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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.
KeywordsBreakdown Voltage Differential Pair Instrumentation Amplifier Versus Sense Input Transistor
1.2 Traditional Solutions
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
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.
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.
- 1.A. Mehta, “Understand low-side vs. high-side current sensing,” http://www.eetimes.com/design/analog-design/4010355/Understand-low-side-vs-high-side-current-sensing.
- 2.F. Rothan, H. Lhermet, B. Zongo etc, “A ± 1.5 % nonlinearity 0.1-to-100A shunt current sensor based on a 6 kV isolated micro-transformer for electrical vehicles and home automation,” ISSCC Dig. Tech. Papers, pp. 112-113, Feb., 2011.Google Scholar
- 3.M. Munzer, W. Ademmer, B. Strzalkowski, and K. Kaschani, “Insulated signal transfer in a half bridge driver IC based on a coreless transformer technology,” Int. Conf. Power Electronics and Drive Systems, vol. 1, pp. 93-96, Nov., 2003.Google Scholar
- 4.K. Kaschani, B. Strzalkowski, M. Münzer and W. Ademmer, “Coreless transformer a new technology for driver ICs,” PCIM 2003, Nürnberg.Google Scholar
- 5.S. Kaeriyama, S. Uchida, M. Furumiya, M. Okada, T. Maeda, and M. Mizuno, “A 2.5 kV isolation 35 kV/us CMR 250 mbps digital isolator in standard CMOS with a small transformer driving technique,” IEEE J. Solid-State Circuits, vol. 47, no. 2, pp. 435-443, Feb., 2012.Google Scholar
- 6.J. H. Huijsing, Operational Amplifiers: Theory and Design Second Edition, New York: Springer, 2011.Google Scholar
- 7.C. C. Enz, and G. C. Temes, “Circuit techniques for reducing the effects of Op-Amp imperfections: autozeroing, correlated double sampling, and chopper stabilization,” Proceedings of IEEE, vol. 84, no. 11, pp. 1584-1614, Nov., 1996.Google Scholar
- 8.C. Menolfi, and Q. Huang, “A fully integrated, untrimmed CMOS instrumentation amplifier with submicrovolt offset”, IEEE J. Solid-State Circuits, vol. 34, no. 3, pp. 415-420, Mar., 1999.Google Scholar
- 9.K.C. Hsieh, P.R. Gray, D. Senderowicz, D.G. Messerschmitt, “A low-noise chopper-stabilized differential switched-capacitor filtering technique”, IEEE J. Solid-State Circuits, vol. 16, no. 6, pp. 708-715, Dec., 1981.Google Scholar
- 10.Wikipedia, “Opto-isolator,” http://en.wikipedia.org/wiki/Opto-isolator.
- 11.H.L. Skolnik, “Design considerations for linear optically coupled isolation amplifiers,” IEEE J. Solid-State Circuits, vol.17, no. 6, pp. 1094-1101, Dec., 1982.Google Scholar
- 12.W. Olschewski, “Optical coupling extends isolation-amplifier utility,” Electronics, Aug., 1976.Google Scholar
- 13.1S0100 Product Data Sheet, Burr-Brown Res. Corp., Tucson, AZ, Mar., 1982.Google Scholar
- 14.J. F. Witte, J. H. Huijsing, and K. A. A. Makinwa, “A current-feedback instrumentation amplifier with 5 µV offset for bidirectional high-side current-sensing,” IEEE J. Solid-State Circuits, vol. 43, no. 12, pp. 2769-2775, Dec., 2008.Google Scholar
- 15.V. Schaffer, M. F. Snoeij, M. V. Ivanov, and D. T. Trifonov, “A 36 V programmable instrumentation amplifier with sub-20 µV offset and a CMRR in excess of 120 dB at all gain settings,” IEEE J. Solid-State Circuits, vol. 44, no. 7, pp. 2036-2046, July, 2009.Google Scholar
- 16.LTC1043, Linear Technology, http://www.linear.com/product/LTC1043, Jan., 1984.
- 17.T. Denison, K. Consoer,W. Santa, et al., “A 2 µW 100nV/√Hz chopper stabilized instrumentation amplifier for chronic measurement of neural field potentials,” IEEE JSSC, vol. 42, no. 12, pp. 2934–2945, Dec., 2007.Google Scholar
- 18.R. Burt, and J.A. Zhang, “Micropower chopper-stabilized operational amplifier using a SC notch filter with synchronous integration inside the continuous-time signal path,” IEEE J. Solid-State Circuits, vol. 41, no. 12, pp. 2729-2736, Dec., 2006.Google Scholar
- 19.M. Belloni, E. Bonizzoni, A. Fornasari, F. Maloberti, “A micropower chopper-correlated double-sampling amplifier with 2 µV standard deviation offset and 37nV/√Hz input noise density,” IEEE ISSCC Dig. Tech. papers, pp. 76-77, Feb., 2010.Google Scholar
- 20.R.Wu, K.A.A. Makinwa, J.H. Huijsing, “A chopper current-feedback instrumentation amplifier with a 1 mHz 1/f noise corner and an AC-coupled ripple-reduction loop,” IEEE J. Solid-State Circuits, vol.44, no. 12, pp. 3232–3243, Dec., 2009.Google Scholar
- 21.Y. Kusuda, “Auto correction feedback for ripple suppression in a chopper amplifier,” IEEE J. Solid-State Circuits, vol.45, No. 8, pp. 1436-1445, Aug., 2010.Google Scholar