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A to D Converters used in Microsystems

There is strong interest in the development of very small integrated microsystems capable precision measurements of environmental parameters and yet able to operate by scavenging energy from their surroundings. The measurement of environmental temperature, pressure, and humidity using solar power or the measurement of blood pressure using kinetic energy derived from the beating heart are possible examples. In order to realize such microsystems, a number of circuit blocks will be needed, including the sensors themselves, the readout and control electronics, a suitable power source, and some means of storing the information and/or reporting it to the outside world. In a microsystem, using capacitive devices, sensing can be accomplished with virtually no expenditure of energy. On-board data storage in static RAM can also be accomplished with modest energy use. The use of oscillators for reading out capacitive sensors can be done with very little energy. Data conversion can be accomplished using ultra low power Analog to Digital Converters (ADC). This paper discusses the application of ADCs in a microsystem and four common ways of implementing the ADC function. The four common ways are sigma delta, successive approximation, sub ranging and flash. The advantages and disadvantages of each ADC circuit and the interplay between accuracy, resolution, sampling rate, size, and power are discussed in the paper also. Operation of an A to D Converter in a Microsystem A microsystem is a unit capable of gathering electronic data from the physical world, processing and acting on the information and transferring the data to other electronic systems which gain intelligence from this process. Capacitive sensors are widely used in application areas for measurement of physical quantities such as pressure. They offer high sensitivity, yet consume no power and can be readout rapidly using low power techniques. In the block diagrams of the readout circuitry in a microsystem is shown in Figure 1. An integrator is used to convert the changes in pressure to an electrical parameter. Pressure changes are detected by a capacitive transducer. The integrator serves as a capacitance to voltage converter. The amplitude of the integrator’s output voltage changes as the capacitance changes on the capacitive transducer. The