APPLICATION NOTE
MODELING RETURN PATH IMPEDANCE EFFECTS
ON 32 OHM CROSSTALK PERFORMANCE
Overview
Current headphone port testing with 32 ohm loads presents new challenges compared to previous test requirements
and methods. The current Microsoft Windows Logo Program version 3 requires crosstalk testing across the entire 20Hz
to 20KHz bandwidth instead of simply at 1KHz or 10KHz. Unfortunately, we can no longer neglect the influence of return
path impedance related to cabling and connector impedance due to the high currents caused by low impedance loads.
Although most systems are more complex than the model presented, we can gain an understanding of the importance
of choosing good connectors and reducing overall return path impedance.
A brief overview of a headphone port
Let’s look at a simplified headphone port. (Figure 1 - Simplified Circuit1) The headphone amplifier (CODEC or discrete
amplifier) drives a trace to a coupling capacitor and another trace to a jack. The amplifier has output impedance, the
trace has some finite impedance, and so does the jack. The capacitor has frequency dependent impedance. To further
simplify our analysis, we will neglect the complex portion (inductance and capacitance) of all of these circuit elements
except for the capacitor and focus only on the real component (resistance) of their impedance. Fortunately, this will not
significantly alter our results.
C1
J1
220uF
1
+
-
V1
V SRC
C2
2
3
220uF
TP1
CHANNEL A
ACTIVE
R1
32
TP2
SIGNAL RETURN
R2
32
REFERENCE
TP3
CHANNEL B
IDLE
Figure 1 - Simplified Circuit
Test equipment is connected to the system under test by plugging a plug and cable into the jack. A 32 ohm load is
applied to the left and right channels and the test equipment measures and compares the voltage across each resistor.
One channel is driven with a tone, while the other channel is driven with silence. As with the system side, there are
impedances associated with the plug, cabling, and test equipment connectors.
Reducing to an equivalent circuit for simulation
To reduce the system to something easy to simulate, we’ll lump many of the resistances together. The amplifier’s output
resistance, trace resistance, jack-plug contact resistance, cable resistance, and test equipment connector resistance
will be represented by a lumped resistor Rs placed at the output of the amplifier. The system ground impedance, sleeve
contact resistance, cable resistance, and resistance between the load common point and the return pin will be
represented by Rg placed at the system ground point.
IDT™
1
MODELING RETURN PATH IMPEDANCE EFFECTS ON 32 OHM CROSSTALK PERFORMANCE
REV A 091806
MODELING RETURN PATH IMPEDANCE EFFECTS ON 32 OHM CROSSTALK PERFORMANCE
RS1
C1
PC AUDIO CODECS
220uF
TP1
CHANNEL A
RG
+
-
ACTIVE
V1
V SRC
R1
32
RS2
C2
220uF
TP2
SIGNAL RETURN
R2
32
REFERENCE
TP3
CHANNEL B
IDLE
Figure 2 - Circuit with Lumped elements
Finally, remove the amplifier symbols and redraw in a slightly more familiar way.
TP1
CHANNEL A
ACTIVE
TP2
SIGNAL RETURN
RS1
C1
220uF
R1
32
R2
32
REFERENCE
TP3
CHANNEL B
+
-
V1
V SRC
IDLE
RG
C2
220uF
RS2
Figure 3 - redrawn for simulation
Notice that the circuit is basically a ladder of resistors and the voltages across the load resistors (R1 and R2 in the
figures above) would be equal if not for the presence of the return path resistance (RG). This model is useful for
studying the crosstalk versus frequency. An example sweep is shown below (Figure 4 - Crosstalk vs. Frequency4) for a
total source resistance of 10 ohms, a load impedance of 32 ohms, and a return path impedance of 0.05 ohms.
IDT™
2
MODELING RETURN PATH IMPEDANCE EFFECTS ON 32 OHM CROSSTALK PERFORMANCE
REV A 091806
MODELING RETURN PATH IMPEDANCE EFFECTS ON 32 OHM CROSSTALK PERFORMANCE
PC AUDIO CODECS
Figure 4 - Crosstalk vs. Frequency
Reducing for calculation at a single frequency
As seen by the above graph, the crosstalk is fairly constant across frequency and actually improves at the low end as
the amplitude response decreases due to the AC coupling caps. So, at any one frequency, we can add the ac coupling
capacitors to RS and further reduce the circuit and aid calculation. The final equivalent circuit is shown below.
TP1
CHANNEL A
ACTIVE
TP2
SIGNAL RETURN
RSL
R1
32
R2
32
REFERENCE
TP3
CHANNEL B
+
-
V1
V SRC
IDLE
RG
RSR
Figure 5 - Final Simplified Circuit
IDT™
3
MODELING RETURN PATH IMPEDANCE EFFECTS ON 32 OHM CROSSTALK PERFORMANCE
REV A 091806
MODELING RETURN PATH IMPEDANCE EFFECTS ON 32 OHM CROSSTALK PERFORMANCE
PC AUDIO CODECS
Calculating the crosstalk
Looking at the final circuit (Figure 5 - Final Simplified Circuit5), we can derive a set of equations to describe the
crosstalk. The test equipment will compare the voltage measured across R2 above to the voltage measured across R1.
I’ll refer to the voltage across R1 as VA , the voltage across R2 as VB, and the voltage between the common point
between R1 and R2 and the signal ground at the system under test as VG. VG is the test equipment’s reference.
RSL (left) and RSR (right) are assumed to be equivalent and are represented as RS.
R1 and R2 are also assumed to be equivalent and are represented as RL.
VA = VSRC ×
RL
 RG (RL + RS ) 
RS + RL + 

 RG + RL + RS 


 RG (RL + RS ) 




RL  

 RG + RL + RS 

VB = VSRC ×  −
×
(
)
RL
+
RS
RG
RL
+
RS




 RL + RS + 

 RG + RL + RS  

VB
− RG
=
VA RG + ( RL + RS )
Since RG << RL, then we may simplify to:
VB
− RG
=
VA ( RL + RS )
The crosstalk (in dB) will be:
 RG 
20 Log 

 RL + RS 
For the above simulation example, RG = 0.05, RL = 32, and RS = 10. The simulated value at 1KHz is -58.497dB, and
the equation above yields -58.486dB. Although the values chosen were crude estimates of actual source and return
impedances, these two results compare well to the -57dB to -59dB results measured on actual systems.
What effect do jack contact resistance and cable resistance have on crosstalk?
Usually, the resistance in the return path itself is very low, because we often use ground planes and short distances
between jacks and the audio devices. However, it is often necessary to locate some jacks, such as the front panel jacks
in a desktop PC, far from the audio device. Minimizing the cable and connector impedance is critical. Even so, the
largest contributor to return path impedance is the contact impedance of the sleeve connection in the 3.5mm stereo
jack. Contact impedance for typical devices ranges from 30 milliohms to 100 milliohms maximum. Unfortunately, even a
few tens of milliohms can cause a system to exhibit more than -70dB of crosstalk (a common goal) as indicated by the
graph below.
IDT™
4
MODELING RETURN PATH IMPEDANCE EFFECTS ON 32 OHM CROSSTALK PERFORMANCE
REV A 091806
MODELING RETURN PATH IMPEDANCE EFFECTS ON 32 OHM CROSSTALK PERFORMANCE
PC AUDIO CODECS
Crosstalk vs. RG
-80
-75
-70
dB -65
-60
-55
-50
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0.1
Ohms
Figure 6 - Effect of return path impedance. RS = 10 ohms
Does the source impedance have an effect?
Naturally, but this effect is generally small unless the output impedance is large compared to the load impedance, which
is not recommended. A graph of crosstalk versus source impedance for 32 ohm loads shows the possible benefits of
large output impedance, but most of the output power will never reach the load and will be dissipated as heat rather
than sound.
Crosstalk vs Source impedance
-70
-68
-66
dB
-64
-62
-60
-58
-56
0
10
20
30
40
50
60
70
80
90
100
Ohms
Figure 7 - Effect of Source impedance. RG = 0.05 ohms
IDT™
5
MODELING RETURN PATH IMPEDANCE EFFECTS ON 32 OHM CROSSTALK PERFORMANCE
REV A 091806
MODELING RETURN PATH IMPEDANCE EFFECTS ON 32 OHM CROSSTALK PERFORMANCE
PC AUDIO CODECS
Conclusion
The return path impedance, and not the inherent crosstalk of the audio device, is often the limiting factor for low
impedance (headphone) crosstalk performance on modern systems. Careful attention to cabling and connectors is
required to meet the stringent requirements currently imposed on the PC industry. Specifically, choose 3.55mm jacks
that guarantee less than 10 milliohms of contact impedance and ensure that any additional cabling and connector
impedance for front panel connectors add less than 30 milliohms to the return path impedance.
IDT™
6
MODELING RETURN PATH IMPEDANCE EFFECTS ON 32 OHM CROSSTALK PERFORMANCE
REV A 091806
MODELING RETURN PATH IMPEDANCE EFFECTS ON 32 OHM CROSSTALK PERFORMANCE
PC AUDIO CODECS
Innovate with IDT and accelerate your future networks. Contact:
www.IDT.com
For Sales
800-345-7015
408-284-8200
Fax: 408-284-2775
Corporate Headquarters
Asia Pacific and Japan
Europe
Integrated Device Technology, Inc.
6024 Silver Creek Valley Road
Integrated Device Technology
Singapore (1997) Pte. Ltd.
IDT Europe, Limited
321 Kingston Road
San Jose, CA 95138
United States
800 345 7015
Reg. No. 199707558G
435 Orchard Road
#20-03 Wisma Atria
Leatherhead, Surrey
KT22 7TU
England
+408 284 8200 (outside U.S.)
Singapore 238877
+65 6 887 5505
+44 1372 363 339
© 2006 Integrated Device Technology, Inc. All rights reserved. Product specifications subject to change without notice. IDT and the IDT logo are trademarks of Integrated Device
Technology, Inc. Accelerated Thinking is a service mark of Integrated Device Technology, Inc. All other brands, product names and marks are or may be trademarks or registered
trademarks used to identify products or services of their respective owners.
Printed in USA