Considerations for High Accuracy Measurement System Design

Improvements in sensors technology, warrants need for high accuracy analog measurement systems. Achieving high accuracy to make use of the state of the art sensors, requires in deep analysis & design skills.

To begin with, choosing an appropriate ADC for the design is a critical design decision. This requires analysis of accuracy requirements, to determine the minimum Effective Number of Bits (ENOB) of ADC. In addition to this, sampling frequency with which the analog inputs needs to be measured also plays an important role.

Further to this, Analog Front End (AFE) Circuit for the analog inputs needs to be selected carefully. AFE might be required to scale down the input voltage or converting current input to voltage type or for buffering. Any external component present in the AFE will affect the measurement accuracy. Thus all the AFE Components needs to be high precision components to reduce impact on accuracy.

Since the sensors are connected to the system from external sources, it is important to have protection against ESD, EFT, Surge etc. The leakage current of the protection circuits can be another factor which can lead to measurement errors. Architecture & components of the protection circuit needs to be selected with the aim of reducing the leakage current to minimum.

Once all the components are finalized, one of the easier analysis method is to prepare an error analysis sheet which includes all the individual errors introduced by the components. One component can have multiple error factors & all the factors from all contributing components needs to be considered for analysis. For example, an ADC can have multiple error inducing parameters such as INL, DNL, Offset Error, Gain Error, Offset Drift, Gain Drift etc.

If there is a scope for calibration, then errors which can be removed using calibration (such as offset drift, gain drift of ADC, Resistor tolerance, Reference Input accuracy) can be neglected during analysis. But the only way to remove these error parameters completely, is to calibrate each individual errors separately. If the calibration is performed as a system (which is the case generally), then these errors cannot be eliminated completely due to inherent limitations of the calibration algorithm.

Once the analysis sheet is prepared, Root Mean Square (RMS) method can be used to predict the system measurement accuracy. This process can be iterated multiple times to analyze the impact of each component or to improve the measurement accuracy.

In addition to this, careful design of board layout is mandatory for achieving good accuracy. Isolation of analog sections from high frequency digital sections, sufficient clearance between analog traces, maintaining uniform grounding are some of the general techniques used to make sure that board level noise doesn’t impact the accuracy.

Designing an analog system with high measurement accuracy requires multi-facet design skills. Selection of proper components, Accuracy analysis at design stage, Careful PCB Layout, Selection of suitable calibration method can help in realizing a high accuracy measurement system.

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