By Len Staller
Embedded Systems Design
(02/24/05, 05:24:00 PM EST)
Embedded Systems Design
(02/24/05, 05:24:00 PM EST)
Absolute error
The absolute error is the total DC measurement error and is characterized by the offset, full-scale, INL, and DNL errors. Quantization error also affects accuracy, but it's inherent in the analog-to-digital conversion process (and so does not vary from one ADC to another of equal resolution). When designing with an ADC, the engineer uses the performance specifications posted in the data sheet to calculate the maximum absolute error that can be expected in the measurement, if it's important. Offset and full-scale errors can be reduced by calibration at the expense of dynamic range and the cost of the calibration process itself. Adding or subtracting a constant number to or from the ADC output codes can minimize offset error. Multiplying the ADC output codes by a correction factor can minimize full-scale error. Absolute error is less important in some applications, such as closed-loop control, where DNL is most important.
Dynamic performance
An ADC's dynamic performance is specified using parameters obtained via frequency-domain analysis and is typically measured by performing a fast Fourier transform (FFT) on the output codes of the ADC. In Figure 8, the fundamental frequency is the input signal frequency. This is the signal measured with the ADC. Everything else is noise—the unwanted signals—to be characterized with respect to the desired signal. This includes harmonic distortion, thermal noise, 1/ƒ noise, and quantization noise. (The figure is exaggerated for ease of observation.) Some sources of noise may not derive from the ADC itself. For example, distortion and thermal noise originate from the external circuit at the input to the ADC. Engineers minimize outside sources of error when assessing the performance of an ADC and in their system design.
Figure 8: An FFT of ADC output codes
Reference:
- http://www.embended.com/
- http://en.wikipedia.org
