Texas Instrumetn TI Electrical Characteristics in An Op Amp DS
Texas Instrumetn TI Electrical Characteristics in An Op Amp DS
Texas Instrumetn TI Electrical Characteristics in An Op Amp DS
in an
Op Amp Datasheet
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Op Amp Basics
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Ideal Operational Amplifier
• Ideal Op Amp
Zero input current
• Infinite input resistance +IN + OUT
• Infinite open loop gain
• Zero output resistance -IN -
• Infinite Slew Rate
Input Current = 0A
Open Loop Gain
Infinite Rout OUT
+IN + +
Rin
0 ohms
Infinite
- -
-IN
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Op Amp Loop Gain Model
network
RF
network =VFB/VOUT
VOUT
VFB
RI
-
VOUT
+ RF
+ VFB
VIN
- RI
VOUT/VIN = Acl = Aol/(1+Aolβ)
If Aol >> 1 then Acl ≈ 1/β
- Aol: Open Loop Gain
VIN +
Aol VOUT β: Feedback Factor
Acl: Closed Loop Gain
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Ideal Operational Amplifier
VINM
-
VINP +
Aol VOUT
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Ideal Operational Amplifier
Irf = (Vout - Vin) / RF
Iri = Vin / RI
Iin- = 0A
Non-Inverting Configuration
Ideal Op Amp
+
Vin 1 Vin 1V Vout 10V
-
Iin- = 0A
RI 10k RF 90k
Iri Irf
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Ideal Operational Amplifier Irf = (Vout - 0V) / RF
Iri = (0V-Vin) / RI
Iin- = 0A
Inverting Configuration
For Ideal Op Amp Irf = Iri
With Feedback and High Open Loop Gain: (Vout - 0V) / RF = (0V-Vin) / RI
+IN is forced to equal -IN Vout / Vin = -RF/RI
Ideal Op Amp
+
Gnd 0V Vout -9V
-
Iin- = 0A
RI 10k RF 90k
Vin 1
Iri Irf
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Intuitive AC Op Amp Model
VO
RO
IN+ K(f) VOUT
x1
+
RIN VDIFF
-
IN-
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Input Specifications
Input Bias Current (Ib) & Input Offset Current (Ios)
Input Offset Voltage (Vos)
Power Supply Rejection Ratio (PSRR): Referred-To-Input Vos
Common Mode Voltage Range (Vcm)
Common Mode Rejection Ratio (CMRR): Referred-To-Input Vos
Small Signal Input Parasitics: Input Capacitance, Input Resistance
Input Noise: Current, Voltage (in, en)
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Input Bias Current (Ib), Input Offset Current (Ios)
Ib = 5pA
Ios = 4pA
Polarity is + or –
Current into or out of inputs
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Input Bias Current (Ib), Input Offset Current (Ios)
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Input Bias Current (Ib) Vout Error
Vinm = 1.5uV 2
Vinm RF 1M
1 RF 1M
Vout
Ib- 3p
Ib- 3p RI 1M Ideal Op Amp
RI 1M Idelal Op Amp
-
- Vout
Vout Rs 1M +
Rs 1M +
3 RF 1M R2 1M 4
RI 1M VIb- 1.5u Ideal Op Amp R3 1M Ideal Op Amp
- -
Vout Vout
Rs 1M + R1 1M
Vout error = 11uV + Vout error = 11uV
VIb+ 7u
+
VIb 5.5u
Vin Ib flow s through feedback and input resistors Vin + Simplified VIb Model
Model as VIb+ and VIb-
VIb = VIb+ - VIb-
Inverting and Non-Inverting Gains create Vout error
Non-Invverting Gain Creates Vout error
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Input Offset Voltage (Vos) Vout Error
RF 1M
RI 1M Ideal Op Amp
-
Vout
+ Vout error = 50uV
Vos 25u
Input Offset Voltage
Creates Vout error
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Input Offset Voltage (Vos) Drift Vout Error
-
Vos_drift 60u Vos 25u Vout
+ Vout error = 170uV
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Power Supply Rejection Ratio (PSRR) Vout Error
RF 1M
DC PSRR in Table
DC PSRR Drift in Table RI 1M Ideal Op Amp
Polarity is + or - -
Vout
+
PSRR is an RTI (Referred-To-Input) specification + Vout error = 20uV
Appears as Input Offset Voltage Vos_PSRR 10u
delta_Vcc 500m
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Power Supply Rejection Ratio (PSRR) Vout Error
AC PSRR in Curve
Frequency of analysis = 20kHz
PSRR AC @ 20kHz = 80dB
Convert PSRR (dB) to PSRR (Linear Gain):
(80dB/20)
10 = 10,000
PSRR is an attenuation so 1V gets attenuated by x10,000
1/10,000 = 1e-4V/V
Now convert numerator to uV:
(1e-4V) (1uV/1e-6V) = 1e-4uV / 1e-6 = 100uV:
PSRR AC @ 20kHz = 100uV/V
20kHz
R4 1M
R5 1M Ideal Op Amp
-
PSSR AC @ 20kHz = 100uV/V
Vout
delta_Vcc_ac = 100mVpp (AC change in Vcc @ 20kHz) +
+ Vout error = 20uVpp @ 20kHz
+
Vos_PSRR_ac = PSSR AC delta_Vcc_ac delta_Vcc_ac
+
Vos_PSRR_ac
Vos_PSRR_ac = 100uV/V 100mVpp = 10uVpp 10uVpp @ 20kHz 100mVpp @ 20kHz
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Common Mode Voltage Range (Vcm)
+
Vin V = 2V max
-13V < Vin < +13V
Vcc 15
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Common Mode Rejection Ratio (CMRR) Vout Error
CMRR DC = 130dB RF 1M
Convert CMRR (dB) to CMRR (Linear Gain):
(130dB/20)
10 = 3.16e+6
CMRR is an attenuation so 1V gets attenuated by x3.16e+6
1/3.16e+6 = 3.16e-7V/V V2 15
Now convert numerator to uV:
Ideal Op Amp
(31.6e-7V) (1uV/1e-6V) = 3.16e-7uV / 1e-6 = 0.316uV: RI 1M
CMRR DC = 0.316uV/V -
Vout
+
+ Vout error = 3.16uV
CMRR DC = 0.316uV/V
Vin = 5V for Non-Inverting Gain Vin =Vcm Vin 5 Vos_CMRR 1.58u V1 15
Vcm = 5V
Vos_CMRR = CMRR DC Vcm
Vos_CMRR = 0.316uV/V 5V = 1.58uV
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Common Mode Rejection Ratio (CMRR) Vout Error
AC CMRR in Curve Frequency of Analysis = 1kHz
CMRR AC = 100dB @ 1kHz
Convert CMRR (dB) to CMRR (Linear Gain):
(100dB/20)
10 = 100,000
CMRR is an attenuation so 1V gets attenuated by x100,000
1/100,000 = 1e-5V/V
Now convert numerator to uV:
(1e-5V) (1uV/1e-6V) = 1e-5uV / 1e-6 = 10uV:
CMRR AC = 10uV/V @ 1kHz
RF 1M
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Cin RF 1M
Small Signal
Input Parasitics Vee
RI 1M
Ccm Rcm
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Input Noise: Current, Voltage (in, en)
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Op Amp Noise Model
Noise Model
(IN+ and IN- are not correlated) OPA277 Data
VN
IN+ IN-
IOP1
fA -
VN IN
* +
*
nV
U1
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Understanding The Spectrum:
Total Noise Equation (Current or Voltage)
1/f Noise Region White Noise Region
(Pink Noise Region) (Broadband Noise Region)
100k
10k
100
10
1
enT = √[(en1/f)2 + (enBB)2] 0.1 1 10 100 1k 10k
Frequency (Hz)
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Real Filter Correction vs Brickwall Filter
Noise BW
where:
fP = roll-off frequency of pole or poles
Small Signal BW
0 fBF = equivalent brickwall filter frequency
Skirt of
1-Pole Filter
Response
Filter Attenuation (dB)
Skirt of
-20 2-Pole Filter
Response
Skirt of
3-Pole Filter
Response
-40
Brickwall
-80
0.1fP 10fP
fP fBF
Frequency (f)
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AC Noise Bandwidth Ratios for nth Order Low-Pass Filters
Number of Poles in Kn
Filter AC Noise Bandwidth Ratio
1 1.57
2 1.22
3 1.16
4 1.13
5 1.12
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Broadband Noise Equation
eBB
BWn = (fH)(Kn)
where:
BWn = noise bandwidth for a given system
fH = upper frequency of frequency range of operation
Kn = “Brickwall” filter multiplier to include the “skirt” effects of a low pass filter
enBB = (eBB)(√[BWn])
where:
enBB = Broadband voltage noise in volts rms
eBB = Broadband voltage noise density ; usually in nV/√Hz
BWn = Noise bandwidth for a given system
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1/f Noise Equation
e1/f@1Hz
e1/f@1Hz = (e1/f@f)(√[f])
where:
e1/f@1Hz = normalized noise at 1Hz (usually in nV)
e1/f@f = voltage noise density at f ; (usually in nV/√Hz)
f = a frequency in the 1/f region where noise voltage density is known
en1/f = (e1/f@1Hz)(√[ln(fH/fL)])
where:
en1/f = 1/f voltage noise in volts rms over frequency range of operation
e1/f@1Hz = voltage noise density at 1Hz; (usually in nV)
fH = upper frequency of frequency range of operation
(Use BWn as an approximation for fH)
fL = lower frequency of frequency range of operation
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Example Noise Calculation
Given:
R2 1k R1 100k OPA627
Noise Gain of 101
V2 15
VG1
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Voltage Noise Spectrum and Noise Bandwidth
50nV/rt-Hz
5nV/rt-Hz
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Example Voltage Noise Calculation
enBB = (eBB)(√BWn)
enBB = (5nV/√Hz)(√248kHz) = 2490nV rms
Note: This example amp doesn’t have 1/f component for current noise.
Gain
IOP1
fA - VF1
U2
* + Req = R1 || Rf
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Example Current Noise Calculation
inBB = (iBB)(√BWn)
inBB = (2.5fA/√Hz)(√248kHz) = 1.244pA rms
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Resistor Noise – Thermal Noise
The mean- square open- circuit voltage (e) across a resistor (R) is:
en = √ (4kTKRΔf) where:
TK is Temperature (ºK)
R is Resistance (Ω)
f is frequency (Hz)
k is Boltzmann’s constant
(1.381E-23 joule/ºK)
en is volts (VRMS)
TK = 273.15oC + TC
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Resistor Noise – Thermal Noise
en density = √ (4kTKR) 3
1 10
468.916
100
23 ( 25 273.15 ) X 9
nV/rt-Hz
0 10
0
23 ( 125 273.15 ) X 10
9
10
100
4 1.3806510 23
( 25 273.15) X 10
9
0
23 ( 55 273.15 ) X 10
9
25C
4 1.3806510 23
( 125 273.15) X 10
9
10
1 125C
4 1.3806510 23 9
( 55 273.15) X 10
-55C
1
0.347 0.1
3 4 5 6 7
10 100 1 10 1 10 1 10 1 10 1 10
10 X 7
10
Resistance (Ohms)
0.347 0.1
10
10
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Example Resistor Noise Calculation
enr = √(4kTKRΔf)
where:
R = Req = R1||Rf
Δf = BWn
enr = √(4 (1.38E-23)
( (273 + 25) (0.99k)(248kHz))
(0.99k)( = 2010nV rms
en-in= √(4kTRΔf) en-out= Gain x (√(4kTRΔf))
f)
* U1
* U1
R2R1
1k R1Rf2k
Gain
nV
nV
IOP1
- VF1
Req = R1 || Rf
+
*
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Total Noise Calculation
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Calculating Noise Vpp from Noise Vrms
2 X rms 32%
3 X rms 13%
4 X rms 4.6%
5 X rms 1.2%
6 X rms * 0.3%
6.6 X rms 0.1%
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Voltage Noise (f = 0.1Hz to 10Hz) Low Frequency
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Frequency Response
Specifications
Open Loop Gain (Aol) & Phase
Slew Rate (SR)
Total Harmonic Distortion + Noise (THD+N)
Settling Time (ts)
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Open Loop Gain & Phase
5.5MHz
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Vout/Vin: R2 9k
Gain Accuracy & Frequency Response
Real Op Amp
R3 1k
-
Vout
+
+
Vin
fcl
1/Beta Aol at any Frequency:
Aol_f = UGBW / f
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Slew Rate Measurement:
Slew Rate 10% to 90% of Vout
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Slew Rate &
Full Power Bandwidth
or
Maximum Output Voltage vs Frequency
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THD + Noise
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THD + Noise = 1% Example
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Settling Time
Slew
Rate
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Settling Time
Settling Time
Large Signal effects:
Slew Rate
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Output Specifications
Voltage Output Swing from Rail
Short Circuit Current (Isc)
Open Loop Output Impedance (Zo)
Closed Loop Output Impedance (Zout)
Capacitive Load Drive
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Voltage Output
Swing From Rail
Loaded Vout swing from Rail 2 1
Higher Current Load Farther from Rail
Higher Current Load Larger Vsat
Vsat = Vs - Vout
+25C Curve:
Op Amp Aol is degraded if on curve 1
Op Amp Aol is okay if left of curve 2
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Short Circuit Current (Isc)
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Open Loop Output Impedance (Zo)
Closed Loop Output Impedance (Zout)
Capacitive Load Drive
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Op Amp Model for Derivation of ROUT
Definition of Terms:
RO = Op Amp Open Loop Output Resistance
ROUT = Op Amp Closed Loop Output Resistance
ROUT = RO / (1+Aolβ)
RF
RI RO VOUT
-IN
-
xAol +
RDIFF VO
VFB VE
IOUT
-
+ 1A
+IN
Op Amp Model
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ROUT vs RO
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RO & CL: Modified Aol Model
RI RF
100k 100k
OPA452
-
- RO
VOUT
+ 28.7
CL
ol
Da
+
fpo1 = 1/(2∙П∙RO∙CL)
VIN
fpo1 = 1/(2∙П∙28.7Ω∙1μF)
-
fpo1 = 5.545kHz
Create a new “Modified Aol” Plot
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RO & CL: OPA542 Modified Aol First Order
120
OPA452
Aol
100
80
60
Gain (dB)
fpo1
40 STABLE
40dB/Decade
Rate-Of-Closure
20 fcl
1/
-40
-60
1 10 100 1K 10K 100k 1M 10M
Frequency (Hz)
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Zo (Open Loop Output Impedance)
Cap Load Drive
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2nd Order Transient Curves
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2nd Order Damping Ratio vs Phase Margin
Publishing Company. Reading, Massachusetts. Third Edition, 1981.
From: Dorf, Richard C. Modern Control Systems. Addison-Wesley
o
23.5
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Closed Loop For Bipolar, Emitter-Follower Output Op
amps like OPA177, open loop output
Output Impedance impedance = RO (purely resistive
inside UGBW)
Closed Loop Output impedance gives an
indication of what source impedance Since ROUT = RO/(1+Aol) and RO is
the closed loop op amp will have to resistive ROUT looks opposite of Aol
drive loads over frequency and increase at higher frequencies
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Power Supply Specifications
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Specified and Operating Voltage Range (VS)
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Quiescent Current (IQ)
+Vs
IQ
+
+
Real Op Amp Vout
-
IQ
-Vs
Quiescent Current:
Supply Current to operate the op amp
Does NOT include load current
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Temperature Range
Specifications
Specified Range
Operating Range
Thermal Resistance (JA)
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Specified and Operating Temperature Range
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Thermal Resistance (JA)
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Thermal Resistance (JA)
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Thermal Model
PD = PIQ + POUT
PD = Total Power Dissipated
PIQ = Power Dissipated due to IQ
POUT = Power Dissipated in Output Stages
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IQ Power Dissipation (PIQ)
IQ
+
+
Real Op Amp Vout
-
IQ
-Vs
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DC Normal Maximum Power Dissipation in Output Stage (POUT)
+Vs 1
Vout = Vs
IQ 2
Real Op Amp
+ Iout_DC
+
Vout
Vin -
2
IQ
Vs
RL POUT_DC =
4 RL
-Vs
RI RF
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DC Short Circuit Maximum Power Dissipation in Output Stage (POUT)
VF1
VG1 -
IQ
-Vs
RI RF
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AC Normal Maximum Power Dissipation in Output Stage (POUT)
For AC Sinusoidal Signals +Vs
IQ
2 Real Op Amp
2 Vs 2 Vs +
+ Iout_AC
+
Vout
Vout peak = POUT_AC = Vin -
2
RL IQ
RL
-Vs
0.25
0.2
2
0.15 2 Vs
POUT_AC =
2
0.1 2 Vs RL
Vout peak =
0.05
0
0 1 2 3 4 5
V(load) peak AC Sinusoidal Voltage
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AC Normal Maximum Power Dissipation in Output Stage (POUT)
For AC Sinusoidal Signals AC Maximum Power Dissipation Formula
based on symmetrical dual supplies
Vcc 5 Vcc
To use formula for single supply circuits
IQ set +Vs = +(Vcc/2) and -Vs = -(Vcc/2)
Real Op Amp
as shown.
+ Iout_AC
+
+
Vout
Vin -
IQ
RL
+Vs 2.5
IQ +Vs = (Vcc/2)
Real Op Amp
RI RF +
+ Iout_AC
+
Vout
Vin -
IQ
RL
2
2 Vs -Vs 2.5
POUT_AC =
2
RL RI RF
-Vs = -(Vcc/2)
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Absolute Maximum Rating
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Absolute Maximum Rating
For Long-Term Reliable Operation use Op Amp below the Absolute Maximum Ratings
Heat is semiconductor’s worst enemy – Keep TJ at least 25C less than TJ Max
For this op amp be sure to limit current into the input terminals to 10mA during electrical
overstress conditions.
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Op Amp Selection Tip
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Choosing an Op Amp?
Focus on Key Concerns for Application to Narrow Search
Voltage? Current? Speed?
Cu Supply Current?
rre Output Current?
n t? Input Bias Current?
-
SSBW @ G=?
Speed?
Slew Rate?
SR(V/us)=2pifVOP1e-6
where: f=Hz
+ e ?
o l tag Supply Voltage?
Input Offset Voltage?
V Output Swing Voltage?
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References
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References
Jim Karki, Senior Applications Engineer, Texas Instruments
“Understanding Operational Amplifier Specifications” White Paper: SLOA011
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