Op Amp Applications Handbook Walt Jung, Editor

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Thus an ideal noninverting op amp stage gain is simply equal to 1/β, or: 1 G = β 9 Op Amp Basics Eq. 1-4 This noninverting gain confi guration is one of the most useful of all op amp stages, for several reasons. Because V IN sees the op amp’s high impedance (+) input, it provides an ideal interface to the driving source. Gain can easily be adjusted over a wide range via R F and R G, with virtually no source interaction. A key point is the interesting relationship concerning RF and RG. Note that to satisfy the conditions of Eq. 1-2, only their ratio is of concern. In practice this means that stable gain conditions can exist over a range of actual RF – RG values, so long as they provide the same ratio. If RF is taken to zero and RG open, the stage gain becomes unity, and VOUT is then exactly equal to VIN. This special noninverting gain case is also called a unity gain follower, a stage commonly used for buffering a source. Note that this op amp example shows only a simple resistive case of feedback. As mentioned, the feedback can also be reactive, i.e., ZF, to include capacitors and/or inductors. In all cases, however, it must include a dc path, if we are to assume the op amp is being biased by the feedback (which is usually the case). To summarize some key points on op amp feedback stages, we paraphrase from Reference 2 the following statements, which will always be found useful: The summing point idiom is probably the most used phrase of the aspiring analog artifi cer, yet the least appreciated. In general, the inverting (−) input is called the summing point, while the noninverting (+) input is represented as the reference terminal. However, a vital concept is the fact that, within linear op amp applications, the inverting input (or summing point) assumes the same absolute potential as the noninverting input or reference (within the gain error of the amplifi er). In short, the amplifi er tries to servo its own summing point to the reference. The Inverting Op Amp Stage The op amp inverting gain stage, also known simply as the inverter, is shown in Figure 1-4. As can be noted by comparison of Figures 1-3 and 1-4, the inverter can be viewed as similar to a follower, but with a transposition of the input voltage VIN. In the inverter, the signal is applied to RG of the feedback network and the op amp (+) input is grounded. V IN R G SUMMING POINT OP AMP G = V /V OUT IN = − R /R F G V OUT Figure 1-4: The inverting op amp stage (inverter) R F
Chapter One The feedback network resistances R F and R G set the stage gain of the inverter. For an ideal op amp, the gain of this stage is: G F =− Eq. 1-5 RG For clarity, these expressions are again included in the fi gure. Note that a major difference between this stage and the noninverting counterpart is the input-to-output sign reversal, denoted by the minus sign in Eq. 1-5. Like the follower stage, applying ideal op amp principles and some basic algebra can derive the gain expression of Eq. 1-5. The inverting confi guration is also one of the more useful op amp stages. Unlike a noninverting stage, however, the inverter presents a relatively low impedance input for VIN, i.e., the value of RG. This factor provides a fi nite load to the source. While the stage gain can in theory be adjusted over a wide range via RF and RG, there is a practical limitation imposed at high gain, when RG becomes relatively low. If RF is zero, the gain becomes zero. RF can also be made variable, in which case the gain is linearly variable over the dynamic range of the element used for RF. As with the follower gain stage, the gain is ratio dependent, and is relatively insensitive to the exact RF and RG values. The inverter’s gain behavior, due to the principles of infi nite op amp gain, zero input offset, and zero bias current, gives rise to an effective node of zero voltage at the (−) input. The input and feedback currents sum at this point, which logically results in the term summing point. It is also called a virtual ground, because of the fact it will be at the same potential as the grounded reference input. Note that, technically speaking, all op amp feedback circuits have a summing point, whether they are inverters, followers, or a hybrid combination. The summing point is always the feedback junction at the (–) input node, as shown in Figure 1-4. However in follower type circuits this point isn’t a virtual ground, since it follows the (+) input. A special gain case for the inverter occurs when RF = RG, which is also called a unity gain inverter. This form of inverter is commonly used for generating complementary VOUT signals, i.e., VOUT = −VIN. In such cases it is usually desirable to match RF to RG accurately, which can readily be done by using a wellspecifi ed matched resistor pair. A variation of the inverter is the inverting summer, a case similar to Figure 1-4, but with input resistors RG2, RG3, etc (not shown). For a summer individual input resistors are connected to additional sources VIN2, VIN3, and so forth, with their common node connected to the summing point. This confi guration, called a summing amplifi er, allows linear input current summation in RF. 3 VOUT is proportional to an inverse sum of input currents. The Differential Op Amp Stage The op amp differential gain stage (also known as a differential amplifi er, or subtractor) is shown in Figure 1-5. Paired input and feedback network resistances set the gain of this stage. These resistors, RF–RG and RF′–RG′, must be matched as noted, for proper operation. Calculation of individual gains for inputs V1 and V2 and their linear combination derives the stage gain. 3 The very fi rst general-purpose op amp circuit is described by Karl Swartzel in Reference 3, and is titled “Summing Amplifi er.” This amplifi er became a basic building block of the M9 gun director computer and fi re control system used by Allied Forces in World War II. It also infl uenced many vacuum tube op amp designs that followed over the next two decades. 10 R
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Page 4 and 5: Op Amp Applications Handbook Walt J
Page 6 and 7: v Contents Foreword ...............
Page 8 and 9: vii Foreword The signal-processing
Page 10 and 11: ix Preface Op Amp Applications Hand
Page 12 and 13: xi Preface and diversity of use. Th
Page 14 and 15: Signal Amplifi ers; Audio Amplifi e
Page 16 and 17: 1928 Harold S. Black applies for pa
Page 18 and 19: CHAPTER 1 Op Amp Basics ■ Section
Page 20 and 21: CHAPTER 1 Op Amp Basics James Bryan
Page 22 and 23: As a precursor to more detailed sec
Page 24 and 25: INPUT 7 Op Amp Basics Note that wit
Page 28 and 29: G2 V 2 V 1 R G ' OP AMP G = VOUT /V
Page 30 and 31: 13 Op Amp Basics It is important to
Page 32 and 33: dB A VOL β G CL Figure 1-7: Op amp
Page 34 and 35: Input Dynamic Range 17 Op Amp Basic
Page 36 and 37: 19 Op Amp Basics dual- supply devic
Page 38 and 39: References: Introduction 21 Op Amp
Page 40 and 41: The previous section examined op am
Page 42 and 43: 25 Op Amp Basics The CFB topology i
Page 44 and 45: GAIN (R1) 1.04Ω - 37.4Ω FEEDBAC
Page 46 and 47: References: Op Amp Topologies 29 Op
Page 48 and 49: This section describes op amps in t
Page 50 and 51: Single Supply Offers: − Lower Pow
Page 52 and 53: +VS 7 (−) 2 INPUTS 3 (+) −VS 4
Page 54 and 55: 37 Op Amp Basics has a maximum inpu
Page 56 and 57: Offset as Low as 50µV Offset TC ~
Page 58 and 59: +VS (+) INPUTS (−) −V S J1 I1 V
Page 60 and 61: (A) (B) (C) +VS +VS NPN NPN -V S V
Page 62 and 63: Figure 1-33: AD8531/AD8532/AD8534 C
Page 64 and 65: 47 Op Amp Basics a device, even wit
Page 66 and 67: BIPOLAR (NPN-BASED): This is Where
Page 68 and 69: Most op amp specifi cations are lar
Page 70 and 71: Figure 1-39: Alternate input offset
Page 72 and 73: Figure 1-42: Noninverting op amp ex
Page 74 and 75: 57 Op Amp Basics Extremely low inpu
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Input Impedance Figure 1-48: Input
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61 Op Amp Basics Some fast op amps
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±10V RAMP 10kΩ 10kΩ V OS -VREF
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65 Op Amp Basics This sine wave sig
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V 1 R1 − + Figure 1-56: Measuring
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GAIN dB OPEN LOOP GAIN, A(s) IF GAI
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GAIN dB G1 G2 Figure 1-62: Frequenc
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Figure 1-65: Johnson noise of resis
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100 10 795 743 NOISE n/√Hz or µV
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20nV/div. (RTI) + 100 90 .... ....
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Nominal Peak-to-Peak 2 × rms 3 ×
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B A V N,R1 ∼ 4kTR1 V N,R3 ∼ 4kT
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V s = Signal Amplitude (RMS Volts)
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85 Op Amp Basics Note inverting mod
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Power Supplies and Decoupling 87 Op
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This section examines in more detai
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91 Op Amp Basics Note that the offs
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93 Op Amp Basics the nulling amplif
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Noise Considerations for Chopper-St
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Introduction High speed analog sign
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99 Op Amp Basics voltage, v, into a
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g 1 f = = = 153MHz u m 2 CP 2 520 1
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v in R1 + v − − gm + I T i = v
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+V S − −VS Q1 Q3 Q5 Q2 Q4 Q6 Fi
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107 Op Amp Basics Note also that th
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Q1 + − Q2 I1 I2 Figure 1-110: Sim
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R1 C2 A − R2 B |A(s)| VFB OP AMP
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High Speed Current-to-Voltage Conve
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4mA C1 20pF 500Ω − VFB 1 fp = =
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Voltage Feedback Op Amps: − Volta
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CHAPTER 2 Specialty Amplifi ers ■
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CHAPTER 2 Specialty Amplifi ers Wal
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Probably the most popular among all
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125 Specialty Amplifi ers buffers i
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127 Specialty Amplifi ers balanced
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129 Specialty Amplifi ers When dual
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V1 (-) 100kΩ V REF + 25kΩ 25k
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-IN ~ 400Ω V CM + VSIG ~ 2 _ + VS
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135 Specialty Amplifi ers The gener
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In Amp DC Error Sources 137 Special
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Figure 2-20: AD620 In amp common-mo
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141 Specialty Amplifi ers summed in
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AD524C AD620B AD621B 1 AD622 AD624C
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R+∆R R-∆R V B R-∆R R G R+∆R
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147 Specialty Amplifi ers A factor
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149 Specialty Amplifi ers Classic C
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Most data acquisition systems with
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153 Specialty Amplifi ers To unders
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Low Noise PGA 155 Specialty Amplifi
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157 Specialty Amplifi ers The OP113
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159 Specialty Amplifi ers Since tra
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Analog Isolation Techniques There a
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163 Specialty Amplifi ers An isolat
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Figure 2-51: AD215 120kHz low disto
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167 Specialty Amplifi ers This prod
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References: Isolation Amplifi ers 1
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CHAPTER 3 Using Op Amps with Data C
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CHAPTER 3 Using Op Amps with Data C
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175 Using Op Amps with Data Convert
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References: Introduction 177 Using
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ADC and DAC Static Transfer Functio
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ADC Input-Referred Noise 183 Using
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75Ω 185 Using Op Amps with Data C
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Introduction Op amps are often used
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Input Common-Mode Voltage Range Out
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~ R EXT V SOURCE R INT 7kΩ SWITCH
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~ R S V S REFOUT/ REFIN V INX R1 2k
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Driving CMOS ADCs with Switched Cap
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V IN + - Figure 3-35: Optimizing si
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INPUT ±1V 1kΩ + 10µF Drivers fo
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Op Amp Applications Handbook Walt Jung, Editor

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