A precision current source is disclosed that includes a voltage setting circuit that precisely sets the voltage across a range setting resistor to set the current flowing in a load resistance connected in series with the range setting resistor. The voltage setting circuit precisely sets the voltage across the range setting resistor as a function of an input reference voltage. The voltage setting circuit includes an instrumentation amplifier that determines the voltage across the range setting resistor and the difference between this voltage and the reference voltage is used drive a drive voltage amplifier. The drive voltage amplifier output adjusts to minimize the difference between the reference voltage and the voltage across the range setting resistor. Other embodiments include the use of a DC blocking capacitor to allow only AC coupling and various nulling. circuits to remove any charge buildup on a DC blocking capacitor.