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Message ID: 2883     Entry time: Thu Apr 28 06:12:14 2022
Author: Chris 
Type: Electronics 
Category: PSOMA 
Subject: Current driver repair/upgrade 

One of the D1200719 current driver boards (S1500270) was in the EE shop for repair after some unexplained current fluctuations were seen.

Previously I measured its output at different knob settings, and saw saturation when driving a 25 Ω load. The saturation happens when the low frequency current source block reaches its voltage limit. (This wasn’t seen in Zach’s previous test, probably because a lower-impedance load was used.)

The board has now been upgraded from v4 to v7, to fix high frequency oscillations and crossover issues. An annotated schematic is uploaded to the board’s DCC traveler.

A few tests were made to check the basic functionality:

  • A 25 Ω dummy load was connected for testing (note: this component must be rated for high power dissipation)
  • The unit’s nominal current draw from the supplies was about 200 mA
  • The response of the coarse and fine adjust knobs was measured. (Saturation occurs above a coarse knob setting of 7, when driving the 25 Ω load)
  • Noise spectra were measured on the SR780 analyzer at the HF and total current monitor outputs, with a coarse knob setting of 4. (Both were nearly white, ~10 nV/rtHz HF and ~20 nV/rtHz total)
  • A wideband transfer function from HF modulation input to HF monitor output was measured, using 4395A analyzer + ZSC-2-1 splitter + HAT-3+ attenuator on the differential input (which was driven single-ended)
  • The transfer function from the DAC input to the total current monitor output was checked (showing the expected 40 dB/decade filtering above 10 Hz)
Attachment 1: knobs.pdf  14 kB  Uploaded Thu Apr 28 07:15:37 2022  | Hide | Hide all
Attachment 2: spectra.pdf  26 kB  Uploaded Thu Apr 28 07:15:53 2022  | Hide | Hide all
Attachment 3: AG4395A_25-04-2022_170840.pdf  17 kB  Uploaded Thu Apr 28 07:16:08 2022  | Hide | Hide all
Attachment 4: data.zip  14 kB  Uploaded Thu Apr 28 07:16:28 2022
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