AN3249
Application note
M24LR series internal capacitance considerations
for antenna tuning
1
Introduction
The datasheet of the M24LR series devices
(a)
specifies a typical internal tuning capacitance
value of 27.5 pF measured at 0.5 V
PEAK
. While this value is correct in production test
conditions, more considerations have to be taken into account for tuning the antenna in an
real application environment. This document describes the M24LR series tuning
capacitance variation as a function of input voltage, explains the M24LR series tuning
capacitance specification and gives a robust tuning capacitance target for the inductive loop
antenna design.
Table 1: Applicable products
Type
Dual Interface EEPROM
Applicable products
M24LR series
(1)
1. While the considerations introduced in this document applies to the entire M24LR series, the M24LR64-R
has been used hereafter for specific measurements purpose.
a. I
2
C/ISO15693 Dual Interface EEPROM IC
December 2013
DocID017760 Rev 2
1/7
www.st.com
Tuning capacitance variation as a function of input voltage
AN3249
2
Tuning capacitance variation as a function of input
voltage
Figure 1
shows an example measurement of the M24LR serial equivalent capacitance as a
function of the V
AC0-AC1
input RF voltage. The capacitance is measured between 0.2 V
PEAK
and 2 V
PEAK
at 13.56 MHz using a network analyzer.
Figure 1. M24LR64-R serial equivalent capacitance C
S
measurement example
The dotted line in
Figure 1
corresponds to the voltage level (0.5 V) at which the serial
capacitance (C
S
) of the M24LR series is measured in production and specified in the
datasheet (27.5 pF).
This voltage level has been chosen on the plateau of the capacitance curve to ensure the
best measurement repeatability.
3
3.1
M24LR series capacitance for antenna tuning
M24LR series equivalent schematic
Figure 2
shows the equivalent circuit of an M24LR mounted on a loop antenna, in the
presence of a magnetic field.
2/7
DocID017760 Rev 2
AN3249
M24LR series capacitance for antenna tuning
Figure 2. M24LR equivalent circuit
V
OC
represents the open circuit voltage available from the antenna. When a sinusoidal
magnetic field (H) flows through the M24LR antenna, the open circuit voltage amplitude
(V
OC
) can be determined using the following equation.
Equation 1
V
OC
=
N
•
S
• μ •
h
• ω
In this equation, N and S are respectively the number of turns and the surface of the M24LR
antenna,
μ
is the magnetic permeability of air, H is the magnetic field amplitude, and
ω
= 2 •
π
• f (f = 13.56 MHz).
R
A
and L
A
represent respectively the resistive part and inductive part of the antenna
impedance. R
S
and C
S
represent the serial equivalent model of the M24LR impedance.
The voltage amplitude (V
AC0-AC1
) available on the M24LR coil pads AC0 and AC1 can be
determined using the following equation.
Equation 2
Z
S
V
AC0
–
AC1
=
V
OC
-------------------
-
Z
A
+
Z
S
In this equation, Z
S
is the complex serial impedance of the M24LR. Z
S
= R
S
+ j • X
S
and
X
S
= –1/(C
S
•
ω).
As a image of the M24LR power consumption, R
S
is a function of V
AC0-AC1
. The variation of
C
S
as a function of V
AC0-AC1
is shown at low power by the curve in
Figure 1.
In the presence of a sinusoidal time varying magnetic field, the voltage V
AC0-AC1
available
across the M24LR antenna pads depends on Equations 1, 2 and the variation of C
S
and R
S
as a function of V
AC0-AC1
.
DocID017760 Rev 2
3/7
7
M24LR series capacitance for antenna tuning
AN3249
Optimizing antenna tuning consists in designing an antenna with the correct impedance for
the correct tuning capacitance.
Considering variations of C
S
according to V
AC0-AC1
, the correct capacitance value used as
reference for antenna design must be defined.
Please refer to AN2972 “Designing an antenna for the M24LRxx-R and M24LRxxE-R dual
interface I
2
C/RFID devices” for more details on antenna tuning criteria and methodology.
3.2
Correct M24LR series capacitance value for antenna tuning
According to
Equation 1,
V
OC
depends on the antenna dimensions, number of turns and the
magnetic field amplitude flowing into the antenna.
As a consequence, for a given antenna, V
OC
increases when the distance between the
M24LR antenna and the reader antenna decreases (because the magnetic field increases).
Figure 3
represents the M24LR input voltage variation as a function of V
OC
for an antenna
tuned on a M24LR serial capacitance value based on the curve shown in
Figure 1.
As
explained above, this case corresponds to a decreasing distance between M24LR antenna
and reader antenna; the longer the distance from the reader antenna, the closer the value of
V
OC
gets to 0 V .
Figure 3. M24LR RF voltage
As V
OC
increases, a sharp increase of V
AC0-AC1
occurs when the serial capacitance of the
M24LR reaches a value that satisfies the resonance condition
L
A
• C
S
•
ω
2
= 1. At resonance, V
AC0-AC1
= Q • V
OC
where Q = R
S
/(R
A +
R
S
) • 1/(R
S
• C
S
•
ω).
As an example, the Q value for the ANT1 reference design is between 30 and 35.
The maximum operating distance from the reader is defined by the maximum distance for
which the M24LR input RF voltage V
AC0-AC1
remains above 1.9 V
PEAK
which is the
minimum voltage for the M24LR to be properly energized and perform read and write
operations. Optimizing an antenna for an operating range consists in designing an antenna
allowing V
AC0-AC1
≥
1.9 V
PEAK
at the largest distance from reader.
4/7
DocID017760 Rev 2
AN3249
Conclusion
To satisfy this condition, antenna inductance must satisfy the resonance criteria
L
A
• C
S
•
ω
2
= 1, where C
S
is the M24LR serial capacitance measured at the minimum
operating voltage 1.9 V
PEAK
(as shown in
Figure 1,
where C
S
is close to 29 pF at
1.9 V
PEAK
).
To demonstrate this,
Figure 4
represents the M24LR RF Input voltage V
AC0-AC1
as a
function of the RF voltage V
OC
available from the antenna for different inductive loop
antennas tuned respectively for C
S1
< C
S2
, C
S2
= 29 pF and C
S3
> C
S2
.
The comparison graphics shown in
Figure 4
assume using the same antenna dimensions
and number of turns.
Figure 4. M24LR RF input voltage different antenna inductance
As shown in
Figure 4,
the minimum RF voltage V
OC
for which V
AC0-AC1
is above 1.9 V
PEAK
(and consequently maximum distance at which the M24LR can operate) is reached for an
antenna inductance L
A
designed for C
S
= 29 pF.
4
Conclusion
Taking into account this information, STMicroelectronics recommends designing antennas
for the M24LR series using C
S
= 29 pF as a reference value, instead of the value specified
in the datasheet which corresponds to production test conditions.
DocID017760 Rev 2
5/7
7
FPGA/CPLD
京公网安备 11010802033920号
Copyright © 2005-2026 EEWORLD.com.cn, Inc. All rights reserved
电子工程世界
