A high precision ADC system for instrumentation

Pulse Density Modulation and especially Sigma Delta techniques are employed preferably for a wide range of applications. No trimming is required and no complicated calibration circuitry has to be implemented. This type of converters provide low cost and reasonably small silicon area is utilized. Although there are limitations in multiplexing and control loop applications, Pulse Density Modulation is the most common principle for low and medium speed converters. Recently, a variety of high performance Sigma Delta ADCs have been developed in the field of instrumentation. An optimized combination of Pulse Width and Pulse Density Modulation has been used for an ADC to obtain an excellent linearity and very high dynamic range. This principle has been employed for an ADC that occupies an area of 17 mm² with pads including two independent modulators, digital filtering, serial interface, oscillator and other functional blocks. At a power dissipation of 6 mW it is suitable for battery supply. Its extremely good linearity, offset and offset drift performance as well as a very low input bias current offer a wide range of applications. The difficult task of testing high precision ADCs has been of great concern. For Pulse Density Modulators the differential linearity is inherently good. Therefore an incremental technique can be employed to measure the transfer characteristic at 21 bit accuracy, using only standard elements. This technique has been successfully used and is referred to in detail