An Integrated Readout for Current Sensing based on a Σ∆ Modulator with Switched Capacitor Feedback

This work presents a current input continuous-time sigma-delta modulator (C-CTΣ∆M) for biomedical application such as electrochemical impedance spectroscopy. Low noise and low area current input digitizers are required to implement high spatial resolution multi electrode arrays. In this work, the use of a switched capacitor with resistor D/A converter (SCR-DAC) for the C-CTΣ∆M is proposed for this purpose. SCR-DACs offer the realization of large resistances on small area, which leads to a very small area consumption combined with good noise performance. A C-CTΣ∆M with an SCR-DAC prototype is fabricated in a 180-nm CMOS process. The implemented ADC occupies an area of only 120 × 200 µm2. In order to increase the dynamic range (DR) of the C-CTΣ∆M, two modes have been implemented. In the low noise configuration an integrated in band noise of 157-pArms in the signal band width of 100 kHz with a peak SNDR of 62 dB is achieved. In the high DR configuration a peak SNDR of 72 dB with an MSA of -4.1 dB corresponding to 9.1 µA is measured. The complete C-CTΣ∆M consumes 2 mW from a 3-V supply and achieves a total DR of 95 dB.