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Entry  Mon Oct 3 13:59:40 2016, awade, DailyProgress, TempCtrl, Modified LIGO temperature sensor interface board 20161002_D980401_PSLTempBox.epstemp_AD590TransImpedanceBox_PSDs.zipD980400-B.pdf
    Reply  Mon Oct 3 18:15:23 2016, not awade, DailyProgress, TempCtrl, Modified LIGO temperature sensor interface board 
       Reply  Tue Oct 4 11:49:53 2016, awade, DailyProgress, TempCtrl, Modified LIGO temperature sensor interface board 20161002_D980401_PSLTempBox_Vrms.eps20161002_D980401_PSLTempBox_Vrms.fig
          Reply  Sun Oct 9 09:01:27 2016, not awade, DailyProgress, TempCtrl, Modified LIGO temperature sensor interface board 
             Reply  Mon Oct 10 11:32:27 2016, awade, DailyProgress, TempCtrl, Modified LIGO temperature sensor interface board 20161002_D980401_PSLTempBox.pdf
    Reply  Fri Sep 1 13:36:20 2017, awade, DailyProgress, TempCtrl, Modified LIGO temperature sensor interface board 
       Reply  Sun Sep 3 14:10:38 2017, awade, Kira, DailyProgress, TempCtrl, Modified LIGO temperature sensor interface board LIGO-Temperature-Sense-Board-SingleChannel.pdf
          Reply  Mon Sep 4 23:07:41 2017, Kevin, DailyProgress, TempCtrl, Modified LIGO temperature sensor interface board TemperatureSensingCircuit.pdfTemperatureBox.jpg
             Reply  Tue Sep 5 23:54:45 2017, Kevin, DailyProgress, TempCtrl, Modified LIGO temperature sensor interface board p1.jpgp2.JPG
          Reply  Wed Sep 6 19:58:21 2017, awade, Kira, DailyProgress, TempCtrl, Modified LIGO temperature sensor interface board 
Message ID: 1898     Entry time: Wed Sep 6 19:58:21 2017     In reply to: 1893
Author: awade, Kira 
Type: DailyProgress 
Category: TempCtrl 
Subject: Modified LIGO temperature sensor interface board 

Yes, sort of.

1) The pickoff for the output should not be in between those resistors, but should be after the 100 Ohm resistor. 

2) You should not put a resistor in between the two LT1125.

Quote:

[awade,  Kira]

So Kira and I had a look at the temperature sensor interface board on Friday afternoon.  The trans impedance stage wasn't the gain that we expected (about 1/3 of expected).  After a bit of investigation we found that one of the 12 V regulators had died (the LM7812).  We replaced it and now the reported voltage with a probe was ~9.3 V which is about right for the choice of 29.4 kΩ resistors.

We still have this issue with just about any loading driving the LT1125 op amps into ~12 kHz ringing.  We traced out the paths on the pcb and the cause is probably in the design of the second stage of op amp circuit.  There is a second amplification stage (also LT1125, the glued upside down chip) that is configured to pick off the output signal from a point within the feedback path from output to non-inverting pin.  I've attached a sketch up of a single channel of the circuit as it is actually wired up.  We're not entirely certain of the value of the caps (can't see any labels on them).  With caps over output to inverting pin it should be low pass filtering the signal as well.  Re: rana's post and subsequent discussion (PSL:1887), low pass filtering is maybe not the best idea going into an ADC and can maybe increase low frequency noise. Would we be better removing caps and digitally filtering signal? Unless the caps are there to compensate lead wire capacitance, which I'm guessing (from google-feeling-lucky-search) is roughly 49 pf/m, then are they just making the signal look nice at high frequency?

From rough back of the envelop calculations this looks like its drawing on order 10 mA for 100 Ω load and about double that for 50 Ω @ 9.3 V output. The max rating for the op amp is 20 mA, so this is probably the source of the issues.  The Acromag XT1211 differential voltage ADC cards say they are 100 kΩ but maybe there is some more complex interaction that I don't understand that causes them to perturb the temp. sense circuits: picking output signal off from within feedback loop seems like a bad idea to me; or, maybe there is something I'm missing. Maybe one side of the diff input on the ADC has some grounding thing so that the op amp see 100 kΩ across terminals but not to the single ended ground that the op amp sees?

I think maybe we should modify the second stage to have a resistor in series between op amp stages and to move the pick off for signal to outside the second op amp feed back path to look more like the usual amplifying/and or active low pass op amp circuit.  Not sure if we should remove or modify the caps?

Schematic attached.

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Link to partsim schematic 

 

 

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