J. Kissel

Context
Recent investigations into how much DC alignment offsets the quad reaction chains (LHO:82669) drove the discovery that the calibration of the current monitor channels, aka FASTIMON channels, I'd modeled in 2024 (LHO:77545) was wrong. Because the collection of circuits involved have a lot of single-ended to differential conversions along the way and factors of two are easy to get wrong, I took the time to measure a QUAD TOP coil driver in the lab with trusty multi-meters and SR785s.

Executive Summary
For all the work validating the model below -- the corrected model of the FASTIMON channels are correct (and a factor of exactly two different from LHO:77545):
-----------------
    calibration_QTOP [V/A] = 2 * 40.00 [V/A] * (10e3 / 30e3)
                           = 26.667 [V/A]
          or 0.0267 [V/mA]
          or 37.5 [mA/V]

    calibration_QTOP [ct/A] = 2 * 40.00 [V/A] * (10e3 / 30e3) * 1 * (2^16 / 40 [ct/V])
                            = 4.3691e+04 [ct/A]
          or 43.691 [ct/mA]
          or 0.0228 [mA/ct]

-----------------
    calibration_TTOP [V/A] = 2 * 29.83 [V/A] * (10e3 / 30e3)
                           = 19.887 [V/A]
          or 0.0199 [V/mA]
          or 50.285 [mA/V]

    calibration_TTOP [ct/A] = 2 * 29.83 [V/A] * (10e3 / 30e3) * 1 * (2^16 / 40 [ct/V])
                            = 3.2582e+04 [ct/A]
          or 32.582 [ct/mA]
          or 0.0307 [mA/ct]

-----------------
    calibration_OTOP [V/A] = 2 * 279.63 [V/A] * (10e3 / 30e3)
                           = 186.42 [V/A]
          or 0.1864 [V/mA]
          or 5.3642 [mA/V]

    calibration_OTOP [ct/A] = 2 * 279.63 [V/A] * (10e3 / 30e3) * 1 * (2^16 / 40 [ct/V])
                            = 3.0543e+05 [ct/A]
          or 305.43 [ct/mA]
          or 0.00327[mA/ct]


Review of the Model

In the EE shop, I pulled the spare TOP driver, S1102666, which happens to be yet another "the only difference is the output impedance," Transmon flavor of TOP driver, D1001650, which has the same output impedance as the QUAD TOP, D0902747. But, just because I chose a "different" TOP driver, gives me the excuse to review the model, and why the output impedance matters for the calibration of the current monitor. 

Recall, for these top drivers, 
    Z_OUT   = (R5+R1) || (R90+R91) 
            = (R5+R1) * (R90+R91) / (R5+R1 + R90+R91)

    Z_OUT^(QuadTOP) = Z_OUT^(TransmonTOP) = (44 * 440) / (44 + 440) = 40.00 [V/A]
    Z_OUT^(TripleTOP) = (32 * 440) / (32 + 440) = 29.83 [V/A]
    Z_OUT^(OMCTOP) = (292 * 6600) / (292 + 6600) = 279.63 [V/A]

Which changes the calibration of the coil current monitor, because 
         ADC [ct]        [                         "R1" [V_SE] ]    1 [V_DF]    2^16 [ct]      
    ----------------   = [ 2 * Z_OUT [V_DF / A] *  ----------- ] *  -------- * ----------- 
    Coil Current [A]     [                         "R2" [V_DF] ]    1 [V_SE]    40 [V_DF] 
where you'll notice *the* difference between the equation here and that in LHO:77545 is the "conversion" from the single-ended output of the current monitor, which is piped to the positive leg of its external differential output while the negative leg is held at 0V. So, the ADC, which reads (positive - negative) will just read out a value that's equivalent to the original single-ended voltage.

Measurement Setup

I first attach diagrams of the measurement setup, so that that's no confusion about factors of two from which pins I read out how.

(1) (pages 1 and 2 of CoilDriver_FASTIMON_Calibration_Diagrams.pdf) Validating calibration at DC :: Using the SR785 and an SR785 accessor box to drive a range of DC voltage offsets differentially into the DAC input of the driver, I measured the voltage across a dummy OSEM set of resistors -- as a proxy for the coil current -- and the differential output voltage of the FASTIMON and the SLOW RMS I MON. Although we didn't expect it, I did this in two configurations to make sure the answer didn't change whether the low-pass switch is ON vs. OFF.

(2) (pages 3 thru 5 of CoilDriver_FASTIMON_Calibration_Diagrams.pdf)Validating calibration at AC (~ 1 to 100 Hz). Using a similar setup, but now making sure the inputs and outputs are read out truly differentially (making sure all BNC shields are connect to the chassis 0V), I drove a swept-sine excitation through the driver from 0.1 to 1000 Hz in various configurations, in order to capture
   (a) The "standard" transconductance measurement, Coil Current per DAC input voltage
   (b) The transfer function we really want, which is the per FAST I MON per Coil Current
   (c) A bonus, FAST I MON per DAC input voltage

Results

The rest of the .pdfs show the results of these set-ups. While mildly interesting, they all agree with the model.