Knowles BU7135 is extensively used in surface acceleration (SAL) measurement in speech related study. By the way, the acceleration data is in the range of milli-gravity (mg). Piezoelectric accelerometers such as Endevco 27AM1 provide noise floor or residual noise value (i.e for a -100 model = 400 micro-gravity). Hence I could calculate the minumum detectable acceleration which will be 4000 micro-gravity. However Knowles manufacturer provide the following info for noise max (input: N/A; output -103dB) [ see link] How can I determine the minimum detectable acceleration from the output noise max (dB) data for BU7135?
http://www.knowles.com/eng/Products/Sensors/Accelerometers
Hi
This is classic misuse of the dB scale. dB really is little more than the logarithm of %, so dB in itself is like stating %.
However, if we go to the Knowles datasheet, we find the sensor sensitivity to be -45 dB re 1 V/g, i.e. 10E(-45/20) = 56.23 mV/g.
Assuming that Knowles uses the same reference (1 g in this case) also for the noise floor, you find the noise floor at 10E(-103/20) = 7E-6 g. That said, this is only guesswork. You must ask them what is their reference or their -103 dB value can be anything.
It may be prudent also to ask them how the compute their dB value as their usually is some confusion as to whether one should use 10Log(Value/Ref) or 20Log(Value/Ref) .
The correct answer is that the dB always refers to power, so when referencing Watt,we use 10Log(Value/Ref) and when we reference field quantities, we use 20Log(Value/Ref) as these must be squared to become proportional to power. http://en.wikipedia.org/wiki/Decibel
Hope this helps
Claes
Thanks Claes. Your response definitely helped.
Endevco 27AM1 and Dytran 3055B2T both with sensitivity of 100mV/g has floor noise at 400 micro-gravity. The floor noise for Endevco 61C13 which is more sensitive at 1000mV/g is at 50 micro-gravity. I will be surprised if Knowles BU7135 accelerometer has floor noise of ~7 micro-gravity.
I have forwarded the question to Knowles representative via their website.
I will post their response once it is available.
Abu Dzarr
This is what I had gathered from my communication with Knowles Staff.
All measurement utilizes 20log(Value/Ref)
The BU-27135 tech sheet provides values for a standard 2-wire connection. It has a sensitivity of -43.5dB (1 kHz) re 1V/g. Upon conversion antilog(-43.5/20) = 6.7 millivolt/g. The 'A' weighted noise (re 1.0V) -103 dB Max. This is equivalant to 7 microvolt. This value can be further converted to unit gravity (7/6700) = 0.001g = floor noise.
However I was also provided a three-wire connection values which is more sensitive (attached) ~30mV/g. Typical minimum g at 1KHz for 6dB signal to noise is 0.003g. 6dB here refers to SNR margin. Hence the floor noise will be antilog(-6/20)*0.003 = 0.0015g
Abu Dzarr
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