6mV5V150 5V remove as much noise as possible the ADuC847146s microcontroller was programmed to employ an averaging algorithm to get better performance Figure 8 shows a typical histogram obt ID: 215539
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+6mV5V– 5V Analog Dialogue 39-12, December (2005) remove as much noise as possible, the ADuC847’s microcontroller was programmed to employ an averaging algorithm to get better performance. Figure 8 shows a typical histogram obtained from a sigma-delta ADC when the analog input is grounded. Ideally, for this xed dc analog input, the output code should be constant. However, due to noise, there will be a spread of codes around the constant analog input value. This noise is due to thermal noise within the ADC and quantization noise inherent in the analog-to-digital conversion process. The code spread is generally Gaussian in nature. Figure 8. Histogram for an ADC measuring a constant analog input. An averaging lter is a good way to reduce random white noise while keeping the sharpest step response. The software for the design discussed here uses a moving-averaging algorithm. Figure 9 shows the basic algorithm ow. DATA 1 DATA 1 DATA 2 DATA 3 DATA M–2 THE AVERAGE VALUE FILTER OUTPUT DATA FROM ADC DATA M MOVING WINDOW DELETE LARGEST AND SMALLEST DATA DATA 2 DATA 3 AVERAGE Figure 9. Averaging algorithm. A moving-average lter averages a number of points from the input signal to produce each point in the output signal. The input to the lter is taken directly from the ADC. Operating on the most recent M data points, the smallest and the largest data points (the outliers) are deleted from the data window. The remaining M – 2 points are averaged, as shown in the equation. y i M xi j j M 1 2 0 3 Using the moving-average technique, the output data rate remains the same as the input data rate. This is rst-order averaging. For higher update rates, second-order averaging is generally used to reduce the waveform dispersion. In that case, the output from the rst stage is averaged through a second stage to further improve results. Figure 10 shows the measured data from the AD7799 after averaging. Comparing this to Figure 5: after averaging there is 180 160 140 120 100 80 60 40 20 0 8447745 8447746 8447747 8447748 1.5 1.0 0.5 0 –0.5 –1.0 –1.5 –2.0 0 300 250 200 150 100 50 MEASUREMENT SEQUENCE (CONVERSION NUMBER) DEVIATION FROM AVERAGE (LSBs) CODE VALUE (24-BIT LSBs) NUMBER OF HITS Figure 10. AD7799 noise performance after ltering at: gain = 64, update rate = 4.17 Hz, reference = 5 V, load-cell input. RMS noise = 0.611 LSBs, p-p resolution = 21.9 bits.