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To operate a SQUID magnetometer in a magnetically unshielded environment, it is important to ensure that a flux-locked loop (FLL) circuit has a very large dynamic range and high slew rate. A digital FLL (D-FLL) system satisfies these conditions: its dynamic range is expanded by jumping a working point on the SQUID’s periodic Φ-V characteristic and counting the number of such events. We have developed a D-FLL system by using a double-counter. It achieves a low-cost and high performance magnetocardiogram (MCG) et al. measurement. Slew rate of the D-FLL with double-counter is greatly limited by its digital feedback loop. To demonstrate its very high ability, it has to be designed optimally. A simplified D-FLL model is shown by Fig. 1. It can increase the slew rate that optimum feedback loop gain is divided into an analog preamplifier A0 and a digital amplifier Kp on a microcontroller. Sampling period of analog-to-digital convertor (ADC) is T. Generally, A0 increases slew rate of the D-FLL. However, too much A0 reduces linear input flux range (Φlin) . Thus, it is desired to designe an optimum A0 with respect to slew rate. It can increase slew rate without reducing Φlin . However, too large Kp is not also desired because it happens a system oscillarion. The optimum is given by

Examination of a Digital FLL System Design for SQUIDs