This article presents a 15-channel orthogonal code chopping instrumentation amplifier (OCCIA) for an area-efficient and low gain-mismatch multi-channel bio-signal acquisition. Orthogonal codes directly modulate each channel and merge into a single signal for sharing IA, while performing dynamic offset compensation with low power consumption. Digitization-before-demodulation (DBD) transmits the combined modulated data directly, which alleviates ripple noise, and completely removes the demodulation and TX encoding overhead from the ASIC. The proposed OCCIA in 0.18-μm 1P6M CMOS, when compared with the recent multi-channel instrumentation amplifiers (IAs), shows the smallest area (0.019 mm ² /Ch.), low gain mismatch (0.43%), with the lowest power consumption (1.97 μW/Ch.) and low crosstalk (<; -51.5 dB) at 490-Hz bandwidth.