------Question for ADXL356CEZ Please Put your question below------
I’m hoping to monitor accelerations up to ~+/- 40 g at frequencies up to 100 Hz. I have a good DAQ which can monitor at 250 kHz sampling frequency. I plan to attach the accelerometer and any breakout/evaluation board to the sample attachment point of my vibrator to allow concurrent collection of acceleration data with output from other sensors through the same DAQ.
As a result, I’d like to be able to collect sufficient data to reconstruct the sinusoidal acceleration curves at the frequencies of interest. I noted that most of the ADXL evaluation boards have a 50 Hz bandwidth constraint, which from what I’ve been able to tell means that any acceleration data oscillating at higher than a 50 Hz frequency will not appear in recorded data, and as a result any vibrational inputs greater than that frequency will not be trackable with this setup. As I’m relatively new to vibration analysis I want to make sure I’m understanding things correctly.
While 100 Hz would be preferred, I can make do with a maximum 50 Hz vibrational frequency, so using the above-mentioned boards may be sufficient. However, I noted that there were multiple versions of the ADXL356C accelerometer, and I want to make sure I select the correct one for use with the EVAL-ADXL356CZ evaluation board.
Finally, while you carry the ADXL358C accelerometer, and I know there is a similar Eval board available, I was unable to find it on you site. Do you carry this eval board as well?
Thank you, and I look forward to your response.
Greetings,
Consider the EVAL-ADXL356CZ for your purposes, and note that the eval board user guide and the component datasheet are useful to read in conjunction with each other.
The bandwidth constraint on the analog-output eval boards is artificial, arising from the external capacitors on the board:
in conjunction with the internal series resistance of the output amplifier:
Together these form a simple RC filter, with a -3dB “cutoff” point at f=1/(2* π * R*C). Remove those capacitors and one has access to the full datasheet bandwidth of the accelerometer, or replace them with a lower value to change the filter response as desired.
If it’s not already part of your background, some reading on basic sampling and filter theory would be informative and relevant to an application of this kind. One key idea is that signals outside a filter’s passband don’t just magically “not appear,” but rather appear with diminished amplitude.
The eval board for the ADXL358 does not appear to be among the DigiKey product listings at this time, and ADI’s site seems to suggest that they’re not in any particular hurry to make them available:
The above-mentioned '356 board is very available however and functionally very similar. As for the different suffix variants of the '356C, these refer to the manner in which the products are packaged for shipment; the parts themselves are exactly the same. This is by far the most common answer to “what does the suffix mean” types of questions, and is the first thing one should check when they arise.
Thank you for the very thorough and informative response! I think I should be able to figure out the filtering aspect with this information and select the correct accelerometer for my needs, I’ll let you know if I have further questions.
–J.C.
Apologies for the double reply, but I was looking at the data sheets for the ADXL356C and ADXL358C and am considering providing the external capacitors through manual soldering on my part. In preparation for that, and my specific use case, I have a few questions:
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Both data sheets state that V1P8ANA (which I plan to use to power the analog outputs) requires a decoupling capacitor. I assume this capacitor is connected between V1PANA and VSS (analog ground). In looking at the application circuit diagram (ADXL358 p. 12, ADXL356 p. 29) it appears that this function is carried out by a 0.1 uF and 1 uF capacitor in parallel. From a little research, it looks like this is done to protect against power spikes. However, I also see in these diagrams another set of parallel capacitors connecting VSupply and V1PANA. Is the purpose of this set of capacitors similar? If so, is this setup recommended for any application of these accelerometers? Should these capacitors be placed on/near the accelerometer (i.e. would also be experiencing the vibration) or would they work just as well soldered to a breadboard connected to the accelerometer by lead wires?
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Likewise for the low-pass capacitors mentioned in your previous reply. Do these need to be adjacent to the accelerometer board (i.e. experiencing vibration/acceleration) or would placement on a vibration-isolated breadboard work just as well?
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In the datasheets, it is noted that accelerometer output is ratiometric to V1P8ANA. My setup will be using a differential ADC to read the output of the accelerometer. Fom the documentation, it appears that V1P8ANA should be used as the reference voltage, while the output from the X/Y/Z channel should be the input to the ADC measured relative to this reference. With this setup, what kind of output can I expect? I had anticipated that using the setup noted above, I would be able to achieve the sensitivities noted in the data sheets (i.e. 20 mV/g for the ± 40 g level, or a reading of ~20 mV when 1 g is being experienced by any given axis). If that’s the case, we should be golden, but the datasheet descriptions make me concerned I may be missing something important.
Thank you again for your help and guiding me through this!
Sincerely,
J.C.
1: These are known as supply decoupling capacitors. Their purpose is to stabilize the voltage at the power supply pin(s) against high-frequency perturbations caused by the operation of the chip and/or external influences. Placement near the IC served is essential to reduce parasitic inductance.
Their use is advisable and widely recommended, to the extent that attaching a 0.1uF ceramic cap is almost a Pavlovian response to seeing a pin designated as a supply input among electronics designers. It’s relatively rare for these placements to get specifically evaluated as to need/effect in any particular circumstance however, so one should not interpret their presence in any particular example circuit as some sort of sacred knowledge. This document offers some more in-depth discussion on the topic.
2: Isolating these from vibration (if used) would be a good idea to reduce microphonic effects, which could be quite significant considering the accelerations involved and the high output impedance of the sensors. Ceramic types vary as to their microphonic traits, with C0G or equivalents being the least susceptible.
3: Yes, use the supply to which the output(s) are proportional (e.g. ratiometric) as the reference source for data conversion. This makes the actual value of the supply irrelevant to a good first approximation and more or less maps the full-scale range of the sensor to that of the ADC.
Awesome, thank you so much for your help!
Hi Rick,
Found a few more questions on the EVAL-ADXL356C which I could use some assistance with.
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My DAQ has a 5 V power supply, so I plan to use a voltage divider to provide 2.5 V to the accelerometer. This is on the lower range of the acceptable voltage input, so is there anything I’ll need to be aware of when using this method as opposed to a direct 3.3 V power input?
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Also on the powering subject, looking at power sequencing for the ADXL356 datasheet (p.23) and the breakout board pinout diagrams, am I correct in understanding that I will need to attach the positive voltage terminal to both VDDIO and VDD (P1 pins 1&3) and the negative voltage terminal to ground (P1 pin 5) to power the accelerometer? Is there any drawback to making a soldered connection between VDD and VDDIO on P1 (as they need to be powered simultaneously anyway)?
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When using the EVAL-ADXL356C, I’m a bit confused on how to set the g-range. I understand I’m supposed to connect the RANGE to the DDIO pin, but I’m unsure how to do this with the breakout board setup. I assume it has to do with the connectors/plugs near the “mode” and “range” labels on the chip, (labeled 202-73 in the schematics on p. 4 of the data sheet). For clarification, what options are controlled/available with these interfaces, and how would I go about selecting the +/-40 g option using this interface? What, if any, additional parts will I need to interface properly?
Just want to make sure I’m using the part properly so I can get to my research!
Thank you for patiently answering my questions.
Sincerely,
J.C.
Re. (1) That’d be a poor design approach in most circumstances, but seeing as how the part consumes almost no current and has a built-in regulator that you’ll be using anyhow, it might actually work here. Keep the total divider resistance on the order of 1K, hang some C off the midpoint, and you might be in business.
Yes.
I suppose there’s a risk that you could burn your fingers by grabbing the wrong end of the iron. Probably not much issue otherwise.
Simply place a jumper on the “range” header in the position corresponding to the range you want to select. Likewise the “mode” header. Or just go ahead and solder that connection too; it’s probably less likely to come off than a friction-fit jumper when you start shaking the bejeebers out of it, and standby mode is rather boring anyhow…
Wonderful, thank you! I’ll let you know if I come up with additional questions as I create the physical experimental setup.
J.C.
Also, I think I found a better alternative than the voltage divider. I can just get a LD1117-3.3 to convert the 5V to a 3.3 V signal, add a few capacitors to stabilize it and be good to go. 
