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Entry  Thu Aug 20 00:21:51 2020, gautam, Update, Electronics, First look at HV coil driver IMG_8724.JPGtimeDomain.pdfHVampNoise.pdf
    Reply  Sun Aug 23 23:36:58 2020, gautam, Update, Electronics, First look at HV coil driver IMG_5379.JPGstabilityCriterion.pdf
       Reply  Wed Aug 26 16:12:25 2020, gautam, Update, Electronics, Test mass coil current requirements coilCurrents.png
       Reply  Tue Sep 1 15:39:04 2020, gautam, Update, Electronics, HV coil driver oscillations fixed testSetup.pdfHVampNoise_driven.pdf
          Reply  Thu Oct 22 11:14:47 2020, gautam, Update, Electronics, HV coil driver packaged into 2U chassis HVampNoise_driven_chassis.pdfHVampNoise_dispUnits.pdfD1900163_measurementSetup.zip
             Reply  Thu Oct 22 13:04:42 2020, rana, Update, Electronics, HV coil driver packaged into 2U chassis 
                Reply  Thu Oct 22 22:01:53 2020, gautam, Update, Electronics, HV coil driver packaged into 2U chassis DACnoiseFilterGain.pdfDACnoiseFilterNoises.pdf
                   Reply  Fri Oct 23 09:03:43 2020, anchal, Update, Electronics, HV coil driver packaged into 2U chassis 
             Reply  Thu Nov 12 14:55:35 2020, gautam, Update, Electronics, More systematic noise characterization powerSupplyNoise.pdfcoherence.pdf
                Reply  Thu Nov 12 15:40:42 2020, Koji, Update, Electronics, More systematic noise characterization 
                   Reply  Mon Nov 16 00:02:34 2020, rana, Update, Electronics, More systematic noise characterization 
                      Reply  Wed Jan 20 10:13:06 2021, gautam, Update, Electronics, HV Power supply bypassing bypassCaps.pdfIMG_9079.jpgIMG_9078.jpgHVampNoise_driven_chassis.pdfprintCoilCurrents.pdf
                         Reply  Mon Feb 1 12:30:21 2021, gautam, Update, Electronics, More careful characterization HVPS.pdfHV_testckt.pdftotalNoise.pdf
                         Reply  Wed Feb 10 21:14:03 2021, gautam, Update, Electronics, Production version of the HV coil driver tested inputDiffRecTF.pdfLVnoises.pdftotalNoise.pdftimeDomainTests.pdf
                            Reply  Fri Feb 26 16:31:02 2021, gautam, Update, Electronics, Production version of the HV coil driver tested with KEPCO HV supplies totalNoise_KEPCO.pdf
                               Reply  Fri Feb 26 20:20:43 2021, Koji, Update, Electronics, Production version of the HV coil driver tested with KEPCO HV supplies 
                                  Reply  Sat Feb 27 17:25:42 2021, gautam, Update, Electronics, Production version of the HV coil driver tested with KEPCO HV supplies 
                                     Reply  Thu May 20 16:56:21 2021, Koji, Update, Electronics, Production version of the HV coil driver tested with KEPCO HV supplies P_20210520_154523_copy.jpg
Message ID: 15536     Entry time: Sun Aug 23 23:36:58 2020     In reply to: 15534     Reply to this: 15542   15552
Author: gautam 
Type: Update 
Category: Electronics 
Subject: First look at HV coil driver 

Summary:

A more careful analysis has revealed some stability problems. I see oscillations at frequencies ranging from ~600kHz to ~1.5 MHz, depending on the voltage output requested, of ~2 V pp at the high-voltage output in a variety of different conditions (see details). My best guess for why this is happening is insufficient phase margin in the open-loop gain of the PA95 high voltage amplification stage, which causes oscillations to show up in the closed loop. I think we can fix the problem by using a larger compensation capacitor, but if anyone has a better suggestion, I'm happy to consider it

Details:

The changes I wanted to make to the measurement posted earlier in this thread were: (i) to measure the noise with a load resistor of 20 ohms (~OSEM coil resistance) connected, instead of the unloaded config previously used, and (ii) measure the voltage noise on the circuit side (= TP5 on the schematic) with some high voltage output being requested. The point was to simulate conditions closer to what this board will eventually be used in, when it has to meet the requirement of <1pA/rtHz current noise at 100 Hz. The voltage divider formed by the 25 kohm series resistor and the 20 ohm OSEM coil simulated resistance makes it hopeless to measure this level of voltage noise using the SR785. On the other hand, the high voltage would destroy the SR785 (rated for 30 V max input). So I made a little Pomona box to alllow me to do this measurement, see Attachment #1. Its transfer function was measured, and I confirmed that the DC high voltage was indeed blocked (using a Fluke DMM) and that the output of this box never exceeded ~1V, as dictated by the pair of diodes - all seemed okay .

Next, I wanted to measure the voltage noise with ~10mA current flowing through the output path - I don't expect to require more than this amount of current for our test masses. However, I noticed some strange features in the spectrum (viewed continuously on the SR785 using exponential averaging setting). Closer investigation using an oscilloscope revealed:

  1. 600kHz to 1 MHz oscillations visible, depending on output voltage.
  2. The oscillations vanish if I drive output above +30 V DC (so input voltage > 1 V).
  3. The oscillations seem to be always present when the output voltage is negative.
  4. No evidence of this offset if circuit is unloaded and voltage across 25k resistor is monitored. But they do show up on scope if connected to circuit side even in this unloaded config.

Some literature review suggested that the capacitor in the feedback path, C4 on the schematic, could be causing problems. Specifically, I think that having that capacitor in the feeddback path necessitates the use of a larger compensation capacitor than the nominal 33pF value (which itself is higher than the 4.7pF recommended on the datasheet, based on experience of the ESD driver circuit which this is based on, oscillations were seen there too but the topology is a bit different). As a first test of this idea, I removed the feedback capacitor, C4 - this seemed to do the trick, the oscillations vanished and I was able to drive the output between the high voltage supply rails. However, we cannot operate in this configuration because we need to roll off the noise gain for the input voltage noise of the PA95 (~6 nV/rtHz at 100 Hz will become ~200 nV/rtHz, which I confirmed using the SR785). Using a passive RC filter at the output of the PA95 (a.k.a. a "snubber" network) is not an option because we need to sum in the fast actuation path voltage at the output of the 25 kohm resistor.

Some modeling confirms this hypothesis, see Attachment #2.  The quantity plotted is the open-loop gain of the PA95 portion of the circuit. If the phase is 0 degrees, then the system goes unstable.

So my plan is to get some 470pF capacitors and test this idea out, unless anyone has better suggestions? I guess usually the OpAmps are compensated to be unconditionally stable, but in this case maybe the power op-amp is more volatile?

Quote:

Need to think more about how to better characterize this noise. An estimate of the required actuation can be found here.

Attachment 1: IMG_5379.JPG  1.968 MB  Uploaded Mon Aug 24 00:46:41 2020  | Hide | Hide all
IMG_5379.JPG
Attachment 2: stabilityCriterion.pdf  131 kB  Uploaded Mon Aug 24 01:00:23 2020  | Hide | Hide all
stabilityCriterion.pdf
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