Texas Instruments | Reducing distortion from CMOS analog switches | Application notes | Texas Instruments Reducing distortion from CMOS analog switches Application notes

Texas Instruments Reducing distortion from CMOS analog switches Application notes
Analog Applications Journal
Industrial
Reducing distortion from CMOS
analog switches
By John Caldwell
Analog Applications Engineer
Figure 1. CMOS transmission gate consisting
of NMOS and PMOS transistors
CMOS analog switches have become ubiquitous on the
inputs and outputs of many electronic systems. They may
be used to select between multiple input channels on data
acquisition systems or to disable outputs during power-up
or power-down events. In fact, analog switches have
become so common that their operation is often taken for
granted. But analog designers should be aware that semiconductor switches exhibit behavior quite unlike their
mechanical cousins. For example, the resistance of a
CMOS switch in the closed position, referred to as the onresistance or RON, changes depending on the input voltage. This behavior is usually undesirable and can
significantly distort the input signal in some applications.
To understand why CMOS switches behave in this manner, it is necessary to understand their basic construction
and operation. A typical solid-state analog switch consists
of two MOSFETs of opposite channel polarity and configured as a transmission gate as shown in Figure 1. The con–
trol voltages (C and C ) at the FET gates are dc voltages of
opposite polarity. The switch is closed when the gate of
the NMOS transistor is high and the gate of the PMOS
transistor is low. Positive input voltages drive the VGS of
the PMOS more negative, decreasing the PMOS on-­
resistance. Therefore, the PMOS is the dominant current
pathway for positive voltages. Conversely, negative voltages applied to the input terminal increase the gate-tosource voltage, VGS, of the NMOS FET, decreasing its
on-resistance and allowing current to flow through the
NMOS pathway.
The basic switch architecture allows for both positive
and negative voltages to be passed, but also causes the
overall resistance of the switch to change with the input
signal. Figure 2 is a plot of the on-resistance of the
TS12A12511 switch versus the signal voltage range[1]. An
RON “flatness” parameter may be included in the datasheet
specification table to quantify the maximum deviation in
the switch on-resistance over the signal range. For example, the RON flatness specification for the TS12A12511 is
1.6 W (typical).
A basic analog output circuit incorporating a CMOS
switch is illustrated in Figure 3. Here the switch is used to
disconnect the load from the output of an operational
amplifier (op amp). Such applications of CMOS switches
are very common in audio applications to suppress clicks
and pops during the power-up or down of preceding
circuitry.
Texas Instruments
C
G
NMOS
S
D
V–
Output
Input
V+
S
PMOS
D
G
C
Figure 2. On-resistance variation of the
TS12A12511 and example of RON flatness
4.8
On-Resistance (Ω)
4.6
4.4
4.2
RON
Flatness
4
3.8
3.6
3.4
With ±5 V
Supplies
3.2
3
–6
–4
–2
0
2
Input/Output Voltage (V)
4
6
Figure 3. Signal distortion in a typical CMOS
switch application
OPA172
–
VOUT
S1
+
+
VIN
4
1 2 TS5A22362
RL
AAJ 1Q 2015
Analog Applications Journal
Industrial
The switch on-resistance forms a voltage divider with
the load resistance RL and the output voltage is:
VOUT = VIN
RL
RON(S1) + R L
=
VIN
RON(S1)
RL
Table 1: Approximate amplitude of fundamental and distortion
harmonics for the example calculation
(1)
RON(S1) ( VIN ) = ∆R × VIN + RO
(2)
In Equation 2, DR represents the change in switch onresistance with input voltage and RO is the resistance for
an input signal of 0 V. In reality, the relationship between
RON and VIN is more complex, but assuming a linear relationship simplifies the analysis while still revealing the
­distortion mechanism.
Inserting Equation 2 for on-resistance back into
Equation 1 for output voltage gives a new equation:
VIN
R
∆R
VIN + O + 1
RL
RL
(3)
A2
sin( 2πft) A
− 2 sin( 2πft)2 +
sin( 2πft)3
B
B
B3
B4
sin( 2πft)4 +
A4
B5
 A3 
3π 

 4  sin  8 πf + 
2
 8B 
A4
16B5
sin (10πf )
R0
+1
RL
(7)
∆R
RL
(8)
Examining the equation for A, it can be seen that if DR = 0,
the harmonic terms will be eliminated. Although this metric is constantly being improved in analog switches, the
on-resistance is never completely independent of input
voltage. An alternate, and more common solution, is to
select a load resistance value that is much larger than the
variations in the on-resistance. This solution is commonly
used on analog inputs, where RL is the input impedance of
data acquisition circuitry and is typically very large.
Unfortunately, other applications that use analog
switches do not have the luxury of ­specifying the load
impedance. An example is switching the outputs to highfidelity headphones. Furthermore, the distortion caused
by even minute variations in switch on-resistance represents a surprising amount of distortion. The total harmonic distortion and noise (THD+N) of the circuit in
Figure 3 was measured with a 2-VPP signal and load resistances of 100 kW and 600 W. According to the TS5A22362
analog-switch datasheet, the on-resistance at 0 V (room
temperature) is about 0.37 W. The on-resistance will vary
approximately 0.115 W over the range of the 2-VPP input
signal.
(5)
(6)
sin( 2πft)5 …
Using the power reduction rules for trigonometric functions
and simplifying the equation, the individual terms for each
harmonic can be grouped together as shown in Table 1.
The Maclaurin series was abbreviated to five terms so the
amplitude for the harmonics are approximations.
Although the on-resistance of a CMOS switch is almost
never linearly related to the input voltage, this example
provides some useful rules for reducing the distortion
from analog switches. Looking at the equations for the
individual harmonics, a reduction in distortion requires
that either the value of A must be very small, or B must be
Texas Instruments
4th Harmonic
A=
Now a sine wave is inserted as the input signal with
x = sin(2pft):
A3
 A2
5A 4 
 3+
 sin(6πf + π)
16B5 
 4B
For B to be large, RO must be much greater than RL. Now
the majority of the signal voltage is dropped across the
switch, rather than the load resistor. The net effect is that
the output signal is attenuated.
In most systems, it is more practical to reduce the value
of A:
R
∆R
and B = 0 + 1,
RL
RL
1
A
A2 3 A3 4 A4 5
x − 2 x2 +
x − 4x +
x …
B
B
B3
B
B5
−
3rd Harmonic
B=
x
(4)
Ax + B
To show the introduction of distortion, this more generic
equation can be written instead as its equivalent Maclaurin
series (shown here to 5 terms):
f [sin( 2πft)] =
 A
π
A3 

 2 + 4  sin  4 πf + 
2
2B 
 2B
very large. The latter option is very un-attractive in most
applications. Recalling the equation for B:
then VOUT = f ( x ) =
f (x) =
2nd Harmonic
5th Harmonic
For simplicity, a generic form of Equation 3 can be generated by substituting the constants A and B for terms in
the above equation:
Let x = VIN , A =
 1 3A 2 5 A 4 
+
 +
 sin( 2πft)
 B 4 B 3 8 B5 
+1
In reality, the value of RON(S1) is not a constant, but is a
function of VIN. As an example, assume that RON(S1) is a
linear function of the input voltage:
VOUT =
Fundamental
5
AAJ 1Q 2015
Analog Applications Journal
Industrial
FFT Output (dBr)
THD+N (%)
The measured THD+N over frequency is given
Figure 4: THD+N measurement of the circuit in Figure 3
in Figure 4 for two load impedances. With the
100-kW load impedance, the THD+N is
1
extremely low. In this case, the measurement is
1-VPK signal level
determined by the noise floor of the instru80-kHz measurement bandwidth
ment, roughly 0.0005%. However, decreasing
0.1
the load impedance to 600 W increases the distortion by an order of magnitude to 0.005%.
This level of distortion may not be acceptable
0.01
in many high-precision analog systems.
600-Ω Load Impedance
The distortion contribution from the switch
is constant over frequency because the voltage0.001
drop across the switch does not change over
100-kΩ Load Impedance
the measured bandwidth.
An FFT of the output signal at 1 kHz into a
0.0001
600-W load (Figure 5) shows that the 2nd har100
1k
10 k
monic is dominant, but spurs are visible above
Frequency (Hz)
the noise floor up to the 5th harmonic. The
­harmonics are due to the RON variations of
the switch.
Conceivably, enclosing the switch inside the
Figure 5. Spectrum of a 1-VPK, 1-kHz sine wave at the
feedback loop of an amplifier allows for the
output of the circuit in Figure 3
additional distortion to be corrected, but this is
not as simple as it may seem. The amplifier’s
feedback loop must still be closed when the
0
switch is open, otherwise the amplifier output
–20
would saturate to one of the power supply rails.
Closing the switch while the amplifier output is
–40
saturated could cause an undesirable transient
–60
voltage at the load.
–80
One solution to this problem is shown in
Figure 6. In this circuit topology, two switches
–100
are used. One switch, S1, is the signal path for
–120
the load. The second switch, S2, allows the op
–140
amp feedback loop to be closed around the first
switch. S2 contributes negligible additional dis–160
tortion in the system because the op amp
inverting input is a very high impedance.
1
2
3
4
5
6
7 8 9 10
20
Frequency (kHz)
With both switches configured as shown in
Figure 6, resistor R1 is in parallel with the pathway through S1 and S2. For minimal distortion,
the dominant feedback pathway should be through the
Figure 6: A dual-switch solution to close the
switches and not through R1. Therefore, the on resistance
amplifier feedback loop
of the switches should be much less than R1:
R1  RON(S1) + RON(S2)
(9)
Considering the 0.37-W on-resistance of an analog switch
such as the TS5A22362, this requirement is easily accomplished. But other switch parts, such as the extremely
popular CD4066B, have typical on-resistances greater
than 100 W.
When the switches are moved to their alternate position
in order to disconnect the load from the amplifier output,
R1 closes the feedback loop of the op amp. Stability must
always be considered when placing a resistor in the feedback path of an op amp. The feedback resistor interacts
with the input capacitance to degrade the feedback-loop
Texas Instruments
R1
200 Ω
S2
–
S1
VOUT
+
+
VIN
6
OPA172
TS5A22362
RL
600 Ω
AAJ 1Q 2015
Analog Applications Journal
Industrial
phase margin. A conservative rule of thumb is
given in Equation 10, the derivation of which
is given in Reference 2.
1
Where fGBW is the op amp gain-bandwidth
product and CCM and CDM are the op amp
common-mode and differential input capacitances, respectively. Inserting the appropriate
values for the OPA172, a precision op amp,
gives a maximum value for R1 of 198.9 W. A
200-W resistor is reasonably close to the calculated value to avoid stability concerns.
1
= 198.9 ≥ R1
20π(10 MHz)(8 pF)
1-VPK signal level
80-kHz measurement bandwidth
(10)
(11)
0.1
THD+N (%)
1
≥ R1
20πfGBW (CCM || CDM )
Figure 7: THD+N measurement of the circuit in Figure 6
0.01
0.001
600-Ω and 100-kΩ Load Impedances
0.0001
100
1k
10 k
Frequency (Hz)
FFT Output (dBr)
The circuit in Figure 6 was tested in the
previously described manner and the results
are given in Figures 7 and 8. By enclosing the
switch inside the feedback loop of the op amp,
Figure 8: Spectrum of a 1-VPK, 1-kHz sine wave at the
output of the circuit in Figure 6
the additional distortion from the RON variation has been effectively eliminated. The
THD+N measurement over frequency for both
0
load impedances (600 W and 100 kW) are
identical, and at the noise floor of the mea–20
surement instrument.
–40
Examining the FFT of the output signal
–60
(Figure 8) shows that the additional harmonics from the TS5A22362 are now below the
–80
noise floor of the measurement instrument.
–100
For high-performance analog systems where
–120
harmonic distortion must be minimized,
enclosing a CMOS analog switch inside the
–140
feedback loop of an op amp can greatly
–160
improve performance. The circuit topology
shown in Figure 6 reduces harmonic distortion
1
2
3
4
5
6
7 8 9 10
20
from the switch and also allows the amplifier
Frequency (kHz)
output to be completely disconnected from the
load. The feedback loop of the op amp is
closed regardless of the switch configuration, preventing
Reference
the amplifier output from saturating and causing
1. TS12A12511 SPDT Analog Switch datasheet, Texas
unwanted voltage transients when the switch is closed.
Instruments, 2015. Available:
Furthermore, a CMOS analog switch with extremely low
www.ti.com/1q15-TS12A12511
RON variation is no longer absolutely crucial, which can
potentially reduce system costs.
Related Web sites
Acknowledgements
www.ti.com/1q15-TS5A22362
The author wishes to acknowledge John Xu, TI analog
field applications engineer, whose idea was the initial
inspiration for this work.
Texas Instruments
www.ti.com/1q15-CD4066B
www.ti.com/1q15-OPA172
7
AAJ 1Q 2015
Analog Applications Journal
TI Worldwide Technical Support
Internet
TI Semiconductor Product Information Center
Home Page
support.ti.com
TI E2E™ Community Home Page
e2e.ti.com
Product Information Centers
Americas Phone
+1(512) 434-1560
Brazil
Phone
0800-891-2616
Mexico
Phone
0800-670-7544
Fax
Internet/Email
+1(972) 927-6377
support.ti.com/sc/pic/americas.htm
Europe, Middle East, and Africa
Phone
European Free Call
International
Russian Support
00800-ASK-TEXAS
(00800 275 83927)
+49 (0) 8161 80 2121
+7 (4) 95 98 10 701
Note: The European Free Call (Toll Free) number is not active in
all countries. If you have technical difficulty calling the free call
number, please use the international number above.
Fax
Internet
Direct Email
+(49) (0) 8161 80 2045
www.ti.com/asktexas
asktexas@ti.com
Japan
Fax
International
Domestic
+81-3-3344-5317
0120-81-0036
Internet/Email
International
Domestic
support.ti.com/sc/pic/japan.htm
www.tij.co.jp/pic
© 2015 Texas Instruments Incorporated. All rights reserved.
Asia
Phone
Toll-Free Number
Note: Toll-free numbers may not support
mobile and IP phones.
Australia
1-800-999-084
China
800-820-8682
Hong Kong
800-96-5941
India
000-800-100-8888
Indonesia
001-803-8861-1006
Korea
080-551-2804
Malaysia
1-800-80-3973
New Zealand
0800-446-934
Philippines
1-800-765-7404
Singapore
800-886-1028
Taiwan
0800-006800
Thailand
001-800-886-0010
International
+86-21-23073444
Fax
+86-21-23073686
Emailtiasia@ti.com or ti-china@ti.com
Internet
support.ti.com/sc/pic/asia.htm
Important Notice: The products and services of Texas Instruments
Incorporated and its subsidiaries described herein are sold subject to TI’s
standard terms and conditions of sale. Customers are advised to obtain the
most current and complete information about TI products and services
before placing orders. TI assumes no liability for applications assistance,
customer’s applications or product designs, software performance, or
infringement of patents. The publication of information regarding any other
company’s products or services does not constitute TI’s approval, warranty
or endorsement thereof.
A021014
E2E is a trademark of Texas Instruments. All other trademarks are the ­property of
their respective owners.
SLYT612
IMPORTANT NOTICE
Texas Instruments Incorporated and its subsidiaries (TI) reserve the right to make corrections, enhancements, improvements and other
changes to its semiconductor products and services per JESD46, latest issue, and to discontinue any product or service per JESD48, latest
issue. Buyers should obtain the latest relevant information before placing orders and should verify that such information is current and
complete. All semiconductor products (also referred to herein as “components”) are sold subject to TI’s terms and conditions of sale
supplied at the time of order acknowledgment.
TI warrants performance of its components to the specifications applicable at the time of sale, in accordance with the warranty in TI’s terms
and conditions of sale of semiconductor products. Testing and other quality control techniques are used to the extent TI deems necessary
to support this warranty. Except where mandated by applicable law, testing of all parameters of each component is not necessarily
performed.
TI assumes no liability for applications assistance or the design of Buyers’ products. Buyers are responsible for their products and
applications using TI components. To minimize the risks associated with Buyers’ products and applications, Buyers should provide
adequate design and operating safeguards.
TI does not warrant or represent that any license, either express or implied, is granted under any patent right, copyright, mask work right, or
other intellectual property right relating to any combination, machine, or process in which TI components or services are used. Information
published by TI regarding third-party products or services does not constitute a license to use such products or services or a warranty or
endorsement thereof. Use of such information may require a license from a third party under the patents or other intellectual property of the
third party, or a license from TI under the patents or other intellectual property of TI.
Reproduction of significant portions of TI information in TI data books or data sheets is permissible only if reproduction is without alteration
and is accompanied by all associated warranties, conditions, limitations, and notices. TI is not responsible or liable for such altered
documentation. Information of third parties may be subject to additional restrictions.
Resale of TI components or services with statements different from or beyond the parameters stated by TI for that component or service
voids all express and any implied warranties for the associated TI component or service and is an unfair and deceptive business practice.
TI is not responsible or liable for any such statements.
Buyer acknowledges and agrees that it is solely responsible for compliance with all legal, regulatory and safety-related requirements
concerning its products, and any use of TI components in its applications, notwithstanding any applications-related information or support
that may be provided by TI. Buyer represents and agrees that it has all the necessary expertise to create and implement safeguards which
anticipate dangerous consequences of failures, monitor failures and their consequences, lessen the likelihood of failures that might cause
harm and take appropriate remedial actions. Buyer will fully indemnify TI and its representatives against any damages arising out of the use
of any TI components in safety-critical applications.
In some cases, TI components may be promoted specifically to facilitate safety-related applications. With such components, TI’s goal is to
help enable customers to design and create their own end-product solutions that meet applicable functional safety standards and
requirements. Nonetheless, such components are subject to these terms.
No TI components are authorized for use in FDA Class III (or similar life-critical medical equipment) unless authorized officers of the parties
have executed a special agreement specifically governing such use.
Only those TI components which TI has specifically designated as military grade or “enhanced plastic” are designed and intended for use in
military/aerospace applications or environments. Buyer acknowledges and agrees that any military or aerospace use of TI components
which have not been so designated is solely at the Buyer's risk, and that Buyer is solely responsible for compliance with all legal and
regulatory requirements in connection with such use.
TI has specifically designated certain components as meeting ISO/TS16949 requirements, mainly for automotive use. In any case of use of
non-designated products, TI will not be responsible for any failure to meet ISO/TS16949.
Products
Applications
Audio
www.ti.com/audio
Automotive and Transportation
www.ti.com/automotive
Amplifiers
amplifier.ti.com
Communications and Telecom
www.ti.com/communications
Data Converters
dataconverter.ti.com
Computers and Peripherals
www.ti.com/computers
DLP® Products
www.dlp.com
Consumer Electronics
www.ti.com/consumer-apps
DSP
dsp.ti.com
Energy and Lighting
www.ti.com/energy
Clocks and Timers
www.ti.com/clocks
Industrial
www.ti.com/industrial
Interface
interface.ti.com
Medical
www.ti.com/medical
Logic
logic.ti.com
Security
www.ti.com/security
Power Mgmt
power.ti.com
Space, Avionics and Defense
www.ti.com/space-avionics-defense
Microcontrollers
microcontroller.ti.com
Video and Imaging
www.ti.com/video
RFID
www.ti-rfid.com
OMAP Applications Processors
www.ti.com/omap
TI E2E Community
e2e.ti.com
Wireless Connectivity
www.ti.com/wirelessconnectivity
Mailing Address: Texas Instruments, Post Office Box 655303, Dallas, Texas 75265
Copyright © 2015, Texas Instruments Incorporated
Was this manual useful for you? yes no
Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Related manuals

Download PDF

advertising