Setup
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Total input current Ic (amps)

LA-55 with ~200 turns, 1k burden.
Voltage output is Vc = (Ic / 1000 * 1k) = Ic
LA-55 output goes to trimmable offset opamp,
  trimmed to 0 when DAC command is 32768.

  -> NOTE: DC offset changes a ton (200mV!) when the LA-55 is
  saturated one way versus the other, then returned to zero.

  Will try implementing a degauss feature, to see if the DC offset
  always settles to the same amount.  Result: seems to work.

R36 = 1k (shouldn't matter)
IC3A:
  R37 = 1k, R38 = 3.3k -> scales up by 4.3
IC3B:
  R39 = 100k
  R40 = 100k
  R41 = 100k
  R42 = 100k
Reference: 
  R43 and R44 are a pot, trimmed so R43 == R44 -> VREF1.25 = 1.25V.
  
So
  ADC12V+ = 1.25 - (4.3 * Vc)
          = 1.25 - 4.3 Ic

PIC input value (PICV) is:
  0    = 0V
  2048 = 2.5V

  PICV = (ADC12V+ * 2048 / 2.5)
  PICV = (1.25 - 4.3 Ic) * 2048 / 2.5
  PICV = 1024 - 3522.6 Ic
  Ic = (1024 - PICV) / 3522.6

Computed by adc12_to_current()

Scaling
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DAC can do about ±20A.  DAC command (DACC) is:
  0     = -20A
  65536 = +20A

  DACC = 32768 * (1 + Ic / 20)

Computed by current_to_dac()

Code
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