Fig 1: An example of
good symmetry in the
layout of an A/D
converter
Sensitivity and selectivity
Simplify communication system design while
increasing available bandwidth. By Clarence Mayott.
I
n modern communications
systems, the more bandwidth
that is available, the more
information that can be
transmitted. As bandwidth
requirements increase, the need for
faster and higher linearity A/D
converters and amplifiers also
increases.
With increased bandwidth, more
noise is introduced into the system
which can overpower low level
signals of interest. This means the
requirement grows for low noise A/D
converters and amplifiers. Increased
bandwidth also means system
linearity becomes more important,
especially in the presence of a
strong interferer which can block
other signals of interest. One
approach to resolve these issues is
to use a high speed, high resolution
A/D converter driven by an equally
fast amplifier. Better sensitivity and
selectivity will improve the quality of
the system.
There are several design trade
offs in any communications system.
Bandwidth, spurious free dynamic
range (SFDR) and sensitivity are all
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important factors, but are difficult to
achieve with one solution. Usable
system bandwidth is dependent on
the A/D converter’s sample rate and
system bandwidth cannot be greater
than half the converter’s sample
rate. More bandwidth means faster
sampling, which limits the range of
practical converter options.
High speed A/D converters are
typically lacking in terms of SFDR
and signal to noise ratio (SNR),
limiting the performance of the
receiver. However, the LTC2107
16bit A/D converter sets a new level
of performance and linearity. With a
sample rate of 210Msample/s, the
usable data bandwidth is nearly
105MHz and, with an SNR of 79dB,
low level signals can be detected.
The LTC2107, coupled with a high
linearity amplifier like the LTC6409,
improves the throughput of modern
communications systems while
simplifying the front end design.
A major challenge in high speed
communications is maintaining good
SNR and wide bandwidth. One
approach is to use slower sample
rates and higher order filters to
“Any deviation
from perfect
symmetry will
cause a
mismatch in
the differential
signals,
manifesting
itself as second
order harmonic
distortion.”
attenuate the out of band noise
before sampling the analogue input
signal. This requires a complex filter
network with several stages of
attenuation and a significant number
of components. High order filters
also tend to ring in response to the
sampling glitches of the high speed
converter. This problem can be
solved by using a converter with a
faster sample rate, which simplifies
analogue filter design.
Using a low order filter allows the
sampling glitches to settle properly
and improves the linearity of the
system. A high speed converter,
such as the LTC2107, has enough
bandwidth for any demanding
communications system. With a
wider bandwidth, the anti aliasing
filter used to reject out of band
signals becomes easier to design.
With more bandwidth as a
guardband, a lower order filter can
be used, simplifying design and
reducing component count.
Wider bandwidth also allows more
noise to be sampled by the
converter, increasing the need for a
device with a high SNR. Along with
an SNR of 79dB, the LTC2107 has
improved SNR and SFDR figures,
allowing the receiver to accept lower
level signals that could be buried in
the noise floor of other parts. This
23 June 2015
www.newelectronics.co.uk
EMBEDDED DESIGN
MIXED SIGNAL & ANALOGUE
adds range to the receiver and
allows signal transmission over
longer distances.
High performance A/D converters
require a high performance
environment in which to operate. With
any direct sampling converter,
nonlinear charge is produced in the
sampling process. This is reflected
into the input network each time the
sampling switches close. A highly
absorptive network is required at the
analogue input to ensure that no
charge is reflected back into the input
“One approach
to resolve
these issues is
to use a high
speed, high
resolution A/D
converter
driven by an
equally fast
amplifier.”
beyond the converter or to the
nearest amplifier or balun
transformer.
To minimise the effects of direct
sampling on the source of the
analogue signal, an amplifier can be
used to absorb the charge from the
sampling process. Since many
feedback amplifiers have low output
impedance, the nonlinear charge is
attenuated before returning to the
converter. If the amplifier is located
as close as possible to the
converter, these reflections can be
boost quality
network, where it could potentially be
resampled. The input network of the
A/D converter should be as close to
50Ω as possible to allow for
maximum absorption of this nonlinear
charge. If this charge is reflected in a
less than ideal reflective network, it
can be resampled by the converter,
resulting in simple or intermodulation
distortion.
Asymmetrical layout
Another potential source of
distortion is an asymmetrical layout
of the input network. Nearly all high
speed A/D converters are
differential by design and, with an
ideal layout, this allows good
common mode rejection and second
order harmonic distortion. Any
deviation from perfect symmetry will
cause a mismatch in the differential
signals, manifesting itself as second
order harmonic distortion. Even a
simple design decision to flood
copper closer on one side of the
differential pair than the other can
cause a difference in ground current
in the adjacent ground planes,
adding distortion. Absolute
symmetry is required for maximum
performance. Figure 1 shows an
example of good layout of a high
speed A/D converter. This symmetry
should extend several centimetres
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reflected a number of times before
the sampling period ends, reducing
the impact of glitches on the
converter’s SFDR. To take advantage
of the available bandwidth, an
amplifier with a large gain bandwidth
product is required. With 10GHz gain
bandwidth, the LTC6409 is a
suitable choice; it has distortion
products better than 90dB out to
100MHz and an input noise density
of 1.1nV/√Hz. When used with the
LTC2107, the LTC6409 provides low
noise and distortion while
maintaining a wide bandwidth.
23 June 2015
Fig 1: Schematic of the LTC6409 and LTC2107
C2
1.5pF
R2
50Ω
R1
200Ω
5V
C1
0.1μF
VOCM
– +
LTC6409
_
S
+ –
R4
50Ω
R3
200Ω
C3
1.5pF
R7
50Ω
R5
50Ω
C5
4.7pF
L3
12nH
C6
4.7pF
R9
50Ω
L1
12nH
LTC2107
R6
50Ω
R8
50Ω
L2
12nH
L4
12nH
C4
4.7pF
R10
50Ω
C7
4.7pF
Interface between the two can be
minimal, further simplifying system
design.
Being a feedback amplifier, the
LTC6409’s gain can be changed by
changing the feedback value. With
the amplifier’s low noise density,
high levels of gain can be achieved
whilst maintaining a good SNR at the
converter. The amp’s increased gain
and high linearity ensure low level
signals can be detected. These
features allow the LTC6409 to drive
an A/D converter with very little
filtering (see fig 2). Most filtering
should be placed in front of the
LTC6409 to limit the frequency
content at its input and to allow for
more traditional filter topologies that
would not work well between the
parts. The LTC6409 also does a
single ended to differential
translation of the input signal, which
allows for filters at the input to be
single ended.
In a communications system,
there will be rogue offenders and
blockers that can appear in band at
any time. If the amplifier or converter
has poor linearity, these unwanted
and unanticipated tones can produce
distortion products which are much
higher than the signal of interest,
causing the receiver to lose that
signal. The more linear the system,
the less likely the interferer will
cause a loss of reception.
The LTC2107 and LTC6409
combination solves many problems
in modern communications systems.
The high sampling rate of the
LTC2107 and high gain-bandwidth
product of the LTC6409 allow signal
bandwidths up to 100MHz with no
degradation in linearity. Ultimately,
this improves the sensitivity of the
receiver and the ability of the
receiver to receive more data and
improve throughput. The
performance of these devices
simplifies the system complexity by
reducing the need for complicated
filters and signal processing.
Author profile:
Clarence Mayott is a mixed signal
products applications engineer with
Linear Technology.
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