A nonlinear A/D converter for smart silicon sensors

An accurately scaled binary-programmable current source has been designed for use in a nonlinear A/D converter. The nonlinear conversion is achieved based on accurate current sources using current mirrors with emitter scaling and analog current switches. The analog output current is the sum of two parts. The first part is used to determine the exponent, while the second is used to determine the mantissa in a floating point representation number. The current source and the required electronics are implemented using a bipolar process in silicon which makes it compatible with silicon sensors. The errors due to the Early effect can be reduced by cascoding the mirror transistors and reducing the voltage swing at the output node. The errors due to mismatch of the transistor parameters can be cancelled to first approximation by choosing the proper layout design. Preliminary research indicates that a current source in the range between about 2 μA and 2 mA is feasible with an absolute error of less than 8 nA and a relative error of less than 0.02%