No battery NFC air pressure sensor

August 12, 2015: I discovered something interesting on the original antenna design today. I had copied the antenna design from a TI reference design without really thinking about it. When I would use the prototype with this antenna design, the range was much worse than other antennas. I calculated the antenna inductance using http://www-smirc.stanford.edu/spiralCalc.html

TI's patch reference had 3 turns of 550 um spacing with 380 um turn width and an OD of 30850 um, leading to something like 600 nH, where the RF430FRL152H datasheet expects 2.66 uH. This is off by a factor of 4.4. Once I realized this, I added a larger capacitor to shift the frequency to where it needed to be. I put a 150 pF cap in parallel with the antenna and got much more range. However, I think that capacitor is effectively shunting a lot of current from the IC itself, reducing power transfer and range. I'll need to redesign the antenna for the next version. Based on the calculator, 6 turns, 200 um space and trace, 33000 um OD will get me closer to 2 uH.

August 11, 2015: I took a little time today to see if I could eliminate vias in the design just to be sure I had a path to do so. With a few minor adjustments, I was able to eliminate all the vias. The biggest assumption is that the reprogramming connector can be eliminated. In the worst case, it could be replaced by pogo pins arranged conveniently around the board. Best case, it's not needed at all and micros can either be gang programmed before population or they can be entirely programmed through the RFID.

I have rigid prototype tags that currently measure a potentiometer, but their range is much worse than the eval kit that has a different antenna. In reading the spec sheet, I think the resonant capacitor on my prototype tags is wrong. I copied the EVM value, which was 39 pF. The datasheet tells me my cap should be about 51.8 pF - 35 pF, or about 16.8 pF. I'm going to try to change this tomorrow to see if my range improves. I only need an inch of Z height, but I also want to make sure it's reasonably tolerant of misalignment.

August 10, 2015: I spent a bit too much time modeling the board in EAGLEUP, but it looks nice. It also made me think about a few things. First, there are a lot of components on this circuit, and I am going to need to think about what can be removed in a more finalized version. First, the TDK pressure sensor die is much smaller than the SO8 package on this prototype. The bypass caps on the unused power supplies can probably be removed as well. The reprogramming header and the RST pullup can probably be removed when reprogramming through the RFID reader is working (it is possible with this micro). It might even be possible to eliminate the op amp if the reduced resolution is adequate for normal use.

One other thing I thought of is that if this is going to go on a low cost printed circuit with conductive ink (like a standard cheap RFID tag), the vias will need to go. That will be pretty hard to do, so I'm going to leave that one alone for right now. I'll have to come back to it after the first generation of prototypes on rigid boards are working.

August 8, 2015: I have completed the first draft of a board layout. The tag is 34 mm in diameter, and includes the pressure sensor, single stage op amp, and RF microcontroller. I've already built analog sensor tags with this antenna design, so I know it works. I will likely need to fine-tune the frequency to maximize the power transfer when it is built though. I will spend a couple more days refining the design and checking all my error calculations to make sure I am not missing anything. Then I'll probably order one of these in FR4 to prove the concept.

August 5, 2015: I've done a bit of thinking about error sources, and I think the few error sources that must be compensated can be done in software when the micro is programmed. A bit more analysis below:

First up is temperature. The NPP-301 has a temperature coefficient of 0.2% of the full scale reading per degree C. For the 700 kPa (101.5 psi) sensor, this is 0.2 psi per degree C. Assuming people won't want to be riding a bike in temperatures below -20C, that means the worst case temperature delta is about 45C, leading to a pressure inaccuracy of 9 psi. That's probably enough to ignore, but it can also be easily compensated using the internal temperature sensor inside the microcontroller. No hardware compensation should be necessary.

Next, the offset voltage. It's +/-10 mV/V. I figure the micro power supply will be about 2 V, so the offset will be +/-20 mV. This is a third of the full scale reading! The good news is that it should be pretty easy to compensate. When the device is first programmed, we can assume it is at about 14.7 psi, and the offset could be zeroed out in software without the use of an external pot. This isn't entirely accurate though, because the sensitivity spec is wide enough to drive a bus through...

Now the sensitivity is specified as 60 mV +/- 20 mV for the full scale pressure. That means 100 psi could show up as 40 mV or 80 mV. That's quite a range, and it's because these pressure sensors are not calibrated (that's why they're so cheap). Calibrated pressure sensors are generally more expensive, but also larger, which is very bad for a tag intended to go in a tire. To calibrate this out, two pressures will need to be measured at the time of programming. This isn't too hard to accomplish, but it is a bit of a pain. A pressure cell with a bike pump could do it. Fortunately, the micro is also programmable through RFID - it may be possible to put the completed tag in a sealed box and write the coefficients wirelessly in an automated process. Either way, there's still no need for extra hardware for compensation - the micro can take care of it all.

All the other variations are less than 0.2% FS, and therefore seem relatively minor compared to these factors. They will not be compensated. The amount of error is definitely allowable for this application.

August 4, 2015: Because I can't find TDK sensors in stock anywhere (in fact, even the TDK website has a dead link under their pressure sensors), I'll prototype with the more expensive (and easier to solder) NPP-301. Digikey has plenty of these in stock for $10.49 each. Too expensive for high volume, fine for prototyping. I think it should work at the ~ 2 V supply from the RFID chip, since it's just a bridge.

The Wheatstone bridge output is unamplified, and will need to be conditioned for the ADC on the microcontroller. There's a nice TI app note on how best to do this:

http://www.ti.com/lit/an/sloa034/sloa034.pdf

For the purposes of what I'm doing, I think I can get by with one op amp. The bridge impedance of the NPP-301 is 5k. I think I can live with the nonlinearity of the single op amp, but I'll calculate this as I go along.

August 3, 2015: I have started to assemble a BOM. The key to a sensor like this is a pressure sensor IC that is small, cheap, and can go up to at least 100 psi for bike tires. Unfortunately, these criteria make the selection pretty thin. I found two candidates: The Nova Sensor NPP-301 and the TDK C32 family. Both are Wheatstone bridge designs and will likely need to be amplified. The TDK sensors are much cheaper, according to Digikey, so I will plan to design with them. http://www.digikey.com/product-search/en?vendor=0&keywords=B58600H8400A038

Unfortunately, none are in stock.