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Application Notes

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  Methods for Measuring Op Amp Settling Time &mdash AN10
  PDF, 277 Kb, File published: 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.
  Extract from the document

  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 …

• Download
  Circuit Techniques for Clock Sources &mdash AN12
  PDF, 150 Kb, File published: Oct 1, 1985
  Circuits for clock sources are presented. Special attention is given to crystal-based designs including TXCOs and VXCOs.
  Extract from the document

  Application Note 12
  October 1985
  Circuit Techniques for Clock Sources
  Jim Williams
  Almost all digital or communication systems require some
  form of clock source. Generating accurate and stable clock
  signals is often a difficult design problem.
  Quartz crystals are the basis for most clock sources. The
  combination of high Q, stability vs time and temperature,
  and wide available frequency range make crystals a
  price-performance bargain. Unfortunately, relatively little
  information has appeared on circuitry for crystals and
  engineers often view crystal circuitry as a black art, best
  left to a few skilled practitioners (see box, “About Quartz
  Crystals”).
  In fact, the highest performance crystal clock circuitry does
  demand a variety of complex considerations and subtle
  implementation techniques. Most applications, however,
  don’t require this level of attention and are relatively easy
  to serve. Figure 1 shows five (5) forms of simple crystal
  clocks. Types 1a through 1d are commonly referred to
  as gate oscillators. Although these types are popular,
  they are often associated with temperamental operation, …

• Download
  Power Gain Stages for Monolithic Amplifiers &mdash AN18
  PDF, 2.5 Mb, File published: 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.
  Extract from the document

  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 …

• Download
  Bridge Circuits &mdash AN43
  PDF, 3.8 Mb, File published: 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.
  Extract from the document

  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 …

Design Notes

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  Updated Operational Amplifier Selection Guide for Optimum Noise Performance &mdash DN140
  PDF, 74 Kb, File published: Oct 1, 1996
  Extract from the document

  Operational Amplifier Selection Guide for
  Optimum Noise Performance – Design Note 140
  Frank Cox
  Eight years ago, George Erdi wrote a very useful Design
  Note (DN6) that presented information to aid in the
  selection of op amps for optimum noise performance,
  in both graphical and tabular form. Design Note 140 is
  an update of DN6. It covers new low noise op amps as
  well as some high speed op amps. Although a great deal
  has changed in eight years, especially in electronics,
  noise is still a critical issue in op amp circuit design
  and the LTВ®1028 is still the lowest noise op amp for
  low source impedance applications.
  The amount of noise an op amp circuit will produce
  is determined by the device used, the total resistance
  in the circuit, the bandwidth of the measurement, the
  temperature of the circuit and the gain of the circuit. A
  convenient п¬Ѓgure of merit for the noise performance
  of an op amp is the spectral density or spot noise. This
  is obtained by normalizing the measurement to a unit
  of bandwidth. Here the unit is 1Hz and the noise is reported as “nV/√Hz.” The noise in a particular application
  bandwidth can be calculated by multiplying the spot
  noise by the square root of the application bandwidth. …

• Download
  Noise Calculations in Op Amp Circuits &mdash DN15
  PDF, 224 Kb, File published: Sep 1, 1988
  Extract from the document

  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 …

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  Operational Amplifier Selection Guide for Optimum Noise Performance &mdash DN3
  PDF, 224 Kb, File published: Oct 1, 1987
  Extract from the document

  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 …

• Download
  Operational Amplifier Selection Guide for Optimum Noise Performance &mdash DN6
  PDF, 137 Kb, File published: Jan 1, 1988

Articles

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  Consider New Precision Amplifiers for Updated Industrial Equipment Designs &mdash LT Journal
  PDF, 154 Kb, File published: Sep 1, 2009
  Extract from the document

  L DESIGN FEATURES Consider New Precision Amplifiers for
  Updated Industrial Equipment Designs
  by Brian Black Introduction
  Industrial equipment is designed for
  long life cycles, so the electronic components used in industrial applications
  are often chosen with significant emphasis on proven performance, quality
  and reliability. Precision amplifiers are
  no exception. Even if new and innovative amplifiers become available over a
  product’s lifetime, a redesigned board
  is often built using the same proven
  op amps in the old board. Even for
  entirely new applications, designers
  will choose amplifiers that have proven
  their mettle in other circuits, making a
  choice based more on familiarity than
  performance.
  Although an amplifier may have
  been tried and proven in a design, it
  is not necessarily the best solution for
  every new design. Many can benefit
  from using more recently released
  amplifiers, which can improve overall
  system performance, reduce power …

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Manufacturer's Classification

• Signal Conditioning > Amplifiers > Operational Amplifiers (Op Amps) > High Voltage Amplifiers (>12V) | Low Bias Current Amplifiers (Ib < 100pA) | JFET Input Amplifiers