Bio-signal instrumentation amplifier (Bio-IA) is one of the most important circuits in the vital sign monitoring system. However, the performance of the Bio-IA using traditional pseudo resistor (PR) is always very sensitive to process, power voltage and temperature (PVT). This project proposed a digitally controlled DC-servo loop (DCDSL) and an improved PR to improve the robustness of Bio-IA. The proposed DCDSL replaces the integrator-based DSL and eliminates the PR in the traditional DSL, which can improve the response speed as well as the noise performance. The proposed Bio-IA was implemented in a standard 0.18μm CMOS process, occupying an active area of 0.474×0.424. The measured results show that the Bio-IA can operate in the temperature range of -20℃~80℃. When the bias current is 500fA, the equivalent resistance of the improved PR can be up to 330GΩ. The proposed Bio-IA can calibrate the DC offset as large as ±300mV with a response time less than 200ms. When the DC offset increases from 0 to 300mV, its input referred noise varies from 0.67μVrms to 1.49μVrms. With 1.2V supply voltage, the mid band gain of the Bio-IA is 40dB.