SCANNING METHOD AND APPARATUS FOR CURRENT SIGNALS HAVING LARGE DYNAMIC RANGE

There is disclosed herein apparatus and a method for scanning information off a processing plane where the information is contained in a current signal having a very small amplitude and which can change signs and vary in amplitude by as much as five orders of magnitude. The preferred embodiment of the apparatus uses a pair of CMOS pass transistors connected to the individual processing elements and the row select lines. The pass transistors, when turned on, couple the output current from the processor containing the desired information to a column line. The column line is connected to a current to voltage converter in the form of a differential input amplifier having a non linear feedback circuit comprised of two diode connected CMOS transistors operating in the subthreshold region. The non linear feedback circuit provides an exponential transfer function which compresses the dynamic range of the output current from the processor to a smaller and more useable output range for an output voltage. The negative feedback to the inverting input coupled to the column line stabilizes the voltage on the column line to virtual ground thereby eliminating the delay associated with driving the parasitic capacitance of the column line with the very small output current from the processor in an attempt to substantially change the voltage of the column line.