Passive Tag

What Is a Passive Tag?

A passive NFC tagNFC tagPassive unpowered device storing data, powered by reader's RF fieldView full → is an NFC transponder with no internal battery. It harvests all energy from the electromagnetic RF field generated by an NFC reader or active device. When the reader's antenna comes within range, its alternating magnetic field at 13.56 MHz induces a voltage in the tag's NFC antenna through inductive coupling, powering the NFC chip to process commands and transmit data.

How Passive Tags Work

1. Field detection: The antenna coil enters the reader's RF field and an AC voltage is induced.
2. Power rectification: On-chip rectifier converts AC to DC; a regulator stabilizes to 1.5-3.3V.
3. Clock recovery: The chip extracts a clock signal from the 13.56 MHz carrier.
4. Command reception: The reader modulates its field using ASK modulation to send commands.
5. Response transmission: The tag responds using load modulation — switching internal load to create detectable variations in the reader's field.

Advantages of Passive Tags

• Unlimited lifespan: No battery degradation. Limited only by EEPROM data retention (10+ years).
• Zero maintenance: No servicing, battery replacement, or recharging.
• Small size: Thin stickers (under 0.5 mm), glass capsules, flexible labels.
• Low cost: $0.03-$0.50 per NFC inlay depending on chip model.
• Environmental: No battery disposal concerns.

Passive Tag vs Active Device

Limitations

• Read rangeRead rangeMaximum communication distance between reader and tagView full →: Limited to 1-5 cm for ISO 14443; up to 1.5 m for ISO 15693 with industrial readers.
• Computation: Harvested power (~1-5 mW) limits cryptographic operation speed.
• No autonomous action: Cannot initiate communication or log data without a reader's field.

Every commercially available NFC tag chip (NTAG, MIFARE, ICODE, ST25T families) is designed for passive operation, distinguishing tags from active devices like smartphones.