Datasheet

Common-Mode Sense
When the headphone jack is used as a line out to inter-
face between other equipment (notebooks, desktops,
and stereo receivers), potential differences between
the equipment grounds can create ground loops and
excessive ground-current flow. The MAX9722A’s INR+
and INL+ inputs are connected together to form a com-
mon-mode input that senses and corrects for the differ-
ence between the headphone return and device
ground (see Figure 8). Connect INR+ and INL+ through
a resistive voltage-divider between the headphone jack
return and SGND of the device. For optimum common-
mode rejection, use the same value resistors for R
1
and
R
F
, and R
2
and R
IN
. For the MAX9722B, R
IN
= 15kΩ
and R
F
= 30kΩ. Improve DC CMRR by adding a
capacitor between SGND and R
2
(see the Typical
Application Circuit). If ground sensing is not required,
connect INR+ and INL+ directly to SGND.
Common-Mode Noise Rejection
Figure 9 shows a theoretical connection between two
devices, for example, a notebook computer (transmit-
ter, on the left) and an amplifier (receiver, on the right),
MAX9722A/MAX9722B
5V, Differential Input, DirectDrive, 130mW
Stereo Headphone Amplifiers with Shutdown
______________________________________________________________________________________ 13
LEFT
AUDIO
INPUT
RIGHT
AUDIO
INPUT
SHDN
R
2
R
1
MAX9722A
Figure 8. Common-Mode Sense Input Eliminates Ground-Loop
Noise
V
NOISE
V
NOISE
0.1Ω
0.1Ω
0.1Ω
0.1Ω
V
REF_IN
= (V
NOISE
x 0.99)
V
IN
= V
AUDIO
+ (V
NOISE
x 0.98)
RESISTOR IS
INSERTED
BETWEEN THE
JACK SLEEVE
AND
GND = 9.8Ω
V
AUDIO
V
AUDIO
GND NOISE COMPONENT IN
OUTPUT = V
NOISE
/100
EXAMPLE CONNECTION:
IMPROVEMENT FROM
ADDING MAX9722 WITH
SERIES RESISTANCE
•
•
•
•
9.8Ω RESISTOR ADDS TO HP CROSSTALK, BUT DIFFERENTIAL
SENSING AT THE JACK SLEEVE CORRECTS FOR THIS (ONE CHANNEL
ONLY SHOWN).
CURRENT FLOW (IN SIGNAL CABLE SCREEN) DUE TO V
NOISE
IS GREATLY REDUCED.
NOISE COMPONENT IN THE RECEIVER OUTPUT IS REDUCED BY 34dB
OVER THE PREVIOUS EXAMPLE WITH THE VALUES SHOWN.
•
•
9.8Ω
0.10Ω RESISTANCE FROM CABLE SCREEN.
0.10Ω RESISTANCE DUE TO GND CABLING AT RECEIVER.
V
NOISE
REPRESENTS THE POTENTIAL DIFFERENCE BETWEEN
THE TWO GNDS.
V
REF_IN
= V
NOISE
/2
V
IN
= V
AUDIO
GND NOISE COMPONENT IN
OUTPUT = V
NOISE
/2
MAX9722A
Figure 9. Common-Mode Noise Rejection