Integrating ADC using a single transistor as integrator and amplifier for very low (1 fA minimum) input currents

In this paper a new analog-to-digital converter (ADC) architecture for the conversion of currents down to the subpicoampere range is presented. A single transistor stage is used to integrate and amplify the input current resulting in simple and area-effective designs. Although the transfer function of the transistor is not linear, inherent linearity of the conversion is achieved by a constant transistor voltage swing and correlated double sampling. The circuit is well suited for highly integrated sensor systems due to its full compatibility with digital CMOS technology and low area requirements. The current-mode design allows low-voltage operation and results in wide dynamic range (140 dB with superimposed bias current) depending only on timing requirements.