1. A method of operating an apparatus comprising an array of sensors arranged in rows and columns, the sensors comprising chemically-sensitive field effect transistors (chemFETs), the method comprising: precharging the chemFETs in a plurality of sensors in the array; in response to a first timing signal, after said precharging, transferring charge from the chemFETs to sample and hold capacitors in sensors in a plurality of sensors in a row of the array; enabling a column output circuit coupled to sensors in a column of the array in response to a column select signal; and outputting signal samples to the column output circuit from the sample and hold capacitors of the sensors in the column in response to respective row select signals.