gareth2020
commented
4 years ago It is hard to tell if the loss of noise performance and immunity will matter in this application. It might not because most local noise (mechanical or electrical) will not correlate between sites.
It is also interesting to note that the Analog Devices evaluation board has two capacitors per voltage pin but they are not different. See https://www.analog.com/media/en/technical-documentation/user-guides/EVAL-ADXL354-355-UG-1030.pdf
i.e. the evaluation board does not seem to match Figure 59 of https://www.analog.com/media/en/technical-documentation/data-sheets/adxl354_adxl355.pdf.
I have sent a query to Analog Devices Technical Support about this mismatch. Analog Devices: Case #(SR-186103-W5C8Y4)
This was the reply: _Hello Gareth, have you experienced such a degradation of performances or is it just hypothetical? As I said before, this breakout board is designed to support different mems with a common BoM.
Also please consider that these EVAL board are not designed to be integrated into a product and they come as it is.
This kit instead is specific for the ADXL-355 https://urldefense.proofpoint.com/v2/url?u=https-3A__www.analog.com_en_design-2Dcenter_evaluation-2Dhardware-2Dand-2Dsoftware_evaluation-2Dboards-2Dkits_EVAL-2DADXL355-2DPMDZ.html&d=DwIFAg&c=jf_iaSHvJObTbx-siA1ZOg&r=BNSi0RLDXywCVDeiXDJV-QTNF_9CBQjfLItfrVCJwho&m=InJ2SnAYkIPzK4qIO0t5JpuxdxNQC41MWLLvil3haCc&s=OZ58kHG_AnoyYVWj_P3e_FZCvnpG0e5nw4wzeJ0_rhg&e=
Sincerely, Rocco Marco Guglielmi EMEA Centralized Applications Center (CAC) Analog Devices CIC.emea@analog.commailto:CIC.emea@analog.com_
perigoso
andygrillo
The noise performance might be worse than the specification because of missing bypass capacitors on the accelerometer chip power supply and voltage reference pins.
Figure 59 in https://www.analog.com/media/en/technical-documentation/data-sheets/adxl354_adxl355.pdf suggests 2 bypass capacitors should be used for each power supply near to the ADXL355. i.e. 0ne 0.1uF (100nF) capacitor and one 1 uF capacitor. However the OpenEEW schematic only seems to show 100nF capacitors connected to pins 5,8,10 and 11 of U3.
Analog Devices, probably characterized the noise spectral density for ±2 g range as 22.5 μg/√Hz by using the circuit shown in Figure 59 of https://www.analog.com/media/en/technical-documentation/data-sheets/adxl354_adxl355.pdf. If the 1u F capacitors are not present, then the noise spectral density might be worse and the signal might be less immune to power supply noise e.g. from other electrical activity on the pcb and from radio frequency interference and from noise on the 5V power input.
For more on why two capacitors per voltage pin (and how to place and rout them) see https://www.analog.com/media/en/training-seminars/tutorials/MT-101.pdf e.g. "Low frequency noise requires larger electrolytic capacitors which act as charge reservoirs to transient currents. High frequency power supply noise is best reduced with low inductance surface mount ceramic capacitors connected directly to the power supply pins of the IC. "
e.g. "The purpose of the large capacitor is to be a reservoir of charge to supply the instantaneous charge requirements of the circuits locally so the charge need not come through the inductance of the power trace"
Reliability of the OpenEEW sensor might also be improved with two capacitors per voltage pin because when one of the capacitors fails it might not causing too much damage to the noise performance. If there was only one capacitor then the noise performance might be damaged too much if it failed.