AAT and Capacitive Wakeup

[LltWc]

I've been reading through the Wake-Up Mode and Automatic Antenna Tuning application notes, and while I understand that AAT and Wake-Up with inductive sensing require more involved software wake-up logic, it doesn't seem to me that capacitive sensing would. For inductive sensing with AAT, the AAT outputs are left floating during Power Down Mode, so upon wake-up, they need time to settle for an accurate measurement.

But the capacitive wake-up is a separate mechanism that has nothing to do with AAT or the NFC antenna. Is there any reason that AAT + capacitive wake-up wouldn't work together?

edit: this is for the ST25R3916, with custom firmware

[Ulysses HERNIOSUS]

Hi LltWc,

theoretically capacitive wake-up and AAT are independent features. Practically I would expect the variable capacitors being located close to the capacitive plates. So I could imagine an interference. But I am not aware of anyone having experimented with it a lot.

As capacitive wake-up is a features rarely used, it got removed from ST25R3916B. ST25R3916B implements also a configurable delay for the wake-up to cope with the variable resistor settling. Maybe ST25R3916B could be an option for you. It is very similar to the non-B, using even the same driver with a compile time switch.

BR, Ulysses

[LltWc]

We intend to use a capacitive sensing antenna rather than plates (and are currently testing antennas), so that should give us more sensitivity than just pads on the board.

The issue with inductive sensing is that we frequently scan NFC cards, and are unable to detect them reliably, presumably because the density of metal in the card antenna is too low. Sensing works best at the corners of the card, and worst at the center, which is where it's most frequently scanned. The antenna setup works very reliably for actual reading.

[LltWc]

Do you have any recommendations for using inductive detection to reliably detect NFC cards? The STEVAL-D25R16B, can, sure, but its antenna is enormous and matched almost perfectly to the card antenna. I can't imagine many products in use are like that. Our device (using a ~20 x 20mm flexi antenna) can detect coin tags reliably, whereas the STEVAL shows a much small amplitude drop. My assumption is that NFC devices in production that are scanning passive tags (not doing contactless payment) would much more frequently be reading cards than coin tags or inlays.

[Ulysses HERNIOSUS]

Hi,

Not sure if I am completely following.

I think it comes all down to coupling. Similar size reader and tag antennas will couple well and lead to good inductive sensing. Different size antennas will couple less.

With your 20x20mm antenna: Which card/tag antenna sizes do you want to detect by inductive wake-up in which range?

BR, Ulysses

[LltWc]

That's what I've been assuming as well based on my measurements.

The difficult part is that we're using a large range of antenna sizes. For in-house testing, typically tokens with 20mm circular antennas. In the field, inlays with 10x17mm antennas or cards with 60.75x40.45mm antennas. Moving forward, we'll primarily be using inlays with 12x12mm antennas. Is there a way to have inductive sense work reliably for all of these?

Right now, we're just sending full-power NFC wake commands to detect if there is a tag nearby, which works for all sizes but of course wastes power.

[Ulysses HERNIOSUS]

Hi,

depending on your timeline also the upcoming ST25R300 may be an option for you. It will bring increased sensitivity for inductive wake-up.

BR, Ulysses