1. A method for operating an apparatus comprising a plurality of chemically sensitive sensors arranged in rows and columns, comprising: generating timing signals, row select signals and column select signals; sampling and holding signal samples in the plurality of sensors in a row of the array in response to the timing signals; enabling column output circuits coupled to corresponding sensors in the row in response to the column select signals; and outputting the signal samples to the column output circuits from the plurality of sensors in the row in response to the row select signals.