An Integrated Fully Differential Current Amplifier With Frequency Compensation for Inductive Sensor Excitation

In this article an integrated fully differential current amplifier is presented. It was designed for inductive sensor excitation, in this case for a fluxgate sensor, however the concept is applicable wherever a low noise and precise current is required. A brief review of some of the basic elements of the circuit is given, followed by the development of a model that takes into account output impedance limitations due to mismatch and stability criteria, an essential consideration in the design of a stable current amplifier for inductive loads. Based on the proposed model, the design and implementation of the current amplifier is outlined, identifying potential difficulties for on-chip integration. The final design was then fabricated using a standard 180 nm CMOS technology. Measurement results show that the circuit draws only 2.8 mA from a 3.3 V supply voltage and occupies a total area of 0.64 mm2. Special efforts were made to accurately evaluate the output impedance, whereby a value of 436 kΩ was recorded. In addition, the current amplifier achieves an output-referred noise current of 2.5 nA/Hz, resulting in a measured signal-to-noise ratio of more than 105.2 dB for a bandwidth of 512 Hz at an output current of 9 mAp-p.