Why and How VCO-based ADCs can improve instrumentation applications

After introducing the basic concepts, we will describe the different VCO-based ADC architectures for instrumentation and biomedical applications published in the recent years. Both open loop and closed loop, high order architectures will be analyzed and compared to other standard instrumentation ADCs. The main solutions given to problems associated with VCO-based ADCs in instrumentation, like VCO's phase noise, linearity and sampling metastability will be reviewed. The comparison with other architectures reveals that the FoM of VCO-based ADCs lags that of conventional ΔΣ modulators. Nevertheless the total power consumption, area and performance make them fairly superior when we consider the total efficiency of the instrumentation system, including signal conditioning pre-amplifiers and references.

I. Introduction

Switched capacitor circuits have dominated instrumentation readout circuits for over three decades. During this time, the advancement of MEMS technology required integrated circuits capable of amplifying and converting all kinds of low level signals. Such MEMS sensors combined with instrumentation amplifiers and switched capacitor ADCs have enabled the inclusion of sensing capabilities in portable devices (e.g. cellular phones and wearables), see Fig. 1(a). A vast knowledge base has been built around this combination in or CMOS technologies at supply voltages above 1.2 V. However, as technology nodes and supply voltages scale down, switched capacitor circuits become increasingly problematic. This is because the switch overdrive voltage is very low and poor opamp DC gain in modern nanometer CMOS processes. This imposes limits on the resolution and power consumption of switched capacitor oversampled data converters.