AbstractBiomagnetic field measurements in unshielded environments require both large dynamic range and high slew rate. Noise amplitudes at 50 Hz and under 10 Hz of significantly more than 1µT are not at all exotic in an industrial or a hospital environment. A digital FLL (D‐FLL) system for a SQUID magnetometer has high resolution and large dynamic range of magnetic field. Slew rate of a D‐FLL system by means of a double‐counter is limited by its digital feedback loop. It can increase the slew rate that optimum feedback loop gain is divided into an analog preamplifier and a digital amplifier on a microcontroller. We show the method of design of their amplifiers. The slew rate of a D‐FLL system with optimum amplifiers is 483Φ0/s, and its noise level is about 50.0 fT/√Hz@120 Hz. © 2007 Wiley Periodicals, Inc. Electr Eng Jpn, 160(4): 1–6, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20526