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This paper covers the design details of an all digital closed-loop interferometric fiber-optic gyroscope (ADCL-IFOG) prototype, constructed in TUBITAK UME, and scale factor comparison between open-loop and ADCL-IFOG prototypes with sine wave biasing modulation. The output of demodulation circuit, proportional to the applied rotation rate, was sampled by AD7714YN analog-to-digital converter (ADC), operated in 16 bit resolution. Error voltage, generated by microcomputer - controlled LTC 1667CG, 14 bit digital to analog converter (DAC), was sent to the phase modulator through a linear summing circuit to make Sagnac phase shift zero, depending on the rotation direction. For this implementation, the ultimate rotation rate of 1.84 (deg/h ) was nullified. The averaged sensitivity of the proposed closed-loop IFOG in unit of error voltage applied to the phase modulator was determined as 132.65 muV/(deg/h ). The scale factors of both the open-loop and ADCL-IFOG prototypes were compared in a range of 1-15270 (deg/h) rotation rate, corresponding to Sagnac phase shifts varying from 0.00115 (deg) to 17.57448 (deg). The maximum peak to peak noise and the bias stability of ADCL-IFOG prototype were determined as 4.97 (deg/h ) and 1.48 (deg/h ) at 23.0degC , respectively.