Low Power Discrete Time Delta Sigma Modulator for Remote Healthcare Devices

Introduction

Today, battery-powered wireless monitoring and diagnostic sensors are the most used and critical elements in the Wireless Body Area Network (WBAN). For wireless wearable medical devices, low power consumption is the most critical requirement. In effect, they need to be with very low power consumption in order to get a long life time network. As every embedded system (Fig.1), analog to digital converter (ADC) is a key building block. As shown in the same figure, it is located in two different places. In fact, for one instance, it serves to digitize the sensed signal from the human body. Also, it occurs in the receiver to bridge the link between the wide band part and the baseband part. Since the received or the sensed signals are analogue, it is important to be converted to the digital form in order to be understood by the treatment unit. The ΔΣ modulator is the most agreed and suitable structure for low power and high precision applications thanks to its oversampling principal and noise shaping technique. Therefore, the operational transconductance amplifier (OTA) is a widely used block in analog and mixed-signal integrated circuit such as; oscillator, Gm-C filter, integrator and so on. In a data converter, owing to the OTA features, the ΔΣ modulator can achieves better resolution. Since the modulator performances are mainly linked to the OTA features, we choose to use the telescopic OTA because of its high features especially in terms of low noise and power consumption. Nevertheless, the main problem of an active integrator based on OTA circuit was the high power consumption caused by this last.

In order to minimize the power consumed by the OTA block, many techniques and methods are used in the literature for designing a delta sigma modulator.

Figure 1.

Internal structure of a wireless medical sensor

In fact, we have found the use of an inverter integrator-based such it is presented in (Chea & Han, 2009). A passive integrator based on passive filter was mainly used for designing a very low power consumption ΔΣ modulator (Fazli Yeknami et al., 2014; Qazi & Dąbrowski, 2015). Also, the MOS parametric integrator use is advantageous to implement a 2nd order Feed Forward (FF) ΔΣ modulator with an acceptable resolution and very low power consumption as presented in (Laouej, Daoud, & Loulou, 2019). In addition, powered the analog circuit under the MOSFET transistor threshold voltage (Vth) was possible thanks to the bulk-driven technique use. In fact, this method was widely used for designing multiple OTA structures such as; one stage folded cascode OTA (Haga et al., 2005), class AB OTA (Kulej & Khateb, 2020), miller OTA (Bhange et al., 2015), differential folded-cascode OTA (Rezaei & Moghaddasi, 2013), fully differential folded cascode OTA (Hyoungdong et al., 2010), and so on.

In (Laouej, Daoud, Mallek et al, 2019), an improved telescopic OTA structure was used for designing a 2nd order FF ΔΣ modulator. In fact, the designed modulator achieves an acceptable resolution. The total power consumption was very low compared with some published works. In order to further reduce the modulator power consumption without degrading the resolution, we propose to apply the bulk driven technique in the chosen OTA architecture.

The aim of this work is to evaluate the performances of a very low power single bit discrete time ΔΣ modulator based on a bulk driven (B-D) telescopic OTA circuit which it is optimized through the PSO program. For this reason, we focus on FF ΔΣ modulator design for IEEE802.15.1/Bluetooth standard requirements. Therefore, a 2ndorder FF ΔΣ modulator was implemented with optimized switched capacitors (SC) circuits to satisfy the targeted performance objective of 0.5MHz signal bandwidth and 12bits resolutions.