Hi,

According to the datasheet for the ADIS16251 rate sensor, the sensor has an angle random walk of 3.6 degrees/sqrt(hr), and the datasheet for what I'm told is one of its recommended replacements, ADIS16265, states that it has a lower angle random walk of 2 degrees/sqrt(hr). However, in testing this myself, I was not able to confirm that this was the case.

To calculate the ARW, I plotted the Allan variance curves of the two sensors using data collected at 3 Hz over a period of about 72 hours. As per IEEE-STD-952-1997 and FAQ: Gyroscope Angle Random Walk, the ARW is obtained by taking the value of the Allan deviation (square root of Allan variance) when the averaging time is 1 second. Applying this method, it seems from the plot below that the ARW of the two sensors is not significantly different, with 0.03081 degrees/sqrt(s) (1.85 degrees/sqrt(hr)) for the 16265 and 0.02903 degrees/sqrt(s) (1.74 degrees/sqrt(hr)) for the 16251.

This approach always seemed a little sketchy to me, however, since that same IEEE standard states that the rate ramp can be found by reading off the value of the plot when the integration time is 3 seconds, yet in the plot I've provided, the slope at 3 seconds is not +1/2 as required.

Next I tried using a system identification approach to determining ARW, where I modeled the total Allan variance of the rate sensor as the sum of the Allan variances contributed by the various error sources in the rate gyro, including quantization noise, ARW, bias drift, rate ramp, and rate random walk. Using this model, I applied a log-least-squares fit to the same data used to make the plot above and found that the ARW of the 16265 was 0.0291 degrees/sqrt(s) (1.75 degrees/sqrt(hr)), whereas the 16251 had an ARW of 0.0228 degrees/sqrt(s) (1.368 degrees/sqrt(hr)), indicating that the older sensor in fact has lower ARW. I've included a plot of the fitted Allan variance curves below.

I've repeated this procedure for two other pairs of sensors of each type and each time, the result I've shown here is repeated. Any idea what can explain this discrepancy?

Thanks.

Great questions! The gyroscope noise was supposed to improve from the ADIS16251 to the ADIS16265, but some overlap in performance would not be surprising, especially in small quantities. After all, the improvement expectation was modest. During the time period between the ADIS16251 and ADIS16265 releases, we focused on improving our characterization approaches for the Allan Variance curves. The key areas associated with this experimentation were with the sample rates and the signal processing (perform averaging inside or outside of the part). In my quick review of the literature available, the final approach used for the ADIS16251 is not clear. Since this part was characterized over 8 years ago and is now obsolete, I am a bit limited on what I have access to.

For the ADIS16265, using 819.2 SPS sample rate setting (SMPL_PRD = 0x0001), the maximum range and little (or no) filtering (SENS_AVG = 0x0402) has been the standard approach that supported the ADIS1626x, ADIS1630x, ADIS16334, ADIS1636x, ADIS1640x and ADIS1644x characterization of this parameter. With respect to the 50Hz vs 330Hz bandwidth option, I would probably use the 330Hz option, but I wouldn't expect this setting to have a large influence.

Does this help?