Like a transformer, a resolver needs an alternating current input to function properly. The resolver receives alternating current from the primary coil (excitation input) and creates a multiple of this input current on the secondary sides (SIN, COS ports). When determining how to generate this alternating current, make sure to understand the op amp abilities and limitations. For the excitation input, the resolver input impedance, stability RMS voltage, and desired frequency must be taken in to account.