http://www.allaboutcircuits.com/technical-articles-preview/36790

Hi John,

Overall it's a very solid article. Here are a few issues for your consideration:

1. Re: inductor vs. ferrite bead

This is my main concern as far as the technical information. The preferred component for IC power-supply filtering is a ferrite bead rather than an inductor. The distinction is important because beads actually dissipate high-frequency energy, whereas inductors simply store it. (You are probably familiar with this issue, but if not you might find this article interesting.) I get the impression that many designers are somewhat loose about using beads vs. inductors, and perhaps this looseness is reflected in the use of L as a reference designator prefix for both types of components (I always use FB for beads). I, on the hand, am very weary of inductors in power lines; I don't like the idea of all that inductance and capacitance sloshing energy back and forth and perhaps causing unexpected ringing.

Anyways, I noticed that in the BOM you identified L1, L2, and L3 as beads, but throughout the article you use the word "inductor" and you mention an "LC" filter (instead of an FBC filter, as I call it).

What are your thoughts?

2. Re: speed vs. throughput in the table

I noticed that the speed for BLE is significantly higher than for classic, but BLE's throughput is much lower. Does that seem right to you? Does BLE have a bunch of extra overhead that slows down the actual rate of data transfer?

3. "BLE is designed primarily for applications requiring simple data (such as sensor data) rather than complex data or streaming audio."

I'm not clear on the distinction between simple data and complex data. In the end it's all ones and zeros. Perhaps you're trying to say that BLE is designed for devices that require only intermittent transmissions of relatively small packets of data.

4. "The use of a module will simplify the design and drastically reduce certification costs—but it will also increase the production cost."

A while ago I was pricing out some IoT concepts for my brother, and I came away thinking that it would be very difficult to significantly reduce BOM cost by using a custom design instead of a module. Have you found that a good designer really can beat the modules when all BOM and assembly costs are factored in?

5. "Inductors L2, L3, and L4 provide some power supply isolation"

I think this should be L1, L2, and L3.

6. "Power and ground planes in the PCB should not run under the device, because they could provide an unwanted thermal path to the sensor."

This issue is a bit confusing, and I'm not sure what to do with it. The datasheet for the MPU-9150 is very clear on two points: the exposed pad should not be soldered to the board, and no traces or vias should be placed under the exposed pad. But with power and ground planes, the situation is not so clear: "Note: For best performance, design a ground plane under the e-pad to reduce PCB signal noise from the board on which the gyro device is mounted."

Also, I think that the thermal path is only part of the story, and saying it this way gives the impression that the copper won't be problematic if there is not significant heat being transferred to the MPU-9250. Part of the problem, and perhaps the more significant part, is that copper somehow interferes with the MEMS stuff (at least the gyro, maybe the magnetometer as well): "Routing traces or vias under the gyro package such that they run under the exposed die pad is prohibited. Routed active signals may harmonically couple with the gyro MEMS devices, compromising gyro response."

7. "An antenna will normally require the use of a pi-network for tuning the antenna (i.e., altering the impedance of the antenna so that it is more compatible with the transceiver's input and output circuitry)"

This is my revision, which was intended to make this sentence a little more clear and informative. Does this look OK to you?

8. Re: Bluetooth Smart

Is Bluetooth Smart the same thing as Bluetooth LE? If so, we should probably mention that somewhere in the article. It will be helpful for readers and good for SEO.

 

Hi Robert,

Excellent feedback, thank you!

My comments are below:

1) You are 100% correct. Those are supposed to be ferrite beads not inductors. I will correct the text.

2) That table must be incorrect. I've always seen the speed for BT Classic listed as 2-3Mbps with BLE listed as 1Mbps. For data throughput then I normally see 0.7-2.1Mbps for BT Classic, and 0.3Mbps for BLE. This is what Wikipedia shows except they show Classic with a speed 1-3Mbps. So the other numbers in your table match Wikipedia except for speed for BT Classic. 0.7Mbps makes no sense and you can't have a throughput higher than the speed so I think that must be a typo.

3) Yes that is what I trying to say but perhaps "simple" and "complex" aren't the best words to use. I'll rewrite it using your phrase if that is okay, so "BLE is designed for devices that require only intermittent transmissions of relatively small packets of data."

4) From my experience the volume must reach a bare minimum of at least 500k units before there is any cost savings from a custom BLE chip design versus a module. In most cases that is also assuming you switch from a chip antenna to a PCB trace antenna. I'll expand on this a bit more in the article.

5) Correct, it should be L1, L2, and L3. Although I will change them to FB1, FB2, and FB3 as you suggested.

6) Actually I think that statement should go under the HDC1080 and not the MPU-9250. I'll review the datasheets again and correct appropriately.

7) I think your change sounds good, although I like this wording a bit more ""An antenna will normally require the use of a pi-network for tuning the antenna (i.e., altering the impedance of the antenna so that it more closely matches the impedance of the transceiver)."

8) Yes, Bluetooth Low-Energy, Bluetooth LE, BLE, and Bluetooth Smart are all the same thing. Good point on adding the term Smart and I'll make that change.

I'll let you know once I've made these changes so you can take a final look.

Thanks again and I appreciate the feedback!

John

 

Hi Robert,

Okay, I've made all of the changes. I also uploaded a new schematic image with L1,L2,and L3 relabeled as FB1,... Comment #6 above was already under the HDC1080 section so I think perhaps you accidently saw it under the MPU-9250.

Please take a look and let me know if all looks good now.

Thanks!
John

 

Thanks for the great feedback, John, I appreciate it.

2) Did you have that table in the original article, or did Kate insert that after you submitted the article? In any event, perhaps we should simply remove the table and replace it with a few prominent specs. The current table might be a little bit of information overload for the reader, and it appears that the information might not be trustworthy.

3) Sounds good.

6) Wow, I don't know what happened there. That information is clearly in the HDC1080 section, but somehow when I was reading my mind applied it to the MPU-9250. Sorry about that. So I don't think you need any revisions for this issue. There's no need to go into layout details for MPU-9250.

7) Sounds good, please revise to your wording.

 

Hi Robert,

Kate must have added the Bluetooth table and I just assumed you did that. I agree it's probably best left out since it doesn't appear accurate. Also this isn't necessarily an article about the differences between BT Classic and LE so I don't think it adds much.

Everything else sounds good I think. Let me know if any other changes are needed after reviewing the latest version.

Best wishes,
John