Texas Instruments | Between the Amplifier and ADC: Managing Filter Loss in Communications Systems (Rev. B) | Application notes | Texas Instruments Between the Amplifier and ADC: Managing Filter Loss in Communications Systems (Rev. B) Application notes

Texas Instruments Between the Amplifier and ADC: Managing Filter Loss in Communications Systems (Rev. B) Application notes
Application Report
SNOA567B – September 2011 – Revised April 2013
AN-2188 Between the Amplifier and the ADC: Managing
Filter Loss in Communications Systems
.....................................................................................................................................................
ABSTRACT
This application report discusses managing filter loss in communications systems.
1
2
3
4
5
Contents
Introduction ..................................................................................................................
Component Selection for Lowest Losses ................................................................................
2.1
Inductors ............................................................................................................
2.2
Capacitors ...........................................................................................................
Filters .........................................................................................................................
3.1
Impedance Transformation .......................................................................................
3.2
Filter Termination ..................................................................................................
3.3
Creative Filter Selection ...........................................................................................
ADC Selection ...............................................................................................................
References ...................................................................................................................
2
2
3
3
4
4
5
6
7
7
List of Figures
1
Filter-Related Losses ....................................................................................................... 2
2
Using Impedance Transform to Realize Voltage Gain ................................................................. 4
3
Frequency Response for Impedance Transform Filter ................................................................. 5
4
Butterworth Filter with 100Ω Input and 200Ω Output Impedances ................................................... 5
5
Butterworth Filter with Different Input Termination Resistance Values — Filter is Designed for 400Ω
Input .......................................................................................................................... 6
6
“Tapped L” Filter
7
Various Input Termination Conditions for the Tapped L Filter — Filter was Designed for 100Ω
Termination .................................................................................................................. 7
............................................................................................................
6
All trademarks are the property of their respective owners.
SNOA567B – September 2011 – Revised April 2013
Submit Documentation Feedback
AN-2188 Between the Amplifier and the ADC: Managing Filter Loss in
Communications Systems
Copyright © 2011–2013, Texas Instruments Incorporated
1
Introduction
1
www.ti.com
Introduction
Filtering plays an essential part in nearly all communications systems. Removing unwanted signals is part
of system design to eliminate unwanted noise or distortion. The signal presented to the analog to digital
converter should have no spurs or distortion products, especially outside the desired Nyquist band. Noise
and distortion spurs that may be safely outside the band of interest may not remain outside of that band
after the ADC sampling process folds them around the sampling frequency.
The primary targets for the final analog filter are broadband noise and harmonic distortion spurs.
Broadband noise, in particular, can be problematic since the bandwidth of many ADCs is typically several
times the bandwidth of the signals to be processed. Likewise, harmonic spurs from the upper end of the
band of interest are typically still within the input bandwidth of the ADC, even though they are far removed
from the signal band. If not filtered out these spurs and noise can fold back onto weaker desired signals
and mask them.
Designing a filter to pass the desired frequencies is fairly easy. However, one of the largest drawbacks to
real filter implementations is the loss of signal through the filter, or insertion loss. This signal loss
contributes dB for dB to the ADC noise figure. What may be even worse is that the amplifier driving the
ADC will generate distortion at multiples of the filter loss. For example, if a filter has 7dB of loss the
amplifier needs to drive a signal 7dB stronger. This will result in second order products with 14dB higher
levels and 3rd order products will be 21dB worse. Some of these distortion products (intermodulation in
particular) cannot be filtered out, so keeping filter loss to a minimum is critical to system performance.
VCC
Filter Impedance
= 2*R
R
R
¼ LMH6522
R
ADC
R
~1-2 dB Voltage Loss
6 dB Voltage Loss
Figure 1. Filter-Related Losses
2
Component Selection for Lowest Losses
One of the first steps of filter optimization is the selection of the center frequency and the bandwidth of the
filter. Once this step has been completed the task becomes one of putting the filter elements on the
printed circuit board (PCB). PCB design is critical to filter performance, but even before we get to this
stage we usually select both the component values and the component sizes we would like to use, this
way we can allocate PCB space for the various components. As will be described later, it is very important
to choose the filter components first, then make the necessary space available on the PCB, rather than
trying to squeeze your filter into the space you have left at the end of the design. The components
selected will be critical to the performance of your filter.
2
AN-2188 Between the Amplifier and the ADC: Managing Filter Loss in
Communications Systems
SNOA567B – September 2011 – Revised April 2013
Submit Documentation Feedback
Copyright © 2011–2013, Texas Instruments Incorporated
Component Selection for Lowest Losses
www.ti.com
2.1
Inductors
Most passive filter designs rely heavily on LC circuit elements. Of all the elements in a filter the inductors
probably deserve the most attention. Inductors are usually larger and more expensive than the capacitors
and resistors used in a given filter. Often inductors are available only in 10% or 5% values as well. By
choosing the inductor first the capacitor values can be chosen to help make up for the more limited
selection of inductor values. One thing that becomes clear when one is shopping for inductors, though, is
that the physical construction of the inductor presents performance compromises that are more severe
than found with the capacitors and resistors that will be used for the filter. For a given value of inductance
the inductors available are usually in a package size larger than you would prefer. Taking into account
frequency limitations and parasitic effects, though, the smaller sizes offered are often unsuitable for the
frequency of operation, and using the wrong components can significantly increase the voltage losses
inside the filter.
When selecting an inductor you really need to focus on a few key parameters. The first, of course, is the
inductance. When looking for inductors of a given value, keep in mind, that of all passive components
inductors have the most constraints with respect to the physical size of the device. While a 100pF
capacitor can be found in nearly every footprint, for example, inductors are far more limited. For example,
in the CoilCraft catalog for an inductor sized 0.080” x 0.050” one series of inductors is available from 2.5
to 51 nH while another series is available from 0.11 to 22 µH (reference [1]). While this may seem to be a
very large range of values, what may not be clear is that the inductor could be specified at a frequency
completely different than the frequency at which you intend to use the inductor. For example: The CS
series of inductors from CoilCraft use frequencies of 250 MHz to 25 MHz to specify the L values, while the
LS series are specified at frequencies of 2.5 MHz to 7.9 MHz.
Finding a component with the correct value of inductance is less than half the battle. One key parameter
that is often misunderstood is the Self resonant frequency (SRF). This represents, ideally, the highest
frequency at which the inductor is really an inductor. At the self resonant frequency, the inductor has equal
amounts of inductive and capacitive reactance. Depending on your application you may be able to use the
inductor up to this frequency or slightly beyond. At the very least make sure to model the filter with realistic
parasitic values to see if the parasitic capacitance causes the filter to loose performance above the SRF.
When looking at inductor specifications at the signal frequency, don’t forget to research the component
performance in the stop band. Having good pass-band performance is important, but having good stop
band performance is important as well. The stop band can easily extend well beyond the passband
frequency.
In filter design many portions of the circuit are composed of dipoles. The dipole is a circuit element where
a capacitor and inductor form a resonant pair that contribute to the frequency response of the circuit. At
and near the resonant frequency of these dipole elements there are large currents flowing in the circuit. In
order to keep resistive losses low the circuit element must have a good quality factor or “Q”. The Q of an
inductor is defined as the ratio of the resistive losses to the inductive reactance. Q must be measured at
the frequency of interest because both the inductive reactance and the resistive losses are frequency
dependent. Most inductor manufacturers will chart Q vs frequency for their inductors. As a rule of thumb, it
is easier to get a high Q inductor when using a physically larger component. This is primarily due to the
ability to use a thicker conductor for the inductor to achieve lower resistance.
Many times, for a given inductor size there will be several construction methods to choose from. Wire
wound inductors can have an air core or ferrite core. The conductor can be wire wound, thin film or
lithographic. Some inductors offer shielding. Other options may include low temperature co-fired ceramic
(LTCC) or other more exotic construction methods. Generally speaking, if the inductor has a satisfactory
L, Q and SRF the construction method is not a concern.
2.2
Capacitors
We looked at inductors first since they are usually the hardest component to select. The capacitors are, of
course, equally critical to the filter design; however, they are usually much easier to select. The key
parameters of a capacitor are very similar to those of the inductors. Capacitors also have quality factor
and self resonant frequency metrics and they should be selected to equal or exceed the values you used
to choose the inductor.
SNOA567B – September 2011 – Revised April 2013
Submit Documentation Feedback
AN-2188 Between the Amplifier and the ADC: Managing Filter Loss in
Communications Systems
Copyright © 2011–2013, Texas Instruments Incorporated
3
Filters
www.ti.com
One recent trend in capacitors is to shift the side plates to the long side of the capacitor to gain a much
higher Q for a similarly sized capacitor of the more conventional construction. AVX corporation, for
instance offers 0204 and 0508 sized capacitors which cut parasitic inductance and resistance (reference
[2]).
When selecting filter capacitors it's also a good time to revisit your supply bypass capacitors for the
amplifier and ADC. New low loss, high SRF technology will be equally valuable on decoupling capacitors.
In order to make the best use of the new capacitor technology it will be very important to ensure that the
decoupling capacitors are close to the amplifier or ADC pins. Vias should be avoided if possible due to the
inductance they add. Power supply decoupling is really just a fancy name for a low pass filter you place on
the device power pins, and before the PCB design is complete is a good time to make other design
changes.
Selecting good components with appropriate values and good performance will help keep the power lost
in the components to a minimum. If your filter is nearly ideal due to high Q inductors and capacitors and
you still find your system performance coming up short there may be some other alternatives for your
system to cut the filter related losses. These techniques are described in the following sections.
3
Filters
3.1
Impedance Transformation
When you design an LC filter you normally chose a characteristic impedance for the filter. High-impedance
filters tend to be susceptible to noise and parasitic PCB capacitance. High-impedance filters also may
have undesirably low values for the capacitors. Designing a low impedance filter usually gives better
capacitor values and gives a filter that is less influenced by PCB capacitance. Filters with very low
characteristic impedance, however, may be difficult or impossible to drive, due to the currents required.
Most communications systems have settled on the range of 50Ω to 400Ω as reasonable impedances for
IF filters. For example, the ADC16DV160 has been tested with filters that range from 100Ω to 400Ω with
good results.
One common trick with filters is to use the input and output impedances to provide some voltage boost.
When there is an ADC on the termination end of a filter you can use voltage gain to offset power loss. The
reason this works is that the ADC doesn’t measure power. An ADC (to a first order) can’t tell the
difference between 2 VPP across 100Ωs and 2VPP across 200Ω. Thus a 100Ω in, 200Ω out, filter could give
you enough voltage boost to almost offset 3dB of the 6dB voltage loss of the termination resistors. This
technique does have some limitations. Filters with more than a 4:1 impedance transformation ratio are
difficult to design and will be sensitive to component tolerances.
VCC
Filter Designed with 100:
input and 200: output
50
100
¼ LMH6522
ADC
100
50
3 dB Voltage Gain ± filter losses
6 dB Voltage Loss
Figure 2. Using Impedance Transform to Realize Voltage Gain
4
AN-2188 Between the Amplifier and the ADC: Managing Filter Loss in
Communications Systems
SNOA567B – September 2011 – Revised April 2013
Submit Documentation Feedback
Copyright © 2011–2013, Texas Instruments Incorporated
Filters
www.ti.com
INSERTION LOSS (dB)
0
100 in 200 out
-5
-10
-15
-20
100M
120M 140M 160M 180M
FREQUENCY (MHz)
200M
Figure 3. Frequency Response for Impedance Transform Filter
RT
C4
L2
L6
C2
50
0.9p
1.3
470n
2.4p
VOUT
+
_
C1
64p
VIN
RT
C6
50
0.9n
L5
1.3
RL
200
L1
18n
L3
C5
470n
2.4p
VOUT-
Figure 4. Butterworth Filter with 100Ω Input and 200Ω Output Impedances
3.2
Filter Termination
There is also another option for getting more voltage out of a filter. Filter termination is required for good
filter response, however, it is possible to use a filter without matching the impedance of the filter exactly.
The degree of filter mismatch will determine the amount of voltage gained and it will also impact the
negative effects on the filter response. Figure 5 shows how the trade off works. It is best to match at least
one side of the filter with the design impedance. With an amplifier like the LMH6521 or LMH6522 it is very
easy to select the filter driving impedance. The LMH6521/22 amplifiers have low output impedance and
the filter termination resistors are external so they can be easily changed, which makes system
optimization easy.
SNOA567B – September 2011 – Revised April 2013
Submit Documentation Feedback
AN-2188 Between the Amplifier and the ADC: Managing Filter Loss in
Communications Systems
Copyright © 2011–2013, Texas Instruments Incorporated
5
Filters
www.ti.com
INSERTION LOSS (dB)
5
400
200
100
0
-5
-10
-15
-20
100M 120M 140M 160M 180M 200M
FREQUENCY (MHz)
Figure 5. Butterworth Filter with Different Input Termination Resistance Values —
Filter is Designed for 400Ω Input
3.3
Creative Filter Selection
Another option for filter flexibility is the topology of the filter. The filter below has an asymmetric response,
incorporates impedance transform and is very forgiving of input impedance termination resistance. From
the filter response curves it can be seen that this filter will even give a slight voltage gain while keeping a
well-behaved response. This filter topology is particularly well-suited for an anti-alias filter since it provides
more attenuation on the high-frequency side of the response. It is on this side that the distortion products
fall and in many cases the bulk of the amplifier noise falls into the higher frequencies as well.
RT1
C5
18
1n
L1
L2
VOUT
+
_
110n
VIN
RT2
C6
18
1n
RL1
100
200n
C2
C1
5.2p
18p
L5
L3
110n
200n
L4
42n
RL2
100
VOUT-
Figure 6. “Tapped L” Filter
6
AN-2188 Between the Amplifier and the ADC: Managing Filter Loss in
Communications Systems
SNOA567B – September 2011 – Revised April 2013
Submit Documentation Feedback
Copyright © 2011–2013, Texas Instruments Incorporated
ADC Selection
www.ti.com
INSERTION LOSS (dB)
5
100
60
40
0
-5
-10
-15
-20
100M
150M
200M
FREQUENCY (MHz)
250M
Figure 7. Various Input Termination Conditions for the Tapped L Filter —
Filter was Designed for 100Ω Termination
4
ADC Selection
The choice of an ADC can significantly ease the system design. With ever-improving distortion
specifications, higher sample rates and noise performance the ADC chosen in a particular application can
enable the use of a lower order filter. This filter will have lower loss and will be less susceptible to
component variations. Higher ADC sample rates will provide a lot of filter flexibility. For instance, a radio
with a 60MHz bandwidth and using a 100 MSPS ADC would gain an additional 25MHz of frequency range
by changing to a 150 MSPS ADC. This extra frequency range can be used for filter skirts.
While advancements in high performance, active circuits are making system designs more robust, there is
still a need for the use of passive components. Using these filter techniques can make the most of that
last passive stage in the signal path.
5
References
1. http://www.coilcraft.com/smind.cfm
2. http://www.avx.com/docs/catalogs/lga2t.pdf
SNOA567B – September 2011 – Revised April 2013
Submit Documentation Feedback
AN-2188 Between the Amplifier and the ADC: Managing Filter Loss in
Communications Systems
Copyright © 2011–2013, Texas Instruments Incorporated
7
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 © 2013, 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