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Opportunity
The future for the operational amplifier. By Reza Moghimi and Craig Wilson.
H
uge advances have been made in
semiconductor processing and circuit
design in the four decades since the first
IC op amp was introduced. IC manufacturers
have employed these advances to design nearly
‘ideal’ amplifiers.
Although unachievable by definition, the
ideal op amp has provided a target for analogue
designers. The ideal amplifier is noiseless, has
infinite gain, infinite input impedance, zero
bias current and zero offset voltage. It is also
free and takes up no space. These assumptions
are made to come up with the simple transfer
functions shown in many text books.
In reality, amplifier selection today is quite
complex. This is due, in part, to the variety of
system design requirements and the multitude
of circuit configurations. There are many
performance trade offs and manufacturers such
as Analog Devices have worked to provide a
portfolio of capabilities to meet user needs.
Amplifier designers continue to push the
technology envelope and this trend will
continue for the foreseeable future.
Manufacturers combine new processes, package
options and manufacturing capabilities to
produce parts that are, in effect, ‘perfect’ for
many of today’s challenging applications. Every
application requires a different combination of
specifications and the number of amplifiers
available continues to expand to fit those
needs.
Comparing equivalent devices, today’s op
amps have wider bandwidth, operate at lower
voltages, consume less current, fill less pcb area
and cost less than older parts. This trend will
ADI’s AD8506 op amp is designed for portable
applications such as medical equipment.
continue as signal to noise requirements
increase and as real world signal processing
takes place in more of our home appliances and
industrial equipment.
Where we’ve been
A quick historical look reveals amplifiers
(LM709) that required external compensation
and external offset adjust components. The
majority of these products were processed on
2in wafers using a bipolar process. Dual inline
and TO-99 can were the only packaging choices
and the major application segment was
industrial instrumentation. Low power meant
drawing a few milliamps current from ±15V
supplies. Manufacturers specified only dc
parameters, yields were low and prices were
high.
In today’s world of precision amplifiers,
small signal designers concerns themselves with
critical factors such as low supply current, low
offset, low noise and low bias current. The latest
amplifiers use innovative designs and processes
to deliver performance that continues to exceed
user’s expectations.
Design engineers push to optimise each
specification using circuit and production test
techniques, including autozero, the Digitrim
trimming process, fuse blowing and laser
trimmed resistors, thus creating amplifiers that
are close to ideal in a few specific parameters.
As an example, state of the art amplifiers
specify offset voltages as low as a few
microvolts.
Significant advances have occurred in all
aspects of process technology. These advances
allow amplifier design engineers to take the
maximum performance and functionality from
every technology. CMOS process technologies
have benefited from advances driven by the
digital microprocessor world and amplifier
designers have used this superior performance
at lower costs.
Traditionally, ultra high performance
amplifiers were designed in the bipolar world.
Now, however, analogue amplifier designers
have been able to overcome the higher voltage
noise inherent to CMOS processes, allowing
them to combine low noise with super low bias
current (made possible by the oxide insulated
gate). Manufacturers continue to develop
“Today’s op amps have wider bandwidth,
operate at lower voltages, consume less current, fill less pcb
area and cost less than older parts.”
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www.newelectronics.co.uk 22 April 2008
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SPONSORED TUTORIAL
Mixed Signal & Analogue
y knocks
proprietary manufacturing processes, such as
industrial CMOS (iCMOS) from Analog Devices.
This process provides the lowest noise CMOS
amplifier (2.7nV/rtHz at 10kHz) and a super low
supply current amplifier (1µA per amplifier).
Many high performance op amps still use
bipolar processes, which offer significant
benefits in analogue design and few
performance tradeoffs. New industrial bipolar
processes, such as ADI’s iPolar trench isolation
process, have shrunk die sizes dramatically by
including state of the art processing techniques
as well as devices such as JFETS.
These new developments in process
technologies have allowed amplifier designers
to develop products with incredible performance
parameters. As an example, the AD8599 ultra
low distortion op amp pushes total harmonic
distortion down to –105dB and broadband noise
down to nearly immeasurable levels (1nV/rtHz
at 1kHz). In the world of high speed amplifiers,
ultra fast processes have allowed devices with
slew rates of 310V/µs and unity gain bandwidths
of 600MHz.
These performance enhancements are all
possible while also continuing to shrink the die
size. This has enabled package sizes to become
incredibly small, to levels that are hardly
detectable by the naked eye.
Where are we going?
For applications powered by single AA cell or
nickel metal hydride (NiMH) batteries, size and
power consumption are primary concerns.
Operating supply voltages are already down to
1.8V and are decreasing. The AD8500, with its
1µA supply current, is the best choice when
precision, low voltage operation and low power
consumption are all required.
The drive towards single cell operation is on
Figure 1: AD8556 block diagram
DIGIN
VCLAMP
VDD
VDD
d/a
converter
logic
VPOS
1
EMI
filter
VDD
A1
2
3
A5
EMI
filter
VSS
VSS
VSS
VDD
A3
EMI
filter
VNEG
EMI
filter
–in
+in
VDD
A2
VDD
EMI
filter
VSS
VOUT
A4
VSS
VSS
VSS
filt/digout
and precision amplifiers will be available when
needed. To save power, many products will
require intelligent shutdown circuits.
Integrating other functions with amplifiers
can also reduce system errors. For example, the
digitally programmable sensor signal amplifier
with EMI filter (see figure 1) has brought many
previously unachievable benefits (gain
adjustment, offset adjustment and fault
detection circuitry) to the sensor signal
conditioning world. Further increases in
integration can include linearity correction,
frequency shaping, or other functions to create
more complete solutions.
Manufacturing and design trends will
continue to bring lower priced, better specified
and smaller parts to users. Existing issues such
as rail to rail performance and power
consumption will see continuous improvement
and amplifiers will get closer to their ‘ideal’
state.
Continued advances in process and
packaging technologies will continue to enable
more integration within smaller packages,
allowing increased functionality and
performance. Amplifiers will begin to integrate
other functions, again within very small
packages. Advances in package material sets will
allow even tighter parametric specifications.
The future will also bring products that are
easier to design into systems. Manufacturers
will place more emphasis on design tools for
amplifiers. Traditional PSPICE models will be
replaced by more elaborate ones that include
more amplifier parameters. Additional tools will
help to analyse stability, ac and dc errors. Using
SPICE models to represent the general behaviour
of a selected amplifier, designers can select a
component, quickly configure a circuit, apply a
signal and evaluate the amplifier’s general
performance on the web.
With online parametric evaluation tools,
users can now conduct real time simulations
quickly and efficiently and troubleshoot
potential problems across various parameters
and architectures. Wizard tools will be developed
for the web to provide expert knowledge to
designers’ problems 24 hours a day. ■
Author profiles:
Reza Moghimi, an Applications Engineering
Manager and Craig Wilson, a Technologist, are with
Analog Devices’ Precision Signal Processing Group.
For more information, go to www.analog.com
www.newelectronics.co.uk 22 April 2008
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