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A Question about your noise characteristic definitions

Question asked by ode45 on Apr 7, 2012
Latest reply on Apr 9, 2012 by NevadaMark

Would youplease explain the difference between "noise density", "output noise" and "Velocity Random walk" parameters in your spec sheets (i.e. adis16407)

 

My best understanding is as follows. I will greatly appreciate if you can comment on it:

 

1. Noise Density: this is the (flat) noise spectral level of the analog sensor output which is later discretized in the front end.

 

2. Output noise: This is the rms noise of the sampled signal (probably after aliasing filters but before any user specified output filter stage).

 

On the other hand, the numbers that are indicated in the spec sheet do not comply with these definitions. Figure 19 of ADIS16407 spec sheet says the internal BW of accelerometer is 330Hz. In this case the rms output noise should be:

0.5mg/sqrt(Hz)*sqrt(330*1.6)=11.48

However the speec sheet says is is 9mg. (page 24)

 

3. What do you mean by "velocity random walk". Apparently, you use the value of Allan variance figures at tau=1 for VRW. (This is also how it is defined in the usual texts). However, in your case this is completely wrong. You have a huge quantization step, which is 3.33mg. Basically your stationary output is nothing but the quantized white noise. The Allan variance of the quantized white noise is completely different (I would rather say arbitrary) than the allan variance of the unquantized noise. Therefore, your apperant use of the term "velocity random walk" is wrong. As a matter of fact, because of the very large quantization step, I doubt that your Accelerometer Allan variance figure (i.e. Fig.8)  represents anything meaningful.

(PS: Quantization error defined for high-end inertial sensor is a completely different phenomenon which manifests itself as a -2 segment in allan curves. I mean "bit quantization" in the above explanations as your sensors do not have an "Qunatization error" in the sense that we use in high-end sensors.)

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