AN555
Optional RF shunt capacitors C5 and C6 may be placed on the left and right audio traces at the headphone
amplifier output to reduce the level of digital noise passed to the antenna. The recommended value is 100 pF or
greater, however, the designer should confirm that the headphone amplifier is capable of driving the selected shunt
capacitance.
The schematic example in Figure 15 uses the National Semiconductor LM4910 headphone amplifier. Passive
components R1 R4 and C7 C8 are required for the LM4910 headphone amplifier as described in the LM4910 data
sheet. The gain of the right and left amplifiers is R3/R1 and R4/R2, respectively. These gains can be adjusted by
changing the values of resistors R3 and R4. As a general guide, gain between 0.6 and 1.0 is recommended for the
headphone amplifier, depending on the gain of the headphone elements. Capacitors C7 and C8 are ac-coupling
capacitors required for the LM4910 interface. These capacitors, in conjunction with resistors R1 and R2, create a
high-pass filter that sets the audio amplifier's lower frequency limit. The high-pass corner frequencies for the right
and left amplifiers are:
fCRIGHT
=
-----------------1------------------
2 R1 C7
fCLEFT
=
-----------------1------------------
2 R2 C8
Equation 2.
With the specified BOM components, the corner frequency of the headphone amplifier is approximately 20 Hz.
Capacitor C1 is the supply bypass capacitor for the audio amplifier. The LM4910 can also be shut down by
applying a logic low voltage to the number 3 pin. The maximum logic low level is 0.4 V and the minimum logic high
level is 1.5 V.
The bill of materials for the typical application schematic shown in Figure 15 is provided in Table 14. Note that
manufacturer is not critical for resistors and capacitors.
4.3. Headphone Antenna Bill of Materials
Table 14. Headphone Antenna Bill of Materials
Designator
LMATCH
C4
D1, D2, D3
U3
R1, R2, R3, R4
C7, C8
C5, C6
R5, R6
F1, F2
C1
R7
Description
IND, 0603, SM, 270 nH, MURATA, LQW18ANR27J00D
AC coupling cap, SM, 0402, X7R, 100 pF
IC, SM, ESD DIODE, SOT23-3, California Micro Devices, CM1210-01ST
IC, SM, HEADPHONE AMP, National Semiconductor, LM4910MA
RES, SM, 0603, 20 k
CAP, SM, 0603, 0.39 UF, X7R
CAP, SM, 0402, C0G, 100 pF
RES, SM, 0603, 100 k
FERRITE BEAD, SM, 0603, 2.5 k, Murata, BLM18BD252SN1D
CAP, SM, 0402, X7R, 0.1 µF
RES, SM, 0402, 10 k
26
Rev. 0.2