Basic VFC Cells
Voltage-to-frequency converter circuit architectures based on multivibrator implementations basically consist on an input voltage-to-current converter followed by a bidirectional current integrator driven by a control circuit, usually a voltage window com
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Basic VFC Cells
Voltage-to-frequency converter circuit architectures based on multivibrator implementations basically consist on an input voltage-to-current converter followed by a bidirectional current integrator driven by a control circuit, usually a voltage window comparator (VWC) or a Schmitt trigger (ST). In this chapter an introduction to each of these three basic cells is done and some implementations are advanced pursuing the requirements set by the driving application of this work, which remember mainly are: (1) low voltage supply, to be powered with the batteries used in WSN nodes, (2) low power consumption, to maximize batteries life, (3) rail-to-rail input operation, to take advantage of the maximum achievable conversion resolution, (4) output frequency range compatible with WSN μC clocks, typically fclk ¼ 1 – 4 MHz, and (5) temperature and supply voltage-independence, to maintain constant VFC performance characteristics. In addition, to facilitate a system-on-chip solution, thus reducing the total area, increasing the operation velocity, avoiding wiring faults and parasitics and, especially, reducing the cost, the implementation will be done in a CMOS technology. In particular, the chosen technology is a standard CMOS process, provided by United Microelectronics Corporation (UMC), with minimum gate length of 0.18 μm, P-substrate/N-well, six metal layers and one polysilicon layer [UMC12]. Thus, voltage-to-current converters are studied in the first section; next, bidirectional current integrators are explained in the second section and control circuits are introduced in the third section. Bias circuits, also presented in this chapter, are considered in the fourth section. Finally, as a closing section, general conclusions are given.
3.1
V-I Converters
The voltage-to-current (V-I) converter is the input stage of the VFC. This cell, which is a basic building block in many analog and mixed signal designs, such as multipliers, continuous-time Gm-C filters, data converters, high-performance C.A. Murillo et al., Voltage-to-Frequency Converters: CMOS Design and Implementation, Analog Circuits and Signal Processing, DOI 10.1007/978-1-4614-6237-8_3, # Springer Science+Business Media New York 2013
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3 Basic VFC Cells
Fig. 3.1 Conventional V-I converters: (a) with passive resistors at the input and (b) conventional OTA-MOS-resistance V-I converter
sensor interfaces or variable gain amplifiers, is critical because the overall system performance depends largely on the V-I converter features. This leads to the need for a time-, temperature-, and voltage level-independent transconductance, with a high linear range and an appropriate bandwidth. Especially critical is the input range: in a VFC, the V-I input range determines the system input operating range. Therefore, attaining rail-to-rail operation for the V-I converter is a must because, if this feature is not accomplished, there will be a loss in the maximum allowable VFC signal-to-noise ratio (SNR). As a consequence, we will not take advantage of the
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