement a precision DC high-voltage source. A candidate that I pulled off an LT application note is shown in Attachment #1.

Requirements:

• The series resistance in the bias path should be $10 k\Omega$, such that the noise from this stage is dominated by the Johnson noise of said resistor, and hence, the current noise contribution is negligible compared to the series resistance in the fast actuation path ($4.5 k\Omega$).
• Since we only really need this for the test masses, what actuation range do we want?
• Currently, ETMY has a series resistance of $400\Omega$ and has a pitch DC bias voltage of -4 V.
• This corresponds to 10 mA of DC current.
• To drive this current through $10 k\Omega$, we need 100 V.
• I'm assuming we can manually correct for yaw misalignments such that 10mA of DC current will be sufficient for any sort of corrective alignment.
• So +/- 120 V DC should be sufficient.
• The current noise of this stage should be negligible at 100 Hz.
• The noise of the transistors and the HV supply should be suppressed by the feedback loop and so shouldn't be a significant contribution (I'll model to confirm).
• The input noise of the LT1055 is ~20nV/rtHz at 100 Hz, while the Johnson noise of $10 k\Omega$ is ~13nV/rtHz so maybe the low-passing needs to be tuned, but I think if it comes to it, we can implement a passive RC network at the output to achieve additional filtering.
• To implement this circuit, we need +/- 125V DC.
• At EX and EY, we have a KEPCO HV supply meant to be used for the Green Steering PZTs.
• I'm not sure if these can do bipolar outputs, if not, for temporary testing, we can transport the unit at EY to EX.

If all this seems reasonable, I'd like to prototype this circuit and test it with ETMX, which already has the high series resistance for the fast path. So I will ask Steve to order the OpAmp and transistors.

 Attachment 1: LT1055_precOpAmp.pdf  84 kB  Uploaded Fri Aug 3 17:14:08 2018
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