Thesis Title: "Custom Circuit Design for MEMS Devices "

Name of the Student: Ms. Disha Chugh

Advisor: Prof. Saurabh Chandorkar, CeNSE

Date: 21st November 2025, (Friday), 4 PM

Venue: Seminar Hall (Hybrid): 

Abstract

Micro-Electro-Mechanical Systems (MEMS) find applications in various domains, including inertial sensors, 
microfluidic devices, RF systems, and optical applications. Depending on the desired application, these 
devices can be operated in resonant or non-resonant modes and transduced through methods such as electrostatic, 
electrothermal, magnetostatic, piezoelectric, or piezoresistive mechanisms. Achieving the desired operation 
for a specific application requires designing peripheral circuitry around these devices. In this context, 
understanding both the device's actual operation and its accurate electrical equivalent is essential for 
designing application-specific circuits, as it allows us to treat the MEMS device and electrical circuit 
as a unified system.

Resonant MEMS, typically modeled as second-order systems, are conventionally represented using R-L-C equivalents, 
such as the Butterworth-Van Dyke (BVD), Mason, or transformer-based models. However, we argue that though 
these models do find application in circuit design, they are not deal for designing peripheral circuitry. 
We highlight the limitations of using passive elements like R, L, and C to describe the electrical equivalent 
of capacitive MEMS devices and emphasize their non-transmission-type nature. We revisit the assumptions 
underlying these mathematical models and propose a new mathematical framework for capacitive MEMS devices. 
Using this framework, we derive a new electrical equivalent that better aligns with the device's physical behavior.

To validate the new mathematical model, we demonstrate its applicability for a standard circuit for sensing 
capacitively transduced signals viz. Transimpedance Amplifier (TIA). Through the lens of the new model, 
we identify shortcomings of the TIA circuit and develop a novel measurement methodology that measures 
voltage of a floating sense electrode. We address the major challenge of stable continuous voltage 
sensing at electrically floating sense terminals that arise due to accumulation of stray charges by 
introducing a control electrode. This Voltage Amplification (VA) methodology outperforms TIA circuits 
built with same underlying amplifying element (OPAMP) in terms of robustness to input parasitic capacitance, 
bandwidth, and noise performance. Moreover, we demonstrate that the noise performance of a combined 
system (resonator and sensing unit) calculated using the new model matches the experimentally observed 
results, a consistency that was previously unattainable with conventional models.

The newly developed equivalent circuit enables parameter estimation directly from measurements, 
eliminating the need for curve fitting. Simulating this equivalent circuit across a broadband frequency 
range shows excellent agreement with experimental measurements. The key distinction of our model is its 
foundation in physical parameters rather than purely empirical fits. Using this new perspective and 
electrical equivalent, we demonstrate that capacitive MEMS devices can achieve a voltage gain and 
operate with minimal or no sensing circuitry, depending on the application.

Additionally, we provide insights into the sources of feedthrough capacitance in epi-sealed devices, 
quantify contributions from various sources, and present a general methodology to estimate feedthrough 
capacitance in other MEMS devices. Furthermore, we propose innovative methods to minimize feedthrough 
capacitance without introducing excessive noise, including the use of split electrodes instead of full 
electrodes for actuation and sensing.

This work provides a comprehensive framework for understanding and designing peripheral circuitry 
around capacitive MEMS, offering solutions that provide more intuition and better understanding of 
the combined system of a MEMS resonator and its sensing unit.