Tuning Notes

This page contains several commissioning and tuning observations. Recommendations are listed at the bottom.

Drive/Sense Channel Noise

Board ID 04 was configured as follows:

  • Drive outputs shorted together
  • Sense inputs shorted together
  • Modulation frequency 13 Hz
  • Drive/sense gain resistors removed
This yields the following noise plots:

drive_sense_noise_shorted.png
Fig. 1: Drive and sense noise PSDs with shorted I/Os

In this configuration, the drive and sense circuits are disconnected and cannot interact. The current feedback loop is closed, and the voltage amplifier is shorted and cannot wander.

Noise for each is roughly commensurate, with a level of approximately . At the ADC, this corresponds to .

Drive Circuit Offsets

There are a couple of DC offsets

Sense Circuit Offsets

The sense circuit registers a small offset when reading out DC with the inputs shorted together. This must be due to a combination of the following sources:

  • regulator drift of v1.25 (applied to U11, the input amplifier)
  • regulator drift of v2.50 (applied to U1, the ADC)
  • input offset voltage at U11 (the input amplifier)
  • output offset voltage at U11 (the input amplifier)
Since the offset varies with gain, the amplifier's input offset voltage (typ. 20uV) must be playing a role. Since it and the output voltage offset are roughly commensurate, it seems reasonable to assume a similar contribution.

These offsets may be tuned with a shorted input, using two different gain resistances to pry the input and output offsets apart. These effects are only relevant at DC, since AC modulation will cause them to be filtered.

Gain
Typ. Max. Typ. Max.
1 33.6 100.7 83.9 503.3
10 335.5 1006.6 83.9 503.3
100 3355.4 10066.3 83.9 503.3
Table 1: LT1168C input offsets (U11), in ADC counts

Recommendations

  • Other todo: investigate

Attachments Attachments
Topic attachments
I Attachment Action Size Date Who Comment
PNGpng drive_sense_noise_shorted.png manage 41.1 K 2011-06-23 - 21:01 GraemeSmecher  
PNGpng sense_no_offset.png manage 48.4 K 2011-06-23 - 19:05 GraemeSmecher  
This topic: CryoElectronics > WebHome > CryoBoardTuning Topic revision: r2 - 2011-06-23 - GraemeSmecher
© 2020 Winterland Cosmology Lab, McGill University, Montréal, Québec, Canada

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\begin{math}\displaystyle 1~\mathrm{ADC}/\sqrt{\mathrm{Hz}}\end{math}
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\begin{math}\displaystyle \sqrt{2} \times 10 / 2^{24} \approx 843 \mathrm{nV}/\sqrt{\mathrm{Hz}}\end{math}
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\begin{math}\displaystyle V_\mathrm{OSO}\end{math}
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\begin{math}\displaystyle V_\mathrm{OSI}\end{math}
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