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Anwendungshinweise

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  Buffered ADC Family Eliminates Signal Conditioning Complexity &mdash AN167
  PDF, 1.3 Mb, Datei veröffentlicht: Sep 20, 2017
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  Application Note 167
  September 2017
  Buffered ADC Family Eliminates
  Signal Conditioning Complexity
  Joe Sousa, Andrew Thomas, Clement Wagner, Mark Thoren
  Engineers often assume that analog-to-digital converter
  inputs are high impedance. Direct-sampling SAR ADC
  inputs will often be high impedance when not converting, but will draw “spikes” of current at the beginning
  of sample acquisition. On average, this behavior can be
  modeled as a crude nonlinear resistance that is inversely
  proportional to the sample capacitor size and sample rate,
  but instantaneously the signal chain must be able to settle
  completely in response to this abuse before acquisition
  ends and a conversion begins. Coupling a signal chain to an ADC is an art form, often requiring a combination of theory
  and experimentation. By contrast, the LTC2358 family of
  multi-channel, buffered high voltage SAR ADCs offers
  truly high impedance inputs that simplify or eliminate the
  need for signal conditioning in many cases. When signal
  conditioning is required, it can be directly connected to
  the LTC2358 inputs without regard to its ability to drive
  a switched capacitor. The following circuits show some
  applications that take advantage of the properties of the
  LTC2358 inputs. …

Konstruktionshinweise

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  Precision Matched Resistors Automatically Improve Differential Amplifier CMRR – Here’s How &mdash DN1023
  PDF, 184 Kb, Datei veröffentlicht: Mar 21, 2012
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  Precision Matched Resistors Automatically Improve Differential
  Amplifier CMRR – Here’s How
  Design Note 1023
  Kris Lokere, Tyler Hutchison, Greg Zimmer
  INTRODUCTION
  Matched resistors are critical to the performance of a large class of differential circuits such as the following:
  R1 R2 R1 VCC
  R3 VA R2 R4 R2
  INST
  AMP VB 5V 5V + VB VOUT – R3 VB –
  OPAMP VA VOUT VA + – +
  DIFFERENTIAL
  AMP VOUT + – R1
  –5V R3 dn1023 F02 R4 Instrumentation Amp Difference Amp dn1023 F07 R4 dn1023 F12 Differential Amp Ideally, the resistors in these circuits are chosen such that R1/R2 = R4/R3. Any mismatch between these ratios will contribute
  a common mode error. The CMRR (common mode rejection ratio) is an important metric in these circuits, as it indicates how
  much of the unwanted common mode signal will appear in the output. The CMRR due to the resistors in these circuits can
  be calculated using the following formula:
  вЋ› 1вЋћ
  вЋњвЋќ вЋџвЋ  (G+1)
  2
  CMRRR _ ONLY ≈
  О”R /R Where CMRRR_ONLY Is CMRR Due Only to the Resistors
  (Ideal Ampliп¬Ѓer Case), О”R/R Is the Resistor Matching …

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  Matched Resistor Networks for Precision Amplifier Applications &mdash DN502
  PDF, 87 Kb, Datei veröffentlicht: Mar 22, 2012
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  Matched Resistor Networks for Precision Amplifier Applications
  Design Note 502
  Tyler Hutchison
  Introduction
  Some ideal op amp configurations assume that the
  feedback resistors exhibit perfect matching. In practice,
  resistor non-idealities can affect various circuit parameters such as common mode rejection ratio (CMRR),
  harmonic distortion and stability. For instance, as
  shown in Figure 1, a single-ended amplifier configured
  to level-shift a ground-referenced signal to a common
  mode of 2.5V needs a good CMRR. Assuming 34dB
  CMRR and no input signal, this 2.5V level shifter exhibits
  an output offset of 50mV, which can even overwhelm
  the LSB and offset errors of 12-bit ADCs and drivers.
  R
  VCC
  R +
  – VIN +
  – R
  2.5V
  R
  DC – + LTC6255 + DN502 F01 –
  VEE Figure 1. A Single-Ended Op Amp Used as a Level Shifter For an op amp, 34dB is a less than ideal CMRR. However, …

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• Signal Conditioning > Precision Resistor Network