Something I've had a lot of questions about recently is an ARTIQ/Sinara current source:

• target is magnetic coils for ions/atoms
• currents from 1A to a few hundred A
• low noise/high-stability
• modulation BW of order 10kHz
• probably an EEM for control by Kasli

Suggestion:

• keep this modular, separate current source from modulation/stabilisation.
• we have a design for a current stabilisation module. Paper
  • This measures the current with a 4-terminal resistor.
  • low-noise differential front-end
  • error subtraction and amplification
  • loop filter
  • loop-filter output controls a transistor used as a current modulator (shunts current away from the coil to stabilise the current through the coil)
  • second actuator that's driven from a DAC used for feed-forwards to remove ambient AC fields (e.g. 50Hz noise). Key different between the two modulators is that the feed-forwards modulator is connected so that it does not change the current measured by the sense resistor
  • for low-ish currents the sense resistor measures the current directly. For higher currents one uses a transducer like a LEM Ultrastab 400 to reduce the current to a range that's easier to measure with a resistor
• works really well (see paper), however has a few issues:
  • not on a convenient Eurocard format
  • no EEM to provide convenient programming/modulation
  • loop factor is analog so inconvenient to tune. Would be much better to use an ADC after the error amplifier to digitise the error signal. Then do loop filter in DSP and program both DACs from microprocessor. This would also make modulation easier. Moreover, with an ADC one can log the error signal while stepping the set point to make loop filter optimisation easier. Note that due to inductance of coils, getting really good performance requires the loop filter to be individually optimised for each setup; a generic loop filter provides quite bad performance (bw limited to a few hz usually)
  • Best would be if the DACs weren't on the eurocard. Instead, it should have a digital output and put the DACs + shunt/modulation transistors near the load
• Plan would be to combine this stabilisation module with a suitable programmable (but not very low-noise or high-stability) current driver. For higher currents, one uses an Agilent current source or similar, and programs the current over ethernet to be somewhat bigger than the required coil current. The modulator shunts away enough current to achieve the desired current
• for lower currents, it would be nice to have a eurocard with a 5-10A programmable source, as well as the stabilisation transistors. The current source is programmed via EEM to give an appropriate current.