Widening the range of targets for ion-sensitive field-effect transistors (ISFETs) fabricated in unmodified CMOS technology has been enabled by the deposition of polymeric ionophore membranes at the surface, requiring specific sensor training. We present a novel ISFET array with on-chip multiplexing capabilities to perform offline training and real-time sensing on a single substrate, enabling analogue averaging for low noise sensing and reducing post-processing. The analogue front-end pixels rely on a modular current-mode spatial averaging (CMSA) circuit, producing an averaged sensor output per cluster based on a switching matrix integrated within the array. At the output stage, a weak inversion active resistor implements an ELIN system to guarantee linearity, reaching an expected sensitivity of 160 mV/pH with a gain of 3, when designed using a 0.18 um standard CMOS technology.