Picoamp-Eingangsstrom, Mikrovolt-Offset, rauscharmer Operationsverstärker
PDF, 277 Kb, Datei veröffentlicht: Jul 1, 1985
The AN10 begins with a survey of methods for measuring op amp settling time. This commentary develops into circuits for measuring settling time to 0.0005%. Construction details and results are presented. Appended sections cover oscilloscope overload limitations and amplifier frequency compensation.
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Application Note 10
July 1985
Methods of Measuring Op Amp Settling Time
Jim Williams
Servo, DAC and data acquisition amplifiers all require
good dynamic response. In particular, the time required
for an amplifier to settle to final value after an input step
is especially important. This specification allows setting
a circuit’s timing margins with confidence that the data
produced is accurate. The settling time is the total length
of time from input step application until the amplifier remains within a specified error band around the final value. 10mV or less for a 10V step is of interest. No generalpurpose pulse generator is meant to hold output amplitude
and noise within these limits. Generator output-caused
aberration appearing at the oscilloscope probe will be
indistinguishable from amplifier output movement, producing unreliable results. The oscilloscope connection
presents additional problems. As probe capacitance rises,
AC loading of the resistor junction will influence observed
settling waveforms. The 20pF probe shown alleviates this
problem but its 10X attenuation sacrifices oscilloscope
gain. 1X probes are not suitable because of their excessive
input capacitance. An active 1X FET probe will work, but
another issue remains. Figure 1 shows one way to measure amplifier settling
time (see References 1, 2, and 3). The circuit uses the
“false sum node” technique. The resistors and amplifier …
PDF, 1.1 Mb, Datei veröffentlicht: Apr 1, 1985
The AN13 is an extensive discussion of the causes and cures of problems in very high speed comparator circuits. A separate applications section presents circuits, including a 0.025% accurate 1Hz to 30MHz V/F converter, a 200ns 0.01% sample-hold and a 10MHz fiber-optic receiver. Five appendices covering related topics complete this note.
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Application Note 13
April 1985
High Speed Comparator Techniques
Jim Williams
INTRODUCTION
Comparators may be the most underrated and underutilized monolithic linear component. This is unfortunate
because comparators are one of the most flexible and
universally applicable components available. In large
measure the lack of recognition is due to the IC op amp,
whose versatility allows it to dominate the analog design
world. Comparators are frequently perceived as devices,
which crudely express analog signals in digital form—a
1-bit A/D converter. Strictly speaking, this viewpoint is
correct. It is also wastefully constrictive in its outlook.
Comparators don’t “just compare” in the same way that
op amps don’t “just amplify”.
Comparators, in particular high speed comparators, can
be used to implement linear circuit functions which are
as sophisticated as any op amp-based circuit. Judiciously
combining a fast comparator with op amps is a key to
achieving high performance results. In general, op ampbased circuits capitalize on their ability to close a feedback
loop with precision. Ideally, such loops are maintained
continuously over time. Conversely, comparator circuits …
PDF, 421 Kb, Datei veröffentlicht: Aug 1, 1985
This note describes some of the unique IC design techniques incorporated into a fast, monolithic power buffer, the LT1010. Also, some application ideas are described such as capacitive load driving, boosting fast op amp output current and power supply circuits.
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Application Note 16
August 1985
Unique IC Buffer Enhances Op Amp Designs,
Tames Fast Amplifiers
Robert J. Widlar
Abstract: A unity gain IC power buffer that uses NPN
output transistors while avoiding the usual problems of
quasi-complementary designs is described. Free of parasitic oscillations and stable with large capacitive loads, the
buffer has a 20MHz bandwidth, a 100V/Ојs slew and can
drive В±10V into a 75О© load. Standby current is 5mA. A
number of applications using the buffer are detailed, and
it is shown that a buffer has many uses beyond driving
a heavy load.
Introduction
An output buffer can do much more than increase the
output swing of an op amp. It can also eliminate ringing
with large capacitive loads. Fast buffers can improve the
performance of high speed followers, integrators and
sample/hold circuits, while at the same time making them
much easier to work with.
Interest in buffers has been low because a reasonably
priced, high performance, general purpose part has not
been available. Ideally, a buffer should be fast, have no …
PDF, 2.5 Mb, Datei veröffentlicht: Mar 1, 1986
This note presents output state circuits which provide power gain for monolithic amplifiers. The circuits feature voltage gain, current gain, or both. Eleven designs are shown, and performance is summarized. A generalized method for frequency compensation appears in a separate section.
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Application Note 18
March 1986
Power Gain Stages for Monolithic Amplifiers
Jim Williams
Most monolithic amplifiers cannot supply more than a few
hundred milliwatts of output power. Standard IC processing
techniques set device supply levels at 36V, limiting available output swing. Additionally, supplying currents beyond
tens of milliamperes requires large output transistors and
causes undesirable IC power dissipation.
Many applications, however, require greater output power
than most monolithic amplifiers will deliver. When voltage
or current gain (or both) is needed, a separate output
stage is necessary. The power gain stage, sometimes
called a “booster”, is usually placed within the monolithic
amplifier’s feedback loop, preserving the IC’s low drift and
stable gain characteristics. 150mA Output Stage
Figure 1a shows the LTВ®1010 monolithic 150mA current
booster placed within the feedback loop of a fast FET
amplifier. At lower frequencies, the buffer is within the
feedback loop so that its offset voltage and gain errors
are negligible. At higher frequencies, feedback is through
Cf, so that phase shift from the load capacitance acting
against the buffer output resistance does not cause loop …
PDF, 330 Kb, Datei veröffentlicht: Jul 1, 1986
Applications often require an amplifier that has extremely high performance in several areas. For example, high speed and DC precision are often needed. If a single device cannot simultaneously achieve the desired characteristics, a composite amplifier made up of two (or more) devices can be configured to do the job. AN21 shows examples of composite approaches in designs combining speed, precision, low noise and high power.
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Application Note 21
July 1986
Composite Amplifiers
Jim Williams
Amplifier design, regardless of the technology utilized, is
a study in compromise. Device limitations make it difficult
for a particular amplifier to achieve optimal speed, drift,
bias current, noise and power output specifications. As
such, various amplifier families emphasizing one or more
of these areas have evolved. Some amplifiers are very good
attempts at doing everything well, but the best achievable
performance п¬Ѓgures are limited to dedicated designs. designed with little attention to DC biasing considerations
if a separate stabilizing stage is employed.
Figure 1 shows a composite made up of an LTВ®1012 low drift
device and an LT1022 high speed amplifier. The overall circuit is a unity-gain inverter, with the summing node located
at the junction of three 10k resistors. The LT1012 monitors
this summing node, compares it to ground and drives the
LT1022’s positive input, completing a DC stabilizing loop
around the LT1022. The 10k-300pF time constant at the
LT1012 limits its response to low frequency signals. The
LT1022 handles high frequency inputs while the LT1012
stabilizes the DC operating point. The 4.7k-220О© divider
at the LT1022 prevents excessive input overdrive during …
PDF, 3.3 Mb, Datei veröffentlicht: Sep 1, 1987
AN22 details the theoretical and application aspects of the LT1088 thermal RMS/DC converter. The basic theory behind thermal RMS/DC conversion is discussed and design details of the LT1088 are presented. Circuitry for RMS/DC converters, wideband input buffers and heater protection is shown.
PDF, 1.5 Mb, Datei veröffentlicht: Jul 1, 1985
This application note describes a wide range of useful applications for the LTC1043 dual precision instrumentation switched capacitor building block. Some of the applications described are ultra high performance instrumentation amplifier, lock-in amplifier, wide range digitally controlled variable gain amplifier, relative humidity sensor signal conditioner, LVDT signal conditioner, charge pump F/V and V/F converters, 12-bit A/D converter and more.
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Application Note 3
July 1985
Applications for a Switched-Capacitor Instrumentation
Building Block
Jim Williams
CMOS analog IC design is largely based on manipulation
of charge. Switches and capacitors are the elements used
to control and distribute the charge. Monolithic п¬Ѓlters, data
converters and voltage converters rely on the excellent
characteristics of IC CMOS switches. Because of the importance of switches in their circuits, CMOS designers have
developed techniques to minimize switch induced errors,
particularly those associated with stray capacitance and
switch timing. Until now, these techniques have been used
only in the internal construction of monolithic devices. A
new device, the LTCВ®1043, makes these switches available
for board-level use. Multi-pole switching and a self-driven,
non-overlapping clock allow the device to be used in circuits
which are impractical with other switches. Conceptually, the LTC1043 is simple. Figure 1 details its
features. The oscillator, free-running at 200kHz, drives a
non-overlapping clock. Placing a capacitor from Pin 16 to
ground shifts the oscillator frequency downward to any
desired point. The pin may also be driven from an external
source, synchronizing the switches to external circuitry. …
PDF, 818 Kb, Datei veröffentlicht: Mar 1, 1989
Presents circuit techniques permitting high efficiency to be obtained with linear regulation. Particular attention is given to the problem of maintaining high efficiency with widely varying inputs, outputs and loading. Appendix sections review component characteristics and measurement methods.
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Application Note 32
March 1989
High Efficiency Linear Regulators
Jim Williams
Introduction
Linear voltage regulators continue to enjoy widespread use
despite the increasing popularity of switching approaches.
Linear regulators are easily implemented, and have much
better noise and drift characteristics than switchers. Additionally, they do not radiate RF, function with standard
magnetics, are easily frequency compensated, and have
fast response. Their largest disadvantage is inefficiency.
Excess energy is dissipated as heat. This elegantly simplistic regulation mechanism pays dearly in terms of lost
power. Because of this, linear regulators are associated
with excessive dissipation, inefficiency, high operating
temperatures and large heat sinks. While linears cannot
compete with switchers in these areas they can achieve
significantly better results than generally supposed. New
components and some design techniques permit retention of linear regulator’s advantages while improving
efficiency.
One way towards improved efficiency is to minimize the
input-to-output voltage across the regulator. The smaller
this term is, the lower the power loss. The minimum input/
output voltage required to support regulation is referred …
PDF, 3.0 Mb, Datei veröffentlicht: Mar 1, 1990
This note presents guidelines for circuits utilizing LTC's switched capacitor filters. The discussion focuses on how to optimize filter performance by optimizing the printed wiring board, the power supply, and the output buffering of the filter. Many additional topics are discussed such as how to select the proper filter response for the application and how to characterize a filter's THD for DSP applications.
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Application Note 40
March 1990
Take the Mystery Out of the Switched-Capacitor Filter:
The System Designer’s Filter Compendium
Richard Markell
INTRODUCTION
Overview
This Application Note presents guidelines for circuits utilizing Linear Technology’s switched-capacitor filter family.
Although the switched-capacitor filter has been designed
into “telecom” circuits for over 20 years, the newer devices are faster, quieter and lower in distortion. These
filters now achieve total harmonic distortion (THD) below
–76dB (LTC®1064-2), wideband noise below 55μVRMS
(LTC1064-3), high frequency of operation (LTC1064-2,
LTC1064-3 and LTC1064-4 to 100kHz) and steep roll-offs
from passband to stopband (LTC1064-1: –72dB at 1.5 ×
fCUTOFF). These specifications make the new generation of
switched-capacitor filters from LTC candidates to replace
all but the most esoteric of active RC filter designs.
Application Note 40 takes the mystery from the design of
high performance active filters using switched-capacitor
filter integrated circuits. To help the designer get the highest performance available, this Note covers most of the
problems prevalent in system level switched-capacitor
filter design. The Note covers both tutorial filter material …
PDF, 1.7 Mb, Datei veröffentlicht: Jun 1, 1991
A wide variety of voltage reference circuits are detailed in this extensive guidebook of circuits. The detailed schematics cover simple and precision approaches at a variety of power levels. Included are 2 and 3 terminal devices in series and shunt modes for positive and negative polarities. Appended sections cover resistor and capacitor selection and trimming techniques.
PDF, 3.8 Mb, Datei veröffentlicht: Jun 1, 1990
Subtitled "Marrying Gain and Balance," this note covers signal conditioning circuits for various types of bridges. Included are transducer bridges, AC bridges, Wien bridge oscillators, Schottky bridges, and others. Special attention is given to amplifier selection criteria. Appended sections cover strain gauge transducers, understanding distortion measurements, and historical perspectives on bridge readout mechanisms and Wein bridge oscillators.
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Application Note 43
June 1990
Bridge Circuits
Marrying Gain and Balance
Jim Williams
Bridge circuits are among the most elemental and powerful
electrical tools. They are found in measurement, switching, oscillator and transducer circuits. Additionally, bridge
techniques are broadband, serving from DC to bandwidths
well into the GHz range. The electrical analog of the mechanical beam balance, they are also the progenitor of all
electrical differential techniques. and stability of the basic configuration. In particular, transducer manufacturers are quite adept at adapting the bridge
to their needs (see Appendix A, “Strain Gauge Bridges”).
Careful matching of the transducer’s mechanical characteristics to the bridge’s electrical response can provide a
trimmed, calibrated output. Similarly, circuit designers
have altered performance by adding active elements (e.g.,
amplifiers) to the bridge, excitation source or both. Resistance Bridges
Figure 1 shows a basic resistor bridge. The circuit is
usually credited to Charles Wheatstone, although S. H.
Christie, who demonstrated it in 1833, almost certainly
preceded him.1 If all resistor values are equal (or the two
sides ratios are equal) the differential voltage is zero. The
excitation voltage does not alter this, as it affects both
sides equally. When the bridge is operating off null, the
excitation’s magnitude sets output sensitivity. The bridge …
PDF, 980 Kb, Datei veröffentlicht: Jul 1, 1998
DAC DC specifications are relatively easy to verify. AC specifications require more sophisticated approaches to produce reliable information. In particular, the settling time of the DAC and its output amplifier is extraordinarily difficult to determine to 16-bit resolution. This application note presents methods for 16-bit DAC settling time measurement and compares results. Appendices discuss oscilloscope overdrive, frequency compensation, circuit and optimization techniques, layout, power stages and a historical perspective of precision DACs.
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Application Note 74
July 1998
Component and Measurement Advances Ensure
16-Bit DAC Settling Time
The art of timely accuracy
Jim Williams
Introduction
Instrumentation, waveform generation, data acquisition,
feedback control systems and other application areas are
beginning to utilize 16-bit data converters. More specifically, 16-bit digital-to-analog converters (DACs) have
seen increasing use. New components (see Components
for 16-Bit Digital-to-Analog Conversion, page 2) have
made 16-bit DACs a practical design alternative1. These
ICs provide 16-bit performance at reasonable cost compared to previous modular and hybrid technologies. The
DC and AC specifications of the monolithic DAC’s
approach or equal previous converters at significantly
lower cost.
DAC Settling Time
DAC DC specifications are relatively easy to verify. Measurement techniques are well understood, albeit often
tedious. AC specifications require more sophisticated
approaches to produce reliable information. In particular,
the settling time of the DAC and its output amplifier is
extraordinarily difficult to determine to 16-bit resolution. …
PDF, 625 Kb, Datei veröffentlicht: Aug 5, 1986
A discussion of circuit, layout and construction considerations for low level DC circuits includes error analysis of solder, wire and connector junctions. Applications include sub-microvolt instrumentation and isolation amplifiers, stabilized buffers and comparators and precision data converters.
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Application Note 9
August 1986
Application Considerations and Circuits for a New
Chopper-Stabilized Op Amp
Jim Williams
A great deal of progress has been made in op amp DC
characteristics. Carefully executed designs currently available provide sub-microvolt VOS О”T drift, low bias currents
and open-loop gains exceeding one million. Considerable
design and processing advances were required to achieve
these specifications. Because of this, it is interesting to
note that amplifiers with even better DC specification
were available in 1963 (Philbrick Researches Model
SP656). Although these modular amplifiers were large
and expensive (≈3" × 2" × 1.5" at $195.00 1963 dollars)
by modern standards, their DC performance anticipated
today’s best monolithic amplifiers while using relatively
primitive components. This was accomplished by employing chopper-stabilization techniques (see Box “Choppers,
Chopper-Stabilization and the LTC®1052”) instead of the
more common DC-differential stage approach.
The chopper-stabilized approach, developed by E. A.
Goldberg in 1948, uses the amplifier’s input to amplitude
modulate an AC carrier. This carrier, amplified and synchronously demodulated back to DC, furnishes the amplifier’s PARAMETER
EOS – 25В°C …
PDF, 495 Kb, Datei veröffentlicht: Jul 1, 1995
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C-Load Op Amps Conquer Instabilities – Design Note 107
Kevin R. Hoskins
Introduction
Linear Technology Corporation has taken advantage of
advances in process technology and circuit innovations
to create a series of C-Loadв„ў operational amplifiers that
are tolerant of capacitive loading, including the ultimate,
amplifiers that remain stable driving any capacitive load.
This series of amplifiers has a bandwidth that ranges from
160kHz to 140MHz. These amplifiers are appropriate for
a wide range of applications from coaxial cable drivers to
analog-to-digital converter (ADC) input buffer/amplifiers.
Driving ADCs
Most contemporary ADCs incorporate a sample-and-hold
(S/H). A typical S/H circuit is shown in Figure 1. The hold
capacitor’s (C1) size varies with the ADC’s resolution but
is generally in the range of 5pF to 20pF, 10pF to 30pF and
10pF to 50pF for 8-, 10-and 12-bit ADCs, respectively. gracefully and accurately drive capacitive loads, such as
Linear Technology’s C-Load line of monolithic amplifiers.
Table 1 lists Linear Technology’s unconditionally stable
voltage feedback C-Load amplifiers. Table 2 lists other
voltage feedback C-Load amplifiers that are stable with
loads up to 10,000pF. …
PDF, 224 Kb, Datei veröffentlicht: Sep 1, 1988
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Noise Calculations1 in Op Amp Circuits – Design Note 15
Alan Rich
Noise calculations in op amp circuits are one of the most
confused calculations that an analog engineer must
perform.
One cannot just look at noise specifications; the total op
amp circuit including resistors and operating frequency
range must be included in calculations for circuit noise. A
“low” noise amplifier in one circuit will become a “high”
noise amplifier in another circuit.
As part of this Design Note, an IBM-PC2 or compatible
computer program, NOISE, has been written to perform
the noise calculations. This program allows the user to
calculate circuit noise using LTC op amps, determine the
best LTC op amp for a low noise application, display the
noise data for LTC op amps, calculate resistor noise, and
calculate circuit noise using noise specs for any op amp.
At the end of this Design Note there are detailed operating
instructions for the computer program NOISE.
To calculate noise for an op amp circuit, one must consider the op amp voltage and current noise density and
1/f corner frequency, the frequency range of interest, and
the resistor noise.
The most comprehensive specification for voltage or current noise is the noise density frequency response curve …
PDF, 71 Kb, Datei veröffentlicht: Aug 1, 1989
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A Single Amplifier, Precision High Voltage Instrument Amp
Design Note 25
Walt Jung and George Erdi
be relatively simple, while at the same time capable of
high performance. Whereas dual summing amplifier
setups can provide high input-voltage qualifications,
a more simple single amp solution is often sought. An
IA topology which achieves all the above objectives
is shown in Figure 1, the “Precision High Voltage IA.”
The circuit employs the virtues of two key parts in
performing its function; the resistor array and the op
amp used with it. Instrumentation amplifier (IA) circuits abound in analog
systems, in fact virtually any linear applications handbook will show many useful variations on the concept (1).
While this may be somewhat bewildering to a newcomer,
all the variations have uses which are differentiated and
valuable. A good working knowledge of the alternate
forms can be a powerful tool towards designing costeffective high performance linear circuits.
A case in point is a single amplifier precision qualified
high voltage IA. This circuit must withstand very high
common mode voltages at the input, yet it should still L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their
respective owners. R5
975k R1 …
PDF, 130 Kb, Datei veröffentlicht: Nov 1, 1989
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A SPICE Op Amp Macromodel for the LT1012
Design Note 28
Walt Jung
Introduction
The Boyle, et al.1, SPICE macromodel for op amps has
proven to be quite useful for fast and efficient computerbased IC circuit analysis, used within its limitations.
Critics of this type of model point out that it is not
optimum for precise transient analysis of amplifiers
using complex compensation. On the other hand, the
Boyle macromodel may have little match in terms of the
computational speed and performance it can achieve,
plus how quickly it can be implemented. These virtues
are particularly true for lower frequency op amps, or
where DC performance parameters are more important.
The Boyle model can be set up to give realistic and
quite reasonable working approximations to a variety
of IC op amps which use various types of differential
transconductance pair front ends. Two fundamental
advantages of this model are the relative simplicity and
the simulation speed (particularly when a minimum
number of junctions are used). Further, the prudent use
of the appropriate transistors at the input can simulate
real input offset voltage and bias current effects, as well …
PDF, 224 Kb, Datei veröffentlicht: Oct 1, 1987
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Operational Amplifier Selection Guide for Optimum Noise
Performance – Design Note 3
George Erdi
The LTВ®1028 is the lowest noise op amp available today.
Its voltage noise is less than that of a 50О© resistor. In
other words, if the LT1028 is operated with source resistors in excess of 50О© , resistor noise will dominate. If the
application requires large source resistors, the LT1028’s
relatively high current noise will limit performance, and
other op amps will provide lower overall noise. The table below lists which op amp gives minimum total
noise for a specified equivalent source resistance. A two
step procedure should be followed to optimize noise: In general, the total noise of any op amp (referred to the
input) is given by: The table actually has two sets of devices: one for low
frequency (instrumentation), one for wideband applications. The slight differences between the two columns
occur because voltage and current noise increase at low
frequencies (below the so-called 1/f corner) while resistor
noise is flat with frequency. total noise = (voltage noise) + (resistor noise) + (current noise R )
2 2 2 eq where, (2) Enter the table to find the optimum op amp. Best Op Amp for Lowest Noise vs Source Resistance resistor noise = 0.13 Re q in nV Hz
and Req = equivalent source resistance
= R2 + R1//R3
R3
R1 – R2 + DN003 F01 Several conclusions can be reached by inspection of the
equation:
(a) To minimize noise, resistor values should be minimized to make the contribution of the second and …
PDF, 70 Kb, Datei veröffentlicht: Dec 1, 1990
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Chopper vs Bipolar Op Amps—An Unbiased Comparison
Design Note 42
George Erdi Table 1 lists the parameters of importance. In all input
parameters (except noise) the advantage unquestionably goes to the choppers. 5ОјV maximum offset voltage, 0.5ОјV/В°C maximum drift are commonly found
Table 1. Chopper Stabilized vs Precision Bipolar Op Amps
ADVANTAGE
PARAMETER
Offset Voltage
Offset Drift
All Other DC Specs CHOPPER BIPOLAR COMMENTS
вњ“
вњ“
вњ“ No Contest Wideband, 20Hz to
1MHz вњ“ See Details in Text Noise вњ“ See Details in Text вњ“ Rail to Rail Swing
2mA Limit on
Choppers Output: Light Load
Heavy Load
Single Supply
Application вњ“ вњ“ Inherent to
Choppers Needs
Special Design
Bipolars В±15V Supply Voltage вњ“ Except LTC1150 Prejudice/Tradition вњ“ Still a Chopper
Problem Cost 08/90/86_conv вњ“ Unless DC …
PDF, 78 Kb, Datei veröffentlicht: Feb 1, 1992
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3V Operation of Linear Technology Op Amps -Design Note 56
George Erdi
The latest trend in digital electronics is the introduction
of numerous ICs operating on regulated 3V or 3.3V
power supplies. This is a logical development to increase
circuit densities and to reduce power dissipation. In addition, many systems are directly powered by two AA
cells or 3V lithium batteries. Clearly, analog ICs which
work on 3V with good dynamic range to complement
these digital circuits are, and will be, in great demand.
Many Linear Technology operational amplifiers work
well on a 3V supply. The purpose of this design note
is to list these devices and their performance when
powered by 3V. The op amps can be divided into two
groups: single and dual supply devices. The single supply
op amps are optimized for, and fully specified at, a 5V
positive supply with the negative supply terminal tied
to ground. Input common mode voltage range goes
below ground, and the output swings to within a few
millivolts of ground while sinking current. Members of
the single supply family are the micropower LTВ®1077/
LT1078/LT1079 single, dual and quad op amps with
40μA supply current per amplifier, the LT1178/LT1179 dual and quad with 13μA per amplifier. The LT1006/
LT1013/LT1014 single, dual and quad have faster speed …
PDF, 137 Kb, Datei veröffentlicht: Jan 1, 1988
PDF, 929 Kb, Datei veröffentlicht: Jun 1, 1994
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C-Load Op Amps Tame Instabilities – Design Note 83
Richard Markell, George Feliz and William Jett
Introduction
By taking advantage of advances in process technology
and innovative circuit design, Linear Technology Corporation has developed a series of C-Loadв„ў op amps which
are tolerant of capacitive loading, including the ultimate,
amplifiers which are stable with any capacitive load. These
amplifiers span a range of bandwidths from 1MHz to
140MHz. They are suited for a wide range of applications
from coaxial cable drivers to capacitive transducer exciters.
The Problem
The cause of the capacitive load stability problem in most
amplifiers is the pole formed by the load capacitance and
the open-loop output impedance of the amplifier. This
output pole increases the phase lag around the loop which
reduces the phase margin of the amplifier. If the phase
lag is great enough the amplifier will oscillate.
External networks can be used to improve the amplifier’s
stability with a capacitive load but have serious drawbacks.
For instance, most designers are familiar with the use of
a series resistor RS between the load and the amplifier
output. The optimum value of RS depends on the load
capacitance, so this approach isn’t useful for ill-defined …