Don’t Let the Headphone Jack Ground-Return-Path Impedance
Threaten Compliance with Vista Windows Logo Program (WLP)
Crosstalk Requirements
By Kymberly Christman (Schmidt), Product Definer,
Maxim Integrated Products Inc., Sunnyvale, Calif.
The Windows Logo Program (WLP) imposes detailed requirements on the audio
performance of PCs and notebook computers using the Windows Vista operating system.
One of these audio performance requirements is crosstalk. This discussion highlights the
new design challenges engineers are faced with when designing a system to meet Vista
crosstalk requirements. A simple design solution is also proposed. Furthermore, although
this crosstalk discussion is targeted at WLP compliance standards, the cause and effect of
crosstalk is universal to all audio applications.
The crosstalk specification for Microsoft’s Windows Vista OS poses new challenges
when compared to historic audio performance requirements and test conditions. Rather
than enforcing crosstalk performance at 1 kHz and 10 kHz, WLP enforces crosstalk
performance across the full audio band (20 Hz to 15 kHz). Additionally, WLP specifies
crosstalk performance into a 32 Ω headphone load. This is a large contrast to the historic
10 kΩ test load condition.
In the 32 Ω load case, the influence of the output jack’s ground-return current can no
longer be neglected. The larger current flow aggravates the effect of the ground-returnpath impedance and poses a great threat to Vista compliance with respect to crosstalk.
One solution to minimizing the headphone ground-return path impedance is to place the
headphone socket directly at the output of the headphone amplifier. However, many
audio designers have very little influence in the placement of the audio sockets since the
placement is often dictated by the industrial design. An amplifier like the MAX9791 from
Maxim, though, can be configured to cancel the effect of the ground return path
impedance – allowing a system to pass Vista crosstalk requirements with minimal design
effort.
Crosstalk Basics
Crosstalk quantifies the amount of signal coupled from one channel to another channel in
a multichannel audio or video system. An ideal multi-channel signal path would have no
crosstalk between channels, but parasitics in the chip layout create some minimum level
of crosstalk. And once the IC is placed on a printed circuit board, additional parasitics
due to the board layout and material negatively impact the system crosstalk performance.
In a typical two-channel (i.e. stereo) system, stereo crosstalk performance is reported as
two separate conditions: (1) crosstalk from the left channel to the right channel and (2)
crosstalk from the right channel to the left channel. An audio crosstalk measurement from
left to right (Figure 1) is the ratio of the signal measured on the right channel over the
signal measured on the left channel, represented in dB. During this measurement, the
signal on the right channel is set to 0 V while the signal on the left channel (the reference
channel) is set to a fraction of the input signal and swept across the audio band. Vista
specifies a −20 dBFS (full scale) signal on the reference channel and requires crosstalk to
measure less than or equal to −50 dB between 20 Hz and 15 kHz, for both a 10 kΩ and
32 Ω load impedance.
Figure 1. Stereo crosstalk is reported as two separate conditions: crosstalk from the left
channel to the right channel, and crosstalk from the right channel to the left channel.
What Causes a Vista Crosstalk Failure?
The top contributor for a crosstalk failure is the layout, whether of the IC or the PCB.
Given the crosstalk limits defined in WLP 3.0, a crosstalk failure is rarely due to the
physical layout of the IC. The MAX9791A, for example, has a crosstalk specification of
< −64 dB into a 32 load, across the defined audio band. The main cause of crosstalk
failure is almost always the PCB layout: either capacitive coupling between the inputs, or
a shared resistive ground return at the output jack.
If a crosstalk measurement has a positive slope as frequency is swept from 20 Hz to 20
kHz, the system has capacitive coupling at the audio-amplifier inputs. Capacitive
coupling becomes apparent when a high-impedance drive is applied at the audioamplifier inputs, or if the inputs of a multi-channel device are routed too close to each
other. However, this capacitive coupling is not the focus of the discussions in this article.
If the crosstalk measurement is relatively flat across the audio band, but the system fails
the crosstalk specification (Figure 2), the typical cause is a shared resistive ground return.
The shared return can be avoided by ensuring minimal resistance in the return path for
the output jack’s ground connection. That resistance can be minimized by shorting out
any series resistance in the return path, by locating the audio amplifier close to the output
jack (to minimize the length of the ground return), and by minimizing contact resistance
to the sleeve of the output jack itself.
Figure 2. If the crosstalk measurement is relatively flat across the audio band, root cause
is a shared resistive ground return.
These three best practices seem relatively simple to implement; however, in reality, a
system engineer may be forced to route the headphone output a far distance from the
audio amplifier. This routing dominates the crosstalk measurement, resulting in decreased
audio fidelity.
Supporting Equations
A simplified circuit of the headphone signal path can illustrate the effect the ground
return resistance has on the crosstalk measurement. Given this simplified circuit,
equations can be derived to quantify this effect.
The headphone amplifier is connected to the headphone jack through a thin, 10-15 mil
trace. The headphone amplifier has some finite output impedance, the trace has a finite
resistance, and the jack has a finite contact resistance. Additionally, a 32 Ω load is
connected (with some finite resistance) between the headphone jack and the ground
sleeve. This ground sleeve is then connected to ground and routed back to the headphone
amplifier ground.
The headphone port model can be simplified by grouping many of the resistances
together (Figure 3). The headphone amplifier output impedance, trace resistance, and
contact resistance of the jack are grouped together to represent the source resistance, RS.
The resistance between the load and the sleeve, the sleeve contact resistance, and the
system ground return resistance are grouped together to represent the ground resistance,
RG.
Figure 3. The headphone port can be simplified by grouping resistances.
The diagram is simplified further into a ladder of resistors in Figure 4. Notice the
voltages across the headphone load resistors (RL) would be equal if the ground return
path resistance (RG) was not included in the circuit.
RS
VOUTL
VINL
RS
VINR
VOUTR
RL
RG
VGND
RL
VSLV
Figure 4. The headphone port can be further simplified to a ladder of resistances.
We can derive a set of equations to illustrate the crosstalk introduced to the system by the
ground return path resistance (RG). Note the source resistance of the left and right
channels is assumed to be equivalent. The load resistors of the left and right channels are
also assumed to be equivalent.
VOUTL
VINL
RL
RS
RL
RG RL RS
RL RS RG
where R'
VOUTR
VSLV
RL
RL
where VSLV
VOUTR
VOUTL
R'
RS
VINL
RG
RL
RG
RL
R'
RS
R'
RS
where RG << R L
VOUTR
VOUTL
RG
RL
RS
Or Crosstalk in dB = 20 log
RG
RL
RS
Assuming a typical source resistance of 5 Ω, the ground return impedance would need to
be limited to 115 mΩ to meet Vista’s 50 dB crosstalk specification (Figure 5). This is
further complicated by the fact that the impedance of the sleeve connection in the 3.5 mm
stereo jack can make up 30-90 mΩ alone. Given a 3.5 mm stereo jack with 60 mΩ of
sleeve contact resistance, the design is already limited to 55 mΩ of return-path
impedance.
Figure 5. Vista-compliant platforms are restricted to only 115m
impedance.
of ground return
Using 1 oz. copper, 55 mΩ of allowable ground-return-path impedance limits Vistacompliant platforms to only 110 square units. If board congestion and numerous vias in
the ground plane limit the ground return path to only 30 mils wide, the trace length will
be limited to 3.3 inches. (Note: Every square of a sheet of 1oz. copper has a resistance of
0.5 mΩ at 25°C.)
If the headphone jack ground is connected close to the audio device ground using a solid
ground plane, the return path resistance can be quite low. However, it is often necessary
to locate the headphone jack far from the audio device. In this case, acceptable crosstalk
performance becomes a challenge.
Silicon can Improve Performance
Although careful board layout can compensate for some crosstalk, the proper choice of
the amplifier can also make a big difference. For example, the MAX9791 includes Sense
and Com inputs that allow the amplifier to evaluate and correct for the difference
between the headphone jack ground and the device ground. That further reduces the
overall crosstalk of the system.
The Sense pin senses the voltage difference seen between the headphone amplifier
ground and the output jack ground. Any voltage difference is then divided down by the
amplifier’s gain and fed directly to the amplifier’s Com pin. The voltage present at the
Com input is subsequently amplified, inverted, and applied to the headphone amplifier
output. At the output jack, the voltage potential introduced by the resistive ground return
is canceled out. Figure 6 illustrates the effect of the Com and Sense pins with 28.5m of
ground return path resistance.
CROSSTALK vs. FREQUENCY
-40
VDD = 5V
VOUT = 30mVRMS
-50
COM and SENSE
DISABLED
RL = 32
CROSSTALK (dB)
RG = 28.5m
-60
MAX9791
RCH TO LCH
-70
-80
MAX9791
LCH TO RCH
-90
-100
10
100
1000
10000
100000
FREQUENCY (Hz)
Figure 6. The Com and Sense inputs on the MAX9791 reduce crosstalk performance by
eliminating effects of 28.5 m of ground return path resistance.
When using common-mode sense, connect the Com input through a resistor (RCOM) to the
quiet analog ground (GND) of the device (Figure 7). For optimum common-mode
rejection, use the same value resistors for RIN2 and RCOM. To improve DC CMRR, add a
capacitor equal to CIN2 in between the chip’s GND pin and RCOM.
Figure 7. The Sense and Com inputs were added at the headphone amplifier of the
MAX9791 speaker and headphone amplifier chip to help reduce system headphone
crosstalk. The MAX4409 offers the same approach in a stand-alone headphone amplifier
chip.