A Low-Power Wide Dynamic-Range Current Readout Circuit for Ion-Sensitive FET Sensors

This paper presents an amplifier-less and digital-intensive current-to-digital converter for ion-sensitive FET sensors. Capacitance on the input node is utilized as a residue accumulator, and a clocked comparator is followed for quantization. Without any continuous-time feedback circuit, the converter performs a first-order noise shaping of the quantization error. In order to minimize static power consumption, the proposed circuit employs a single-ended current-steering digital-to-analog converter which flows only the same current as the input. By adopting a switching noise averaging algorithm, our dynamic element matching not only mitigates mismatch of current sources in the current-steering DAC, but also makes the effect of dynamic switching noise become an input-independent constant. The implemented circuit in 0.35 $\mu$ m CMOS converts the current input with a range of 2.8 $\mu$ A to 15 b digital output in about 4 ms, showing a DNL of $+0.24/-0.25\ \mathrm{LSB}$ and an INL of ${\;+\;}1.98/-1.98\ \mathrm{LSB}$ while consuming 16.8 $\mu$ W.