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Entry  Sat Jan 20 01:20:37 2018, gautam, Update, Electronics, Whitening filter D990694 whiteningBoardLimitations.pdf
    Reply  Sat Jan 20 15:57:11 2018, rana, Update, Electronics, Whitening filter D990694 
       Reply  Tue Jan 23 01:33:23 2018, gautam, Update, Electronics, Whitening filter D990694 
          Reply  Wed Jan 24 00:48:47 2018, gautam, Update, Electronics, Whitening filter D990694 
Message ID: 13563     Entry time: Sat Jan 20 01:20:37 2018     Reply to this: 13564
Author: gautam 
Type: Update 
Category: Electronics 
Subject: Whitening filter D990694 

We use D990694 in various places. Today, Rana alerted me to an important consideration to be kept in mind when we use this board, which I found quite interesting. I still don't understand the problem at the BJT level, but I think one can appreciate the problem without going to the transistor design of the LT1125. I'm attaching an annotated schematic of the whitening section in question. If the following assumptions are valid, then I think my picture is valid.

  1. The switch used to bypass the various whitening gain stages, namely the ADG333ABR, has infinite impedance in the "OFF" state, such that when the 24dB gain stage is bypassed, U28A (or in general one of the 4 quad op-amps) is forced to drive it's output voltage across 1.0665 kohms of resistance.
  2. The individual LT1125 Op Amps can drive a maximum of 30mA of current.

Then, as one can see in the attached schematic, when we set the gain of any input to <24dB, we must ensure that the input voltage is less than approximately 2V. Otherwise, by asking too much of the first stage op-amp in the quad IC LT1125, we may be messign around with all the 4 op amps in the quad! Even the 0dB setting is not immune to this problem, as it uses one of the 4 op amps.

I don't think the usual rules of calculating the gain of a non-inverting amplifier (G = 1 + Rf/Ri) remain valid even when the op-amp is forced to drive more output current than it can, and I don't have a way to quantify the possible interference between the 4 op amps in the quad - but does this seem like a valid conclusion? If so, we must check signal levels of various LSC signals. AS55 signals currently have the 0dB gain setting - I had turned this down from 6dB some months ago, because it seemed like the ADC was saturating at the 6dB gain setting, which suggests that the input voltage is ceratinly > 2V, and AS55_Q is what is used for MICH control in the DRMI. All of my noise budgeting work over the last few months used this setting, I wonder if they are all invalid surprise


Now that I think about this a bit more - this problem shouldn't be significant for the usual LSC degrees of freedom when in lock, as the huge DC gain of the loop should squish large DC values of the error signals, and so there shouldn't be any danger of overloading the LT1125. But I don't know if we are being hurt by this effect when flashing through resonances, when the PDH horn-to-horn voltage can be quite high (which is in principle a good thing?). I don't know if there is any "hysterisis" effect where the overloaded quad IC has some relaxation time before it returns to normal operation, and if we are being limited in our ability to catch lock because if this effect. 

The concerns remain valid for th ALS demodulated error signals though, for which the signals will remain large throughout.

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