Current-Mode CMOS A/D Converter for Pico Ampere-Range with a Potentiostatic Input

The IC design using sub-micrometer CMOS processes continues to increase the applicability from the relative accuracy applications as those used in communication, to high absolute accuracy applications as those used in smart sensors. One of the desirable and significant applications of biosensors is to measure the parameters on-line as a part of an implant unit.

These sensors would make it possible to produce therapeutic instruments that could correct body malfunctions such as closed-loop insulin pumps for diabetics.Another application of such equipment is found in chemical analytic instruments: a small amount of a certain gas in a solution, such as oxygen in blood, can be determined by measuring the concentration-dependent current.

The circuit that maintains the electrochemical stability in the sensor, as well as buffering the current output is called potentiostat.The DC current at the output of the potentiostat is very low, in the pico Ampere range. The spectra of the DC output signal and the noise overlap, thus amplifying the input signal would result in a severe degradation of the dynamic range.The analog-to-digital converter requires high resolution, reduced offset levels, and good linearity.

In this context conversion techniques of the integration-type are preferred, such as dual-slope, multi-slope, or sigma-delta modulation. The performances of the comparator, clock-feedthrough effect, and the offset compensation are the main problems. Furthermore, if an analog circuit is designed as a building block in a cell library, there is no way to control the noise coupling through the substrate, supply lines, and clocks.Different conversion methods are analyzed as a prestudy of the project.

The circuit is optimized by SPICE simulations from the transistor level, and by proper full-custom mask lay-out. The influence and methods to reduce the nonlinearities of the A/D converter are discussed. At the input of the integrator was implemented a bridge of 4-switches in order to measure the low current with minimum charge injection, and to overcome the drift problems.A new conversion technique is proposed: folded-dual-slope.

This method offers direct advantage for such application:- each switch is operated a constant number of times for each conversion, regardless of the input signal. The nonlinearities from the charge injection, the transient response delay of the comparator, and the integrator capacitor give a constant offset in all readings.

This offsets can be easily removed by applying a zero input periodically;- requires simple circuitry, and a single reference current.