EM4095
Appendix
Equations
Peak to peak voltage on antenna is defined by following
equation:
Antenna resonant frequency f0:
IANT
πf0C0
VANTpp
=
(6)
1
f0 =
(1)
2π LAC0
To ensure correct operation of the AM demodulation
chain, the AC peak to peak voltage on DEMOD_IN pin
(VDMOD_INpp) has to be inside common mode range. Once
peak to peak voltage on antenna is known the capacitor
divider division factor can be calculated:
Where C0 is resonant capacitor composed of CRES, CDV1
and CDV2
:
C
DV1∗CDV 2
C0 = CRES
+
(2)
C
DV1 + CDV 2
CDV1
VDMOD _ INpp = VANTpp
(7)
C
DV1 + CDV 2
Usually antenna coil is specified by its inductance (LA) and
Q factor (QA). Serial resistance of antenna is defined by
following equation:
Power dissipation is composed of power dissipated on
ANT drivers and internal power consumption:
2πf0 LA
P = 2⋅ IRMS 2 ⋅ RAD + IDDon
V
(
DD −VSS
(7)
RANT
=
(3)
)
QA
Temperature increase of die due to power dissipation is:
The equations which follow are valid for bridge
configuration as defined on Figures 1, 2 and 3. For figures
1 and 2 RSER has to be considered 0.
∆T = P ⋅ RTh
(8)
The AC current amplitude at resonant frequency is
defined as follows:
Where RTh is Package thermal resistor.
4
Vdd −Vss
π RANT + RSER + 2RAD
IANT
=
(4)
RMS antenna current (important for power dissipation
calculation):
IANT
IRMS
=
(5)
2
EM Microelectronic-Marin SA cannot assume responsibility for use of any circuitry described other than circuitry
entirely embodied in an EM Microelectronic-Marin SA product. EM Microelectronic-Marin SA reserves the right to
change the circuitry and specifications without notice at any time. You are strongly urged to ensure that the
information given has not been superseded by a more up-to-date version.
© EM Microelectronic-Marin SA, 07/02, Rev. Ê/490
10
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Copyright 2002, EM Microelectronic-Marin SA
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