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Tips of the Month
Noise Rejection Performance of Integrating A/D Converters
The November 2005 issue of NetSOL Online highlighted some examples of functions (filters and the moving average) available on measuring instruments (the µR series of recorders) for suppressing noise contained in input signals.
This issue provides a simple explanation of just how noise is removed in the A/D circuits when the input signals are converted from analog to digital.
µR series instruments employ integrating A/D converters for converting the measured input from analog signals into digital signals. The integrating A/D converter integrates the measured values at the specified time width. If a specified time interval (the integral time) matches the period of the noise, the areas under the curve on the plus and minus sides in the figure below cancel each other out (offset one another), the average value becomes zero, and the noise is rejected.
For example, if the integral time is 20 ms, noise having frequencies of 50 Hz and integer multiples thereof can be rejected. Likewise, if the integral time is 16.67 ms, noise frequencies of 60 Hz and integer multiples thereof can be rejected. If the integral time is 100 ms, noise of 10 Hz and its integer multiples can be rejected. The commercial power supply is one of the main sources of noise. By setting these integral times, commercial power noise of 50 Hz or 60 Hz can be eliminated.
The relationship between the integral times of the A/D converter on the µR10000/µR20000 and the corresponding rejected noise (frequencies rejected), as well as the relationship between integral times and measurement interval are summarized in the tables below.
1: Factory default setting
1: Factory default setting
The factory default setting for two of the older µR series dot models (µR1000/1800) was 100 ms. As you can see above, the µR series dot models (µR10000 and µR20000) use an AUTO mode. Please keep in mind that the integral time can switch.
Now take a look at the frequency characteristics graph below to see how noise is actually removed. Figure below shows a frequency characteristics graph when a 20 ms integral time is applied.
Integral time of 20 ms (logical value)
The vertical axis represents the amplitude ratio (gain) and the horizontal axis represents the frequency. The amplitude ratio refers to the ratio of the output signal amplitude to the input signal amplitude and is indicated in decibels (dB). A negative dB value means that the input signal is attenuated before it is output (0 dB means the input signal is not attenuated).
From the figure above, we can clearly recognize that the frequency components of 50 Hz and its integer multiples have been sufficiently attenuated (i.e., removed).
- User's Manual MX100 Data Acquisition Unit, Yokogawa Electric Corporation
- Technical Information MX100 Performance Specifications, Yokogawa Electric Corporation
- Yokogawa Test and Measurement Instruments General Catalog '94, Yokogawa Electric Corporation