Monochromators are essential components in electron microscopy and spectroscopy for improving spatial and energy resolution. Their use in scanning electron microscopes (SEMs), however, remains limited due to high cost and operational complexity. Using a thin-deflector analysis of a homogeneous electrostatic deflector, we show that conventional monochromators exhibit extreme sensitivity to power-supply drift and mechanical imperfections. Meeting these stringent tolerances typically requires additional correction elements, which further increase system complexity and cost. We demonstrate that fringe-field deflectors are inherently less sensitive to these limitations. Based on this insight, we propose a simple and cost-effective monochromator architecture relying solely on fringe fields. The design achieves optimal energy resolution by incorporating short-range deceleration lenses surrounding the main deflector, eliminating the need for auxiliary correction elements. Such a fully electrostatic configuration is compatible with MEMS fabrication, offering a compact, robust, and accessible pathway for high-performance energy filtering in SEMs.