October 13, 2015: Over the past two days, I've made huge progress. I tuned the antenna to provide 3 cm read distance with my phone. I did this by putting 3.3 uH in parallel with the 15.9 uH antenna inductance to yield about 2.7 uH. The IC has an internal 35 pF cap, so I added another 15 pF to this to yield 50 pF total capacitance. Then, 1/(2*PI*sqrt(2.7E-6*15E-12)) = 13.62 MHz, pretty close to the 13.56 MHz reader frequency. Note that this is all possible using only the datasheet numbers. I tried a couple caps on each side of 15 pF and confirmed the read distance dropped off. That told me that 15 pF was the sweet spot.

After that, I applied various analog voltages to the ADC used as the pressure sensor input and confirmed that it acted as expected.

I put the pressure sensor on today and I put the whole assembly in a little box that I built for pressurizing. I tested it at 0, 10, 20, 30, 40 and 50 psi, and got an excellent linear response, just as expected. When I back out the gain of the amplifier and the voltage applied to the bridge, I am within a reasonable distance of the actual pressure. There's still a bit of calibration that needs to happen though.

So what's next? First, I may jump ahead and try to eliminate the op amp chip. The RF430 has internal op amps, so I might be able to save a lot of space. Then there's a big durability concern to deal with now. This tag needs to go inside a tire tube and get rolled over repeatedly without dying. These two items are next on my list.