A CMOS current-to-LCD interface for portable amperometric sensing systems [biomedical measurement applications]

A new design of a portable system for reading current from amperometric sensors and displaying the results on an LCD is presented. The system integrates most of the required components: a front-end circuit, an A/D converter, a calibration unit and a digital control block. The exceptions are two external devices: a 2 MΩ resistor and a 1 nF capacitor. Input current from 0-1000 nA can be measured with a resolution of 1 nA. An internal bandgap voltage reference generates a polarization voltage for biasing a sensor. The voltage can be adjusted from 0.1 V to 0.8 V in 0.1 V steps. The A/D converter uses the quad-slope conversion technique, which eliminates the effects of offsets. Simulation shows that the system is virtually independent of the ambient temperature and is able to operate from a 2.0 to 5.0 V supply. The prototype circuit has been designed in a 0.7 μm CMOS technology.