The low-voltage circuit is the key interfacing part between the electronic current transformer（ECT） and the relay and protection equipment. Specially designed， it has realized the analog output and the digital output based on the digital signal processor（DSP）， whilst the frequency measurement and harmonic analysis can also be achieved owing to the powerful processing capability of DSP. The calibrating method of ECT is studied， and the calibration module is then embedded in the DSP system， thus implementing the on-the-spot calibration. All the functions have been tested and verified using the fast ECT checking algorithm. The application field of ECT can be expanded by virtue of the modularization and implementation of multifunction of the low-voltage circuit.