This post entry provides a guide to designing antennas for the NTAG I2C plus. This article has been structured as follows:
The NTAG I2C plus is what we call a connected NFC tag. It combines a memory, a passive NFC interface and a contact I2C interface. Additionally, it has more features such as:
As such, it supports bidirectional communication between an NFC-enabled device and the host MCU and it is an ideal solution for Industrial applications, IoT nodes, meters, consumer electronics and accessories among others.
To enable the NFC interface, the chip needs to be connected to an antenna coil using the two dedicated antenna pins. How to design this coil is the main goal for today.
The NTAG I2C plus support package includes development kits, demo apps, sample code, application notes, and, the design files of the Class 4 PCB antenna, and the Class 6 Flex antenna, which are available for direct and free download from the website. These design files
Therefore, if you do not have any antenna size or shape constrains in your application, the easiest is to just copy & paste these reference antennas.
On the other hand, if you need to design your custom antenna, NXP also offers a coil design Excel sheet to help you. I will talk more about it along the article.
The NTAG I2C plus is an 8-pin package, with:
wo antenna pins
The NTAG I2C plus equivalent circuit can be represented with:
For the NTAG I2C plus, this capacitance is 50pF for both the 1k and 2k memory versions. Precisely, the chip capacitance is the most important factor for the antenna tuning.
The antenna coil itself is a resonant circuit with an input impedance. The electrical equivalent model of the antenna coil consists of:
The actual impedance value depends on:
When the NTAG I2C chip and the antenna coil are assembled, we can consider a parasitic resistance and capacitance generated by the connections between the chip and the antenna. This parasitic impedance depends on the assembly process used and the antenna material.
As a result, what we can observe in the schematic of the figure is that the NTAG I2C plus capacitance together with the parasitic connection capacitance and the antenna capacitance forms a resonance circuit with the inductance of the antenna
The self-resonance frequency of a system is given when the imaginary part of the circuit equivalent impedance is null, and the system is only purely resistive.
Considering the antenna loop inductance, the parallel equivalent capacitance and the parallel equivalent resistance of the tag, the resonance frequency and the quality factor of the tag can be calculated by these formulas.
The antenna design procedure for the NTAG I2C plus tags is:
As part of the ISO14443 standard, six PICC antenna classes are defined. Per each of the antenna class, the physical characteristics and dimensions are defined. For instance, Class 1 is the largest, with a size comparable to the size of a regular credit card, and Class 6, which is the smaller one. In addition, Class 3 to Class 6 define two antenna shapes: a rectangular and a circular one. However, tag manufacturers are not constrained to conform to any of these dimensions. Therefore, its use is optional and rather intended to improve interoperability. As such, you may consider using these antenna sizes as a reference for your designs.
The major parameter of the antenna coil is the inductance. This inductance can be estimated based on geometrical parameters and the material properties such as:
To avoid cumbersome formulas, NXP offers you an Excel-based coil calculation tool to estimate the inductance of rectangular and circular antennas. This tool uses some parameters related to the material used and the antenna dimensions. And with it, it estimates the antenna inductance for you. Typically, the coil design steps include:
The antenna characterization can be done using a network analyzer connected to the antenna pads, isolated from the rest of the circuit. For our case, a low-end solution, such as the miniVNA PRO is sufficient. This device is cheap compared with the high-end devices like Agilent but still, accurate enough for our needs.
As a remark, it is fundamental that this characterization is done with the antenna placed at its final mounting position, so that all environment effects, like metal plates or others, are considered.
We use a network analyzer to measure the system resonant frequency after connecting the NTAG I2C plus to the antenna coil. As I explained before, the self-resonant frequency of the tag is given when the system is purely resistive. Most likely, the actual resonant frequency will not be 13.56MHz as we would like, but some other value. If that is your case, calculate the system capacitance at the current resonant frequency based on the equation derived from the NFC tag equivalent circuit shown previously.
At this point:
With this data, we can use once again, this formula to calculate which is the resulting capacitance that would make our tag resonate to our target resonant frequency. Knowing the required total capacitance and the actual capacitance, we can calculate the extra capacitance missing. This is given by this formula:
Regarding the target resonant frequency, for single tag operation, a tuning slightly above 13.56 MHz would lead to maximum read-/write distance. However, due to manufacturing tolerances, a nominal frequency up to 14.5 MHz would still operate well.
Therefore, the last steps are:
If the resonant frequency measured is not the target one, repeat the process by fine tuning the capacitor value. If the frequency is higher than expected, you can increase the capacitor value. On the other hand, if the frequency is lower than expected, you can decrease it.
Based on a real lab exercise, this section illustrates the steps to adjust the tuning of an antenna for the NTAG I2C plus.
As described before, we need to start by characterizing our antenna coil. In this lab exercise, we have used a PCB antenna of 54 by 27 mm and, we have connected our miniVNA PRO to the antenna pads. The results that we have obtained from this measurement are that our PCB antenna has an inductance around 895 nH.
After characterizing the antenna coil:
In this case, it returns a resonant frequency near 24 MHz. Using the formula, we calculate that the tag capacitance at 24 MHz is almost 50 pF. Note that, the actual capacitance is basically the chip capacitance as the antenna and connection capacitance is usually not impacting significantly.
Obviously, a resonant frequency of 24MHz is way too high for a ISO14443 NFC tag like our NTAG I2C plus. Therefore, we need to add some capacitance to the system so that we can bring this resonant frequency down. As an example, for this lab exercise, we are adjusting the tag to around 13.6 MHz, intentionally a bit higher than the NFC operating frequency. With a target resonant frequency to 13.6MHz and an antenna inductance is around 895nH, the result is that the tag needs a total capacitance of around 153 pF. This means that we need to solder an extra capacitance of 100pF to bring down the resonant frequency. So we go to our component box, and select the closer commercial value (100pF).
As a last step, it is worth to measure how well adjusted is our system after adding the 100pF. We connect the miniVNA to the system including the IC, the antenna and the 100pF. Now, the results obtained are that the resonant frequency is 13.8 MHz. In our case, we consider this as good enough. However, you are always free to repeat this process as many times as needed until you obtain the accuracy that you need.
The antenna tuning steps for the NTAG I2C plus that we followed are:
As you can see, the antenna tuning process is quite straight forward. Basically, it is a matter of adjusting the capacitance of the tag until the operating frequency is the right one.
You can find more information about NFC in:
On 25 July 2018, a live session explaining this topic was delivered. You can watch the recording here: