I wanted to investigate my coil driver noise measurement technique under more controlled circumstances, so I spent yesterday setting up various configurations on a breadboard in the control room. The overall topology was as sketched in Attachment #1 of the previous elog, except for #4 below. Summary of configurations tried (series resistance was 4.5k ohm in all cases):

1. Op 27 with 1kohm input and feedback resistors.
2. LT1128 with 1kohm input and feedback resistors.
3. LT1128 with 400 ohm input and feedback resistors.
4. LT1128 with 400 ohm input and feedback resistors, and also the current buffer IC LM6321 implemented.

Attachments:

Attachment #1: Picture of the breadboard setup.

Attachment #2: Noise measurements (input shorted to ground) with 1 Hz linewidth from DC to 4 kHz.

Attachment #3: Noise measurements for full SR785 span. 

Attachment #4: Apparent coupling due to PSRR.

Attachment #5: Comparison of low frequency noise with and without the LM6321 part of the fast DAC path implemented.

All SR785 measurements were made with input range fixed at -42dBVpk, input AC coupled and "Floating", with a Hanning window. 

Conclusions:

• I get much better agreement between LISO and measurement at a few hundred Hz and below with this proto setup. So it would seem like the excess noise I measure at ~200 Hz in the Eurocrate card version of the coil driver could be real and not simply a measurement artefact.
• I am puzzled about the 10 Hz comb in all these measurements:
  • I have seen this a few times before - e.g. elog13655.
  • It is not due to the infamous GPIB issue - the lines persist even though I disconnect both power adaptor and GPIB prologix box from the SR785.
  • It does not seem to be correlated with the position of the analyzer w.r.t. the DC power supply (Tektronix PS280) used to power the circuit (I moved the SR785 around 1m away from the supply).
  • It persists with either of the two LN preamp boxes available. 
  • It persists with either "Float" or "Ground" input setting on the SR785.
  • All this pointed to some other form of coupling - perhaps conductive EMI.
  • The only clue I have is the apparent difference between the level of the coupling for Op27 and LT1128 - it is significantly lower for the latter compared to the former. 
  • I ruled out position on the breadboard: simply interchanging the Op27 and LT1128 positions on the breadboard, I saw higher 10 Hz harmonics for the Op27 compared to the LT1128. In fact, the coupling was higher for the DIP Op27 compared to an SOIC one I attached to the breadboard via an SOIC to DIP adapter (both were Op27-Gs, with spec'ed PSRR of 120 dB typ).
  • To test the hypothesis, I compared the noise for the Op27 config, on the one hand with regulated (via D1000217) DC supply, and on the other, directly powered by the Tektronix supply. The latter configuration shows much higher coupling.
  • I did have 0.1uF decoupling capacitors (I guess I should've used ceramic and not tantala) near the OpAmp power pins, and in fact, removing them had no effect on the level of this coupling 
  • As a quick check, I measured the spectrum of the DC power used to run the breadboard - it is supplied via D1000217. I used an RC network to block out the DC, but the measurement doesn't suggest a level of noise in the supply that could explain these peaks.
  • The regulators are LM2941 and LM2991. They specify something like 0.03% of the output voltage as AC RMS, though I am not sure over what range of frequencies this is integrated over.
  • But perhaps the effect is more subtle, some kind of downconversion of higher frequency noise, but isn't the decoupling cap supposed to protect against this?
• The 19.5 kHz harmonics seem to originate from the CRT display of the SR785 (SVGA).
  • The manual doesn't specify the refresh rate, but from a bit of googling, it seems like this is a plausible number.
  • The coupling seems to be radiative. The box housing the Busby preamp provides ~60dB attentuation of this signal, and the amplitude of the peaks is directly correlated to where I position the Busby box relative to the CRT screen.
  • This problem can be avoided by placing the DUT and preamp sufficiently far from the SR785.

Punchlines:

1. The actual coildriver used, D010001, doesn't have a regulated power supply, it just draws the +/- 15V directly from Sorensens. I don't think this is good for low noise. 
2. The LM6321 part of the circuit doesn't add any excess noise to the circuit, consistent with it being inside the unity gain feedback loop. In any case, with 4.5 kohm series resistance with the coil driver, we would be driving <2.5 mA of current, so perhaps we don't even need this?