Users of analogue-to-digital converters (ADC) often are unaware of how crucial the accuracy of the converters can depend upon the wiring of the modules. In particular, the influence of the wiring can be very decisive when dealing with high signal bandwidths and high signal dynamics.
Improvement of a 16-bit data acquisition card by circuit optimization
The figure shows the integral non-linearity of a 16-bit ADC data acquisition card measured at PTB. The red curve represents the behaviour of the first specimen of the sub-assembly. On the basis of the measurement results, the error sources could be identified and the card optimized. The green curve represents the result of optimization where the linearity is very good. An integral non-linearity of 0 LSB (least significant bit) in the entire code area is the theoretical limiting value for an ideal ADC.
PTB has developed a new measuring set-up to perform high-standard metrological characterization of analogue-to-digital converters and complete ADC sub-assemblies.
Modern measurement and data acquisition systems demand for ADCs and ADC subassemblies with ever higher resolution and linearity at high bandwidth. In practice, however, simply substituting higher resolution ADCs (e. g.: 16-bit or even 24-bit converters) for lower resolution types (e. g.: 12-bit converters) often does not improve the accuracy to the anticipated extent: Proper wiring of ADC modules becomes increasingly important with higher resolution. This is often underestimated. The higher the aspired accuracy is the more important are proper and stable mass and voltage supplies, exact timing, correct routing of the critical signals, layout of interference suppression, and the shielding or partial shielding of important components.
Increasingly more time and effort must be spent on testing ADC sub-assemblies. Dynamic testing of sub-assemblies with test signals that vary in duration is especially difficult at high scanning rates. The measurement adapter required for the test as the link between ADC sub-assembly and measuring set-up must be modified, too.
Engineers who design the circuits often are not in a position to carry out these important measurements by themselves because the suitable technical equipment is not readily available. Investigations and tests of ADC modules and complete sub-assemblies can now be carried out with high metrological quality at the new PTB measuring set-up. Such investigations of ADC sub-assemblies have already been carried out, for example, for the automobile industry. In most cases, the measurement results lead to a marked improvement of the ADC sub-assemblies.
Contact at PTB:
Dr. Allard Schnabel, ORR
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