8
(a) A parallel equivalent circuit of an RFID IC input impedance
where (b) is a series equivalent circuit of the chip input
impedance.
R C
Rseries
Cseries
(b) (a)
70 RFID Handbook: Applications, Technology, Security, and Privacy
The ?¬?nal chip impedance seen by the antenna is also affected by the technique used to
attach the RFID IC to the tag. Generally, there are two different types of attachments
employed. When the IC is in a ?¬‚ip-chip package (which is the industry standard technique
for low-cost packaging) as shown in Figure 4.9, the RFID ICs can be directly attached to the
antenna. The RFID ICs may also be obtained as a ??????strap??™??™ where the IC is connected to two
mounting pads with a thin superstrate as shown in Figure 4.10.
Typically a resistance R of around 1300 V in parallel with a 1.1 pF capacitor C??”which is
that quoted for an Alien Class I Gen I RFID IC fabricated with CMOS technology and at the
threshold of operation of the IC (Alien Technologies, 2005)??”resulting in a series equivalent
circuit impedance of 18.95??“155.8j V can be expected from an RFID IC strap. Generally, it is
good practice before designing an antenna to measure the input impedance of the chip at
various operating frequencies using a network analyzer. Such a measurement method is
outlined in detail in Eunni (2004).
Maximum power transfer requirements dictate that the antenna impedance should be a
conjugate match to ensure the greatest possible performance from the RFID label (measuring
the performance of an RFID label is discussed in Section 4.
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