december 2013 docid025583 rev 1 1/8 AN4409 application note using the same antenna for m24lrxxe-r and m24srxx-y dual interface eeprom families introduction the purpose of this document is to explain why the different tuning capacitance allows customers to use the same 13.56 mhz antenna design for m24lrxxe-r iso15693 and m24srxx-y iso14443 dynamic nfc/rfid tag ic families. this document clarifies also the reference value of the tuning capacitance for m24lrxxe-r and m24srxx-y antenna design. table 1 lists the products concerned by this application note. table 1. applicable products type applicable products nfc/rfid tag ics m24lr, m24sr series www.st.com
antenna tuning frequency and performance AN4409 2/8 docid025583 rev 1 1 antenna tuning frequency and performance m24lrxxe-r and m24srxx-y ics are passive rfid memories powered by the rf magnetic field generated by the reader. tag to reader communication is achieved by mean of m24srxx-r and m24srxx-y impedance modulation mechanism, called load modulation or backscattering. maximizing the tag operating distance requires maximizing the powering distance but also the tag to reader link. to achieve these requirements, stmicroelectronics recommends a tuning frequency: ? 13.56mhz and 14mhz for rfid tags based on m24lrxxe-r family, ? 14mhz and 14.4mhz for rfid tags based on m24srxx-r family.
docid025583 rev 1 3/8 AN4409 tuning capacitance dependency for using the same antenna design 8 2 tuning capacitance dependency for using the same antenna design figure 1 here after shows the equivalent circuit of an m24lrxxe-r or an m24srxx-r mounted on a loop antenna in the presence of a sinusoidal magnetic field. figure 1. m24lrxxe-r or m24srxx-r tag equivalent circuit 0 6 9 9 2 & 5 $ / $ 5 6 & 6 $ q w h q q d 0 / 5 ; ; ( 5 r u 0 6 5 ; ; < $ & $ & 9 $ & $ & v oc represents the open circuit voltage available from the antenna. it depends on the magnetic field strength, the antenna size and number of turns. the tag antenna impedance is given by z a = r a + j.l a . figure 1 is defined by the following equation 1 : equation 1 let?s consider now, the tuning frequency of an m24lrxxe-r based tag (f tun_m24lr ) and the tuning frequency of an m24srxx-y based tag (f tun_m24sr ) when using the same antenna. they are respectively given by equation 2 and equation 3: f tun 1 2 l a c s ------------------------------- =
tuning capacitance dependency for using the same antenna design AN4409 4/8 docid025583 rev 1 equation 2 equation 3 dividing equation 2 by equation 3 leads to: equation 4 or, after simplification, to equation 5 : equation 5 respecting the tuning frequencies defined in section 1) leads to the following relationship: the m24srxx-y and m24lrxxe-r internal tuning capacitance has been designed following this rule offering to customers a simple way to use the m24lrxxe-r or m24srxx-y dual interface eeproms in their application without changing their antenna design. f tunf m24lr 1 2 l a c s m24lr ? f tun m24sr 1 2 l a c s m24sr ? f tun m24lr f tun m24sr -------------------- - 2 l a c s m24sr ? 2 l a c s m24lr ? c s m24sr c s m24lr ------------------- == c s m24sr c s m24lr ------------------- f tun m24lr f tun m24sr -------------------- - ?? ?? ?? 2 = c s m24sr c s m24lr ------------------- 13.8mhz 14.2mhz ------------------------ - ?? ?? 2 0.94 ?
docid025583 rev 1 5/8 AN4409 tuning capacitance specification vs tuning reference value 8 0 6 9 & 6 9 $ & $ & s z &zu]v ^z s z &zu]v s ??z ? ??z ? 3 tuning capacitance specification vs tuning reference value figure 2 shows the serial equivalent tuning capacitance typical variation of either the m24lrxxe-r or m24srxx-y according to the v ac0-ac1 rf input voltage: figure 2. typical tuning capacitance variation of m24lrxxe-r and m24srxx-y v rf_min corresponds to the minimum rf voltage between ac0 and ac1 rf inputs of the rfid memory chips. the corresponding tuning capacitance c s_vrf_min must be taken as reference for antenna design: in other words, customers have to design antennas which inductance satisfies the following relationship, showed by equation 6: equation 6 vdatasheet corresponds to the voltage level used for better correlation with measurements in production. f tun m24sr 1 2 l a c s vrf min ------------------------------------------- l a 1 c s vrf min
2 -------------------------------- ==
tuning capacitance specification vs tuning reference value AN4409 6/8 docid025583 rev 1 table 2 here after summarizes the m24lrxxe-r and m24srxx-y tuning parameters for more information about the target tuning capacitance for antenna design, please refer to the application note an3942 ?m24lr series internal capacitance considerations for antenna tuning? that discusses this point in details for the m24lr series. table 2. tuning parameters for m24lrxxe-r and m24srxx-r m24lrxxe-r m24srxx-r antenna tuning frequency 13.56mhz-14mhz 14mhz-14.4mhz c datasheet 27.5pf 25pf c s_vrf_min 29pf 27.3pf
docid025583 rev 1 7/8 AN4409 revision history 8 4 revision history table 3. document revision history date revision changes 19-dec-2013 1 initial release.
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