Rocky100 major improvements: Sensitivity (Part I)

Sensitivity is probably the most important unique feature of an RFID chip.

In standard, only identification RFID tags the sensitivity is the lowest power you must provide to the chip for it to wake up and reply with its unique ID number.

Note that you want to have a feature that is dependent on the chip, not any other external part. To calculate the sensitivity you perform measurements with the chip alone. You directly input power. Using an antenna would alter the values since the antenna will have a gain and you do not want to add that up.

For your understanding, the RFID communication is dependent on two links:

  • Forward link – reader to tag. In this link two things need to happen:
    • Power transfer. Enough power needs to be transferred to the chip so that it can wake up, process data and prepare the reply
    • Communication – The communication must be strong enough for the chip to be able to understand the message.
  • Backward link – tag to reader. In this link only the communication is necessary since the RFID reader is already powered.

In general, for passive RFID tags the limitation is the power transfer in the forward link.

So far, so good. For only identification tags, the sensitivity of the chip tells you the minimum amount of energy that needs to go into the chip for it to reply.

However, this gets more complicated when you are thinking about RFID chips with POWER OUTPUT. We are talking about RFID chips that harvest energy, reply with a unique ID number but they can also provide power to external devices.

That’s how battery free RFID sensors and actuators are created. The RFID chip shares part of the energy with the external device for it to take measurements or perform an action. Everything is triggered from the RFID reader and the RFID chip is able to reply with the ID plus extra information coming from the external device.

What would the sensitivity of that RFID chip look like? How do you interpret that value?

Note that the power that needs to be input to the chip to reply with the ID is a lot lower than the power required to reply with the ID plus sensor data. To get sensor data, the RFID chip needs to harvest enough energy to power up the external sensor.

We at Farsens believe that RFID chips should have multiple values depending on the what for is the tag design for. This helps users understand and compare chips to develop their solutions. Sensitivity being defined as the lowest energy needed to reply to the reader with the desired value, different values can be shared such as:

  • Sensitivity for just ID in passive
  • Sensitivity for ID + A sensor
  • Sensitivity for ID + B sensor
  • Sensitivity for ID + A actuator

You would never be able to put all sensors in the market – nor it makes sense to do it. All of them share the same basic requirement: power. The more power you need for the sensor, the lower the sensitivity value of the chip plus sensor is going to be.

In our opinion, the information in the graph below makes a lot more sense when talking about RFID chip sensitivity – when power output is available:

Rocky100 major improvements: Sensitivity depending on conditions

Note that we used a value for BAP mode too. In this mode, both the chip and any external device are powered from a battery so the sensitivity is not dependent on the power consumption but on the communication link.

The limitation here is the ability of both the RFID chip and the RFID reader to work with low energy signals:

  • Forward link. In the forward link communication, the receiver is the RFID tag. The quality of the receiving circuit in the chip will define the sensitivity value for the BAP sensitivity. Basically, it all comes down to how soft signals the receiving circuit is able to process. The softer the signals it can process, the longer the communication range.
  • Backward link. In the backward link communication, the receiver is the RFID reader. The quality of the receiving circuit in the reader will define the sensitivity. Since this device is powered and can use more power for its electronics, the sensitivity is a lot higher than that of the chip.

That being said, note that passive RFID works with backscattering. The RFID tag does not transmit data but rather “bounces” it. This means the signal level of the transmission is exactly the level of the reception.

We will continue this explanation in our next post.

Stay tunned!