In　the　audio　analog　to　digital　converter　system　using　sigma-delta　modulator　structure,　an　improved　fifth-order　2-2-1　cascaded　sigma-delta　modulator　and　robust　filter　was　studied　to　solve　the　restriction　of　the　uncertainty　of　integrator　parameter　to　the　cascaded　sigma-delta　modulator.　Based　on　MARKOV　interval　algorithm,　the　nonlinear　relationship　of　uncertain　factors　was　transformed　into　linear　interval　matrix.　A　linear　matrix　inequality　approach　was　developed　to　obtain　the　parameters　of　a　robust　filter.　The　robust　filter　was　designed　based　on　method　of　optimized　CSD　coding.　The　results　indicate　that　the　robust　filter　outperformed　the　nominal　filter　with　an　improve　signal-to-noise　ratio　about　20　dB,　which　achieves　a　signal-to-noise　ratio　of　114.5　dB　and　a　resolution　of　18.72　bits.　The　robust　filter　has　the　advantage　of　low　cost　and　little　using　of　resource.　This　filter　is　sufficient　for　the　audio　analog　to　digital　converter　applications.