# Introduction

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## Abstract

In many measurement situations, the signal of interest is small, possibly in the range of tens of microvolts, and is superimposed on a much larger DC common-mode (CM) signal, possibly in the range of several volts.

## Keywords

Breakdown Voltage Differential Pair Instrumentation Amplifier Versus Sense Input Transistor## 1.1 Problem

*R*

_{sense}in series with the battery. Thus, the current can be determined from the DC voltage drop

*V*

_{sense}across the resistor. To minimize its power consumption,

*R*

_{sense}is usually very small (hundreds of milliohm) and thus,

*V*

_{sense}is also small, typically ranging from tens of microvolts to hundreds of millivolts. This requires a readout amplifier with low offset and low 1/

*f*noise. Moreover,

*V*

_{sense}is accompanied by a large CM voltage, which can be as large as 30 V in the case of a laptop battery. This is far beyond the supply voltages of normal CMOS circuitry. Thus, novel circuit techniques to reject this large CM voltage and accurately measure

*V*

_{sense}must be found. This problem becomes more challenging as CMOS technology advances, since this has historically been accompanied by a steady decrease in supply voltages.

## 1.2 Traditional Solutions

*f*

_{chop}) [6, 7, 8, 9]. In [2], the choppers are also driven via an isolating transformer. The input chopper converts the DC differential voltage

*V*

_{sense}into a square wave, and the output chopper converts the amplified square wave back to DC. In this way, the differential signal is first modulated to high frequency by the input chopper and so can be coupled to the input of the readout amplifier via a transformer. The DC CM voltage, however, will not be modulated and thus will not be coupled to the readout amplifier. Furthermore, the offset and 1/

*f*noise of the readout amplifier will be up-modulated by the output chopper and so can be filtered out. A big disadvantage of this approach, however, is that transformers are difficult to integrate on chip. Although integrated microtransformers [2] can be realized in some processes, they tend to occupy a lot of chip area.

*f*noise are also up-modulated and so are removed from the base band. However, the maximum CM voltage that can be handled is determined by the differential pair’s supply. So handling a large CM voltage requires a large supply voltage and, consequently, high-voltage input transistors in the input stage. This increases the power consumption considerably [14, 15]. Moreover, the limited output impedance of the input transistors will reduce the circuit’s CM immunity, while their offset and 1/

*f*noise will reduce its DC precision. Finally, the common-mode voltage range (CMVR) of the circuit shown in Fig. 1.5 will not cover both the negative and positive rails [14, 15] (Fig. 1.3).

From the above introduction, it seems that none of these approaches is very satisfactory. Thus, a new approach is required.

## 1.3 A Promising Solution: Capacitively Coupled Chopper Amplifier

*f*noise of the input stage.

*R*

_{1,2}and so the amplifier can be implemented with low-voltage circuitry. At the amplifier’s output, an output chopper demodulates the signal back to the base band. The offset and 1/

*f*noise of this amplifier, however, are blocked/filtered by the output capacitors. To prevent its output from saturating, the gain of the (transconductance) amplifier is limited by the output resistors

*R*

_{out1,2}. However, the amplifier cannot be used as an operational amplifier (opamp) due to its low gain or an instrumentation amplifier (IA) due to its inaccurate gain (

*G*

_{m1}*

*R*

_{out1,2}).

*R*

_{out1,2}and

*C*

_{out1,2}, the open-loop gain of the amplifier can be quite large. A chopped capacitor feedback path ensures that the gain of the IA is accurately defined as

*C*

_{in1,2}/

*C*

_{fb1,2}. Since the amplifier was intended for biomedical applications, a DC servo loop was employed (an SC integrator and

*C*

_{hp1,2}) to give it a high-pass characteristic. The up-modulated offset and 1/

*f*noise of

*G*

_{m1}was suppressed by a second stage (not shown) which acts as a low-pass filter. Although not designed for high input CM voltages, this CCIA demonstrated the feasibility of realizing on-chip capacitively coupled precision IAs. In this thesis, the design of capacitively coupled chopper IAs and opamps that can handle large input CM voltages, in some cases even larger than their supply voltages, will be explored.

## 1.4 Challenging Issues

Although the use of capacitively coupled chopper amplifiers seems to be a promising way of obtaining both wide CMVR and high DC precision, there are still many issues to be solved. The first issue is the robust implementation of the input chopper, which must handle the high CM voltage present before the input capacitors. Otherwise, the maximum input CM will be limited by the input chopper and not by the breakdown voltage of the input capacitors.

The input impedance of the amplifier is also an issue. Since the input capacitors are constantly switched between *V* _{in+} and *V* _{in−}, they are constantly being charged and discharged, which requires a certain amount of input current. The resulting impedance depends on the input capacitance and the chopping frequency, and typically ranges from hundreds of kilo-ohms to tens of megaohms, which may not be high enough for some applications. Thus, techniques to boost the input impedance are required.

Since the input DC CM voltage is blocked, the DC CM level of the amplifier’s input stage (*G* _{m1} in Fig. 1.9) must be fixed by something else. This can be done by using biasing resistors (*R* _{1,2} in Fig. 1.9). However, as will be shown later, these resistors introduce noise, and so must be rather large (hundreds of megaohms). Thus, another challenge is that of realizing such large resistors in an area-efficient and relatively accurate manner.

A fourth issue of capacitively coupled chopper amplifiers is the up-modulated offset and 1/*f* noise of the amplifier. Without the AC-coupling output capacitors shown in Fig. 1.9, the amplifier’s offset will be up-modulated by the output chopper and result in ripple. This is usually not acceptable and, thus, must be suppressed effectively. A low-pass filter (Fig. 1.10) is the most straightforward way but it often involves the use of large passive components. In recent years, more effective ripple-reduction techniques [18, 19, 20, 21] have been developed. However, care must be taken to minimize the extra area and power required, especially in low power and cost sensitive applications.

Finally, when a ripple-reduction technique is applied, it is very likely to introduce a notch at the chopping frequency in the amplifier’s transfer function. This notch limits the amplifier’s useable bandwidth and, moreover, results in a step response that will exhibit a certain amount of undesirable ringing. Thus, techniques of dealing with such transfer function notches must be devised.

## 1.5 Organization

In Chap. 2, the chopping technique, which has been frequently mentioned in the above, is introduced in detail. Several other commonly used dynamic offset cancelation techniques are also introduced. Like chopping, these techniques can also be used to achieve low offset and 1/*f* noise. As mentioned above, the up-modulated offset and 1/*f* noise due to chopping results in ripple, which must be sufficiently suppressed. Thus, ripple-reduction techniques will also be discussed.

In Chap. 3, the proposed capacitively coupled chopper topology for both opamps and IAs will be described. Its operating principles will be discussed in detail as well as its strengths and weaknesses.

As mentioned earlier, input capacitive coupling is not the only prerequisite to obtaining wide input CMVR. Thus, specially designed input choppers with wide CMVR are proposed, which will be presented in Chap. 4.

In Chaps. 5, two opamp prototypes employing the capacitively coupled chopper topology are presented. First, an overview of the state of the art is given. Later, the design considerations of the prototypes on both system level and transistor level are described. A single-path capacitively coupled chopper opamp is presented first, followed by a multipath capacitively coupled chopper opamp that improves the high-frequency performance and step response of the single-path opamp. Measurement results of each opamp will be given.

In Chaps. 6 and 7, two IA prototypes employing the capacitively coupled chopper topology will be proposed. The first IA features a ±30 V input CMVR and high power efficiency and is designed for HV current-sensing applications. The second IA is wireless sensor node, where power efficiency and small chip area are critical. It can be operated in two modes: a DC mode for DC and low frequency signals sensing; and an AC mode for biomedical signals such as ECG sensing. As in Chap. 5, a state-of-the-art overview will be given at the beginning of each chapter. The systematic and transistor level designs will be presented as well as experimental results.

The thesis ends with conclusions and future work, which are presented in Chap. 8. Two capacitively coupled chopper analog-to-digital converters (ADC) are proposed as future work, which can directly digitize a signal source with high CM voltage. The original contributions of the author are listed at the end of the chapters.

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