What is the traditional approach to analog front-end solutions, and how do sigma delta solutions overcome their shortcomings?
Traditional approaches to the problem of measuring signals from transducers have been to use a high resolution Analog to Digital Converter (ADC) as the core element in the process. These were generally integrating ADCs or voltage to frequency converters (VFC) that provided high resolution for low input bandwidth signals. This ADC had to be surrounded by a considerable amount of analog signal conditioning circuitry to boost the transducer output signal so that the full dynamic range of these converters could be used to achieve the required performance. The performance of this signal conditioning circuitry, rather than the high-resolution converter, often determined the system success in measuring low-level signals. In designing gain stages, a number of factors have to be taken into account; the first and most critical is noise, both in the measurement environment and in the components that constitute the high gain stage. Common sources of noise in the circuit design environment are mains frequency noise and power supply noise. Other causes for concern include common mode rejection of the gain stage as the transducer output may sit on a large DC signal, therefore necessitating conversion from a differential input signal to a single ended output. Offset in the amplifiers along with drift performance play havoc with circuit performance, leading to the use of expensive chopper stabilized amplifiers. Programmability was difficult to design into these systems and calibration was performed mainly with the use of the system micro-controller. This leads to the requirement for external memory to store calibration coefficients.
Advances in design have enabled highly accurate and integrated solutions to be developed. The advantages that an integrated solution offers the system designer include signal conditioning necessary for direct transducer interface integrated on chip that greatly reduces analog circuit design and layout complexity. Integrated solutions also offer better control of specifications and error budgets than those of discrete solutions. Sigma delta converters provide enough dynamic range to allow direct transducer interface, thus removing the need for a high gain signal conditioning stage in front of the ADC.
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