A Highly Digital VCO-Based ADC Architecture for Current Sensing Applications

Abstract

This paper presents a voltage-controlled oscillator (VCO) based current to digital converter for sensor readout applications. Second order noise shaping of the quantization error is achieved by using implicit capacitance of the sensor to realize a passive integrator and a VCO-based quantizer. The non-linearity in voltage to frequency conversion of the VCO is tackled by placing the VCO in a loop consisting of a simple digital IIR filter and a passive integrator. The IIR filter provides large gain within the signal bandwidth and suppresses VCO input swing. As a result, non-linearity of the VCO is not exercised, thus greatly improving the proposed architecture's immunity to VCO nonlinearity. The use of a digital filter instead of an analog loop filter eases the design and makes it scaling friendly. Designed for an ambient light sensor application, the proposed circuit achieves 900 pA accuracy over an input current range of 4 A. Fabricated in a 0.18 m CMOS process, the readout circuit consumes a total of 77.8 A current, and occupies an active area of 0.36 mm .Index Terms-VCO based ADC, time based ADC, analog-to-digital conversion, current sensor, delta-sigma modulation, digital readout circuits, ambient light sensor, sensor applications.

A 0.5V time-domain instrumentation circuit with clocked and unclocked ΔΣ operation

“…TD systems encode information with respect to the timing intervals of asynchronous digital signals to perform mixed signal processing while extensively using standard logic and oscillators that do not suffer from analogue complications. Many recent publications will indicate the potential for exceptional dynamic range like in the recording system of [3] or highly compact instrumentation like the potentiostat in [4]. We argue that a key advantage for these systems is that supply voltage is utilized more effectively from a fundamental aspect without being impeded by linearity.…”

Advances in Voltage-Controlled-Oscillator-Based ΔΣ ADCs

“…Apart from applications in filter and power management, there also have been vigorous research efforts in using VCO-based techniques in sensor interfaces. Published works have cover temperature sensing [34], current sensing [35], capacitance sensing [36], [37], as well as biomedical and neural interfaces [38]- [40]. Noteworthily, open-loop VCO-based quantizers exhibits high suitability for biomedical sensing, where signal amplitudes are small.…”

Current Measurement Transducer Based on Current-To-Voltage-To-Frequency Converting Ring Oscillator with Cascade Bias Circuit

“…For precise measurement of the glucose concentration, a current measurement transducer with a resolution of a few nanoamperes should be designed [3]. There are several voltage-controlled oscillator based analog-to-digital converters that convert the amperometric input to the frequency output with great linearity characteristics and resolution [4,5]. However, large area and power consumption relative to the glucose sensor causes overhead to the integrated device as the size of the glucose sensor unit is µm scale [2].…”