This paper presents a novel window comparator circuit whose error threshold can be adaptively adjusted according to its input signal levels. It is ideal for analog online testing applications. Advantages of adaptive comparator error thresholds over constant or relative error thresholds in analog testing applications are discussed. Analytical equations for guiding the design of proposed comparator circuitry are derived. The proposed comparator circuit has been designed and fabricated using a CMOS 0.18mu technology. Measurement results of the fabricated chip are presented.