A 300-mV ΔΣ Modulator Using a Gain-Enhanced, Inverter-Based Amplifier for Medical Implant Devices

Abstract

An ultra-low-voltage low-power switched-capacitor (SC) delta-sigma (∆Σ) modulator running at a supply voltage as low as 300 mV is presented for biomedical implant devices, e.g., cardiac pacemakers. To reduce the supply voltage, an inverter-based amplifier is used in the integrators, whose DC gain and gain-bandwidth (GBW) are boosted by a simple current-mirror output stage. The full input-feedforward loop topology offers low integrators internal swing, supporting ultra-low-voltage operation. To demonstrate the concept, a second-order loop topology was chosen. The entire modulator operates reliably against process, voltage and temperature (PVT) variations from a 300 mV˘10% supply voltage only, while the switches are driven by a charge pump clock boosting scheme. Designed in a 65 nm CMOS technology and clocked at 256 kHz, the simulation results show that the modulator can achieve a 64.4 dB signal-to-noise ratio (SNR) and a 60.7 dB signal-to-noise and distortion ratio (SNDR) over a 1.0 kHz signal bandwidth while consuming 0.85 µW of power.

Ultra-Low-Voltage Inverter-Based Amplifier with Novel Common-Mode Stabilization Loop

“…The relatively small dc gain of inverter-like amplifiers can be overcome by using SC architectures capable of boosting the overall dc gain to the square [5] or even the cube (using two inverter stages) [6] of the original inverter gain. Considering this fact, as already stated, the ideal application of this kind of amplifiers lies just in SC circuits, such as discrete-time integrators, which are the main building blocks of state-variable filters and ΔΣ modulators [7].…”

A Standard-Cell-Based CMFB for Fully Synthesizable OTAs

“…Even if all these innovative techniques are very interesting from a research point of view, the most common approach to implementing analog building blocks suitable for automatic place and route exploits the digital standard cells as basic analog amplifiers [32][33][34][35][36][37]. In fact, the simplest digital gate (the inverter) behaves as a common source amplifier [38], and several inverter-based OTAs [39][40][41][42][43][44][45][46][47][48][49][50] have been proposed in the literature. However, differently from custom-designed inverters, the standard-cell inverter is typically optimized for area footprint or symmetrical slew rate, and as a consequence, it exhibits a systematic offset in its input-output dc transfer characteristic which impacts the output static voltage and strongly degrades the performance of standard-cell-based cascaded amplifiers.…”