1. An apparatus, comprising: an array of sensors arranged in rows and columns, a row including a plurality of sensors, each sensor in the plurality including a chemically-sensitive field effect transistor (chemFET) and having an output, configured to use the chemFET to produce a signal on the output in response to a row select signal; and a plurality of column output circuits coupled to the outputs of corresponding columns of sensors in the array, and coupled to a circuit to apply a reference voltage to the column output circuits during a calibration operation.