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1 of 78
Deciphering Electrical Characteristics in an
Op Amp Datasheet
2 of 78
Op Amp Basics
3 of 78
-
+
-
+
Open Loop Gain
Rout
Rin
OUT+IN
-IN
-
+
Ideal Op Amp
OUT+IN
-IN
Infinite0 ohms
Infinite
Input Current = 0A
Input Current = 0A Ideal Op Amp
Ideal Operational Amplifier
• Zero input current• Infinite input resistance• Infinite open loop gain• Zero output resistance• Infinite Slew Rate
4 of 78
Op Amp Loop Gain Model
+
-
RF
RI
VIN
+
-
network
Aol+
-
VOUTVIN
VFBVOUT
VFB
RF
RI
=VFB/VOUT
VOUT
network
VOUT/VIN = Acl = Aol/(1+Aolβ)
If Aol >> 1 then Acl ≈ 1/β
Aol: Open Loop Gain
β: Feedback Factor
Acl: Closed Loop Gain
5 of 78
Ideal Operational Amplifier
Aol+
-
VOUTVINP
VINM
VOUT = (VINP – VINM) * AolVOUT / Aol – VINP = -VINM
If Aol = ∞ (for an Ideal Op Amp) then:-VINP = -VINM
orVINP = VINM
6 of 78
Ideal Operational Amplifier
For Ideal Op Amp
With Feedback and High Open Loop Gain:
+IN is forced to equal -IN
Non-Inverting Configuration
Irf = (Vout - Vin) / RF
Iri = Vin / RI
Iin- = 0A
Irf = Iri
(Vout - Vin) / RF = Vin / RI
Vout / Vin = 1 + RF/RI
-
+
Ideal Op Amp
Vout
RF 90kRI 10k
Vin 1 Vin
IrfIri
Iin- = 0A
1V 10V
7 of 78
Ideal Operational Amplifier
-
+
Ideal Op Amp
Vout
RF 90kRI 10k
Vin 1
Gnd
IrfIri
Iin- = 0A
0V -9V
Irf = (Vout - 0V) / RF
Iri = (0V-Vin) / RI
Iin- = 0A
Irf = Iri
(Vout - 0V) / RF = (0V-Vin) / RI
Vout / Vin = -RF/RI
For Ideal Op Amp
With Feedback and High Open Loop Gain:
+IN is forced to equal -IN
Inverting Configuration
8 of 78
Intuitive AC Op Amp Model
+
-
K(f)
VDIFF
IN+
IN-
RIN
RO
VO
VOUTx1
9 of 78
Input SpecificationsInput Bias Current (Ib) & Input Offset Current (Ios)Input Offset Voltage (Vos)Power Supply Rejection Ratio (PSRR): Referred-To-Input VosCommon 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)
10 of 78
Input Bias Current (Ib), Input Offset Current (Ios)
Ib = 5pAIos = 4pA
Polarity is + or –Current into or out of inputs
-
+
Ideal Op Amp
Ib- 3p
Ib+ 7p
Vout
Ib = Ib+ + Ib-
2
Ib = 7pA + 3pA
2 = 5pA
Ios = Ib+ - Ib-
Ios = 7pA - 3pA = 4pA
11 of 78
Input Bias Current (Ib), Input Offset Current (Ios)
25C Specs in TableOften Curves for Temperature Specs Polarity is + or –
12 of 78
-
+
Ideal Op Amp
VoutR1 1M
+Vin
R2 1M
R3 1M
VIb 5.5u
Vout error = 11uV
Simplif ied VIb Model
VIb = VIb+ - VIb-
Non-Invverting Gain Creates Vout error
-
+
Ideal Op Amp
VoutRs 1M
+
Vin
RF 1M
RI 1M VIb- 1.5u
VIb+ 7u
Vout error = 11uV
Ib flow s through feedback and input resistors
Model as VIb+ and VIb-
Inverting and Non-Inverting Gains create Vout error
-
+
Ideal Op Amp
Ib- 3p
Ib+ 7p
VoutRs 1M
RF 1M
RI 1M
Vinm
Vinp
Vinm = 1.5uV
Vinp =7uV
Ib f low s through feedback and input resistors
View Vout and Vin as low impedance
Vinm = Ib- (RF // RI)
Vinp = Ib+ (Rs)
Vin
Vout
Input Bias Current (Ib) Vout Error
-
+
Idelal Op AmpIb- 3p
Ib+ 7p
VoutRs 1M
+
Vin
RF 1M
RI 1M
Ib causes errors at Vout
1
2
3 4
13 of 78
Input Offset Voltage (Vos) Vout Error
25C Specs in TableOften Histograms show distribution of VosPolarity is + or –
-
+
Ideal Op Amp
Vout
RF 1M
RI 1M
Vos 25u
Vout error = 50uV
Input Offset Voltage
Creates Vout error
14 of 78
Input Offset Voltage (Vos) Drift Vout Error
Vos Drift Specs in TableOften Histograms show
distribution of Vos DriftPolarity is + or -
-
+
Ideal Op Amp
Vout
RF 1M
RI 1M
Vos 25uVos_drift 60u
Vout error = 170uV
Initial Vos + Vos Drift creates Vout error
Operating Temperatue = 25C to 85CT = 85C - 25C = 60C
Vos_drift = T dVos
dT
Vos_drift = 60C 1uV/C = 60uV
15 of 78
Power Supply Rejection Ratio (PSRR) Vout Error
DC PSRR in TableDC PSRR Drift in TablePolarity is + or -
PSRR is an RTI (Referred-To-Input) specification Appears as Input Offset Voltage
-
+ +
Ideal Op Amp
Vcc 5
delta_Vcc 500m
RF 1M
RI 1M
Vos_PSRR 10u
Vout
Vout error = 20uV
PSSR DC = 20uV/V
delta_Vcc = 500mV (DC change in Vcc)
Vos_PSRR = PSSR DC delta_Vcc
Vos_PSRR = 20uV/V 500mV = 10uV
PSSR reflects as Vos_PSRR & creates Vout error
16 of 78
20kHz
Power Supply Rejection Ratio (PSRR) Vout ErrorAC PSRR in Curve
PSRR is an RTI (Referred-To-Input) specification Appears as Input Offset Voltage
-
+ +
Ideal Op Amp
Vcc 5
R4 1M
R5 1M
Vout
+ delta_Vcc_ac+
Vos_PSRR_ac
Vout error = 20uVpp @ 20kHz
100mVpp @ 20kHz10uVpp @ 20kHz
PSSR AC @ 20kHz = 100uV/V
delta_Vcc_ac = 100mVpp (AC change in Vcc @ 20kHz)
Vos_PSRR_ac = PSSR AC delta_Vcc_ac
Vos_PSRR_ac = 100uV/V 100mVpp = 10uVpp
Frequency of analysis = 20kHz
PSRR AC @ 20kHz = 80dB
Convert PSRR (dB) to PSRR (Linear Gain):
10(80dB/20) = 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
PSRR AC reflects as Vos_PSRR_ac & creates Vout error
17 of 78
Common Mode Voltage Range (Vcm)
Common Mode Voltage Range
For: Non-Inverting Gain Vinp = Vinm
So: Vin_CM = Vin
From Vcm spec Vin must stay 2V aw ay from either
supply for op amp to operate as a linear gain block
Vin_CM = Voltage Common to Vinp & Vinm
VcmSame for DC & ACAC peak voltage < Vcm
-
+ +
Ideal Op Amp
Vcc 15
Vee 15
+
Vin
RF 1M
Vout
Vinm
Vinp
V = 2V max
V = 2V max-13V < Vin < +13V
18 of 78
Common Mode Rejection Ratio (CMRR) Vout Error
CMRR is an RTI (Referred-To-Input) specification Appears as Input Offset Voltage
CMRR DC in TablePolarity is + or -
CMRR DC reflects as Vos_CMRR & creates Vout error
CMRR DC = 0.316uV/V
Vin = 5V for Non-Inverting Gain Vin =Vcm
Vcm = 5V
Vos_CMRR = CMRR DC Vcm
Vos_CMRR = 0.316uV/V 5V = 1.58uV
-
+ +
Ideal Op Amp
V1 15
V2 15
RF 1M
RI 1M
Vout
Vin 5 Vos_CMRR 1.58u
Vout error = 3.16uV
CMRR DC = 130dB
Convert CMRR (dB) to CMRR (Linear Gain):
10(130dB/20) = 3.16e+6
CMRR is an attenuation so 1V gets attenuated by x3.16e+6
1/3.16e+6 = 3.16e-7V/V
Now convert numerator to uV:
(31.6e-7V) (1uV/1e-6V) = 3.16e-7uV / 1e-6 = 0.316uV:
CMRR DC = 0.316uV/V
19 of 78
Common Mode Rejection Ratio (CMRR) Vout Error
CMRR AC = 10uV/V @1kHz
Vin = 20Vpp for Non-Inverting Gain Vin =Vcm_ac
Vcm_ac = 20Vpp
Vos_CMRR_ac = CMRR AC Vcm_ac
Vos_CMRR_ac = 10uV/V 20Vpp = 200uVpp
CMRR AC reflects as Vos_CMRR_ac & creates Vout error
CMRR is an RTI (Referred-To-Input) specification Appears as Input Offset Voltage
AC CMRR in Curve
-
+ +
Ideal Op Amp
Vcc 15
Vee 15
RF 1M
RI 1M
Vout
+
Vin
+
Vos_CMRR_ac
Vout error = 400uVpp @ 1kHz
20Vpp @ 1kHz
200uVpp @ 1kHz
Frequency of Analysis = 1kHz
CMRR AC = 100dB @ 1kHz
Convert CMRR (dB) to CMRR (Linear Gain):
10(100dB/20) = 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
20 of 78
Vee
Vcc
-
+ +
Ideal Op Amp
Rdiff
Rcm
Rcm
Ccm
Cdiff
Ccm
-In
+In
Vout
RF 1M
RI 1M
Cin
Small Signal
Input Parasitics
Small SignalInput Parasitics
Rdiff > 200G for Bipolar InputsRcm > 40M for Bipolar InputsEven greater for JFET or MOSFET inputs
Ccm and Cdiff can be a problem:Ccm and Cdiff form CinCin & RF form a Loop Gain pole unwanted oscillations depending upon UGBW and value of RF.
Ccm, Cdiff in TableRcm, Rdiff in Table if specified
21 of 78
Input Noise: Current, Voltage (in, en)
22 of 78
Op Amp Noise Model
OPA277 Data
VVNN
IINN--IINN
++
Noise Model
(IN+ and IN- are not correlated)
Tina Simplified Model
*n
VU
1
*fA
U2
-
+
IOP1
INVN
23 of 78
Understanding The Spectrum:Total Noise Equation (Current or Voltage)
0.1 1 10 100 1k 10k1
10
100
1k
10k
100k
)V
olt
age
No
ise
(nV
/H
z
1/f Noise Region(Pink Noise Region)
White Noise Region(Broadband Noise Region)
en1/f calculation
fHfL
Frequency (Hz)
enBB calculation
enT = √[(en1/f)2 + (enBB)2]
where:enT = Total rms Voltage Noise in volts rms en1/f = 1/f voltage noise in volts rmsenBB = Broadband voltage noise in volts rms
24 of 78
fP fBF
Small Signal BW
Noise BW
Skirt of1-Pole FilterResponse
Brickwall
Frequency (f)
0
0.1fP 10fP
-20
-40
-80
Fil
ter
Att
en
ua
tio
n (
dB
)
Skirt of2-Pole FilterResponse
Skirt of3-Pole FilterResponse
Real Filter Correction vs Brickwall Filter
where: fP = roll-off frequency of pole or polesfBF = equivalent brickwall filter frequency
25 of 78
Number of Poles in Filter
KnAC Noise Bandwidth Ratio
1 1.57
2 1.22
3 1.16
4 1.13
5 1.12
AC Noise Bandwidth Ratios for nth Order Low-Pass Filters
Real Filter Correction vs Brickwall Filter
BWn = (fH)(Kn) Effective Noise Bandwidth
26 of 78
Broadband Noise Equation
enBB = (eBB)(√[BWn])
where:enBB = Broadband voltage noise in volts rmseBB = Broadband voltage noise density ; usually in nV/√HzBWn = Noise bandwidth for a given system
BWn = (fH)(Kn)
where:BWn = noise bandwidth for a given systemfH = upper frequency of frequency range of operationKn = “Brickwall” filter multiplier to include the “skirt” effects of a low pass filter
eBB
27 of 78
1/f Noise Equation
en1/f = (e1/f@1Hz)(√[ln(fH/fL)])
where:en1/f = 1/f voltage noise in volts rms over frequency range of operatione1/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
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
e1/f@1Hz
28 of 78
Example Noise Calculation
Given:OPA627 Noise Gain of 101
Find (RTI, RTO): Voltage NoiseCurrent NoiseResistor Noise
V1 15
V2 15
-
+ +U1 OPA627/BB
R1 100kR2 1k+
VG1
VF1
29 of 78
Unity Gain Bandwidth = 16MHz
Closed Loop Bandwidth = 16MHz / 101 = 158kHz
Voltage Noise Spectrum and Noise Bandwidth
50nV/rt-Hz
5nV/rt-Hz
30 of 78
Example Voltage Noise Calculation
Voltage Noise Calculation:
Broadband Voltage Noise Component:BWn ≈ (fH)(Kn) (note Kn = 1.57 for single pole)BWn ≈ (158kHz)(1.57) =248kHz
enBB = (eBB)(√BWn)enBB = (5nV/√Hz)(√248kHz) = 2490nV rms
1/f Voltage Noise Component:e1/f@1Hz = (e1/f@f)(√f)e1/f@1Hz = (50nV/√Hz)(√1Hz) = 50nV
en1/f = (e1/f@1Hz)(√[ln(fH/fL)]) Use fH = BWn
en1/f = (50nV)(√[ln(248kHz/1Hz)]) = 176nV rms
Total Voltage Noise (referred to the input of the amplifier):enT = √[(en1/f)2 + (enBB)2]enT = √[(176nV rms)2 + (2490nV rms)2] = 2496nV rms
31 of 78
Example Current Noise Calculation
Note: This example amp doesn’t have 1/f component for current noise.
Gain
Req = R1 || Rf
*
Rf 3k
-
+
IOP1
VF1
*fA
U2
R1 1k Rf 3k
-
+
IOP1
VF1
*fA
U2
R1 1k
en-out= Gain x (in)x(Req)en-in= (in)x(Req)
32 of 78
Example Current Noise Calculation
Broadband Current Noise Component:BWn ≈ (fH)(Kn)BWn ≈ (158kHz)(1.57) =248kHz
inBB = (iBB)(√BWn)inBB = (2.5fA/√Hz)(√248kHz) = 1.244pA rms
Req = Rf || R1 = 100k || 1k = 0.99k
eni = (In)( Req) = (1.244pA)(0.99k) = 1.23nV rms
Since the Total Voltage noise is envt = 2496nV rms the current noise can be neglected.
neglect
33 of 78
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)
To convert Temperature Kelvin to
TK = 273.15oC + TC
34 of 78
Noise Spectral Density vs. Resistance
Resistance (Ohms)
Noi
se S
pect
ral D
ensi
ty v
s. R
esis
tanc
e
nV/r
t-H
z
10 100 1 10 3 1 10 4 1 10 5 1 10 6 1 10 70.1
1
10
100
1 10 3468.916
0.347
4 1.38065 1023 25 273.15( ) X 10
9
4 1.38065 1023 125 273.15( ) X 10
9
4 1.38065 1023 55 273.15( ) X 10
9
10710 X
1000
10 100 1 103 1 104 1 105 1 106 1 1070.1
1
10
100
1 103468.916
0.347
4 1.380651023 25 273.15( ) X 10
9
4 1.380651023 125 273.15( ) X 109
4 1.380651023 55 273.15( ) X 109
10710 X
25C
125C
-55C
en density = √√ (4kTKR)
Resistor Noise – Thermal Noise
35 of 78
eenr = √(4kT = √(4kTKKRRΔΔf) f)
where:where:
R = Req = R1||RfR = Req = R1||Rf
ΔΔf = BWf = BWnn
eenr = √(4 ( = √(4 (1.38E-23) (273 + 25) (0.99k)((273 + 25) (0.99k)(248kHz)) = 2010nV rms
Example Resistor Noise Calculation
Gain
Req = R1 || Rf
*
R1 2kR2 1k
-
+
IOP1
VF1
*nV
U1
*nV
U1
RfR1
en-out= Gain x (√(4kTR√(4kTRΔΔf)f))en-in= √(4kTR√(4kTRΔΔf)f)
36 of 78
Total Noise Calculation
Voltage Noise From Op-Amp RTI:env = 2510nV rms
Current Noise From Op-Amp RTI (as a voltage):eni = 1.24nV rms
Resistor Noise RTI:enr = 2020nV rms
Total Noise RTI:en in = √((2510nV)2 + ((1.2nV)2 + ((2010nV)2) = 3216nV rms
Total Noise RTO:en out = en in x gain = (3216nV)(101) = 325uV rms
37 of 78
Calculating Noise Vpp from Noise Vrms
Peak-to-PeakAmplitude
Probability of Havinga Larger Amplitude
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%
Relation of Peak-to-Peak Value of AC Noise Voltage to rms Value
*Common Practice is to use: Peak-to-Peak Amplitude = 6 X rms
38 of 78
Voltage Noise (f = 0.1Hz to 10Hz) Low Frequency
Low frequency noise spec and curve:Over specific frequency range: 0.1Hz < f < 10HzGiven as Noise Voltage in pp units
Measured After Bandpass Filter:
0.1Hz Second−Order High−Pass
10Hz Fourth−Order Low−Pass
39 of 78
Frequency Response Specifications
Open Loop Gain (Aol) & PhaseSlew Rate (SR)Total Harmonic Distortion + Noise (THD+N)Settling Time (ts)
40 of 78
Open Loop Gain & Phase
Gain-Bandwidth Product = UGBW (Unity Gain Bandwidth)
G=1 Stable Op Amps
5.5MHz
Open-Loop Voltage Gain at DCLinear operation conditions NOT the same as Voltage Output Swing to Rail
41 of 78
Vout/Vin:Gain Accuracy & Frequency Response
fcl
1/Beta
Vout/Vin
-
+
Real Op Amp
Vout
+
Vin
R2 9k
R3 1k
Aol at any Frequency:
Aol_f = UGBW / f
Aol @ 1kHz = 5.5MHz / 1kHz = 5500
Aol @ 1kHz = 20LOG10(5500) = 74.8dB
Gain Accuracy at any frequency:
Frequency of analysis for Gain Accuracy = 1kHz
Vout / Vin = Aol
1+Aol
Vout / Vin = 5500 / (1+ 5500 0.1)
Vout/ / Vin = 9.98185
Vout / Vin ideal = 10
Gain Error = ((10 - 9.98185) / 10) 100 = 0.18%
Vout/ Vin Frequency Response
1/ = 10
20LOG10(10) = 20dB
Vout / Vin = Aol
1+Aol
fcl is w here Aol = 1
f > fcl: Loop Gain < 1 so Vout/Vin = Aol
42 of 78
Slew Rate Slew Rate Measurement:10% to 90% of Vout
43 of 78
Slew Rate &Full Power Bandwidth orMaximum Output Voltage vs Frequency
Maximum Rate of change of sinew ave is at zero cross
Highest Frequency Op Amp can track sinew ave limited by:
Frequency, Output Voltage, Slew Rate
SR (V/us) = 2fVop(1e-6)
w here:
SR = Slew Rate in V/us
f = frequency of interest
Vop = Vout peak voltage
Given Slew Rate = 2V/us
What is max f for sinew ave of 2.5Vpp?
SR (V/us) = 2fVop(1e-6)
2 = 2f(2.5Vpp/2)(1e-6)
Solving for f:
fmax = 254.6kHz
44 of 78
THD + Noise
Larger Closed Loop Gain Loop Gain to correct for Op Amp Non-Linearities and Noise
45 of 78
THD + Noise = 1% Example
Fundamental f = Input FrequencyFundamental f = 99% Vout AmplitudeHarmonics due to Op Amp non-linearitiesNoise due to Op Amp Input Noise (en, in)
Harmonics + Noise < 1% of Vout
46 of 78
Settling Time
Note: Settling Time includes Slew Rate time
SlewRate
47 of 78
Settling Time
Settling TimeLarge Signal effects:Slew Rate
Small Signal effectsLarge Gain = Less closed loop BandwidthLarge Gain = Less Loop Gain (AolB) to correct for errorsLarge Gain = Longer Settling Time
48 of 78
Output Specifications
Voltage Output Swing from RailShort Circuit Current (Isc)Open Loop Output Impedance (Zo)Closed Loop Output Impedance (Zout)Capacitive Load Drive
49 of 78
Voltage Output Swing From Rail
Loaded Vout swing from RailHigher Current Load Farther from RailHigher Current Load Larger VsatVsat = Vs - Vout
+25C Curve:Op Amp Aol is degraded if on curveOp Amp Aol is okay if left of curve
12
1
2
50 of 78
Short Circuit Current (Isc)
Output shorted Current Limit engagedFor Graph shown TJ max is okayIf using larger voltages (i.e. +5V, Gnd) use Short-Circuit Current values & analyze power dissipation and TJ max
51 of 78
Open Loop Output Impedance (Zo)
Closed Loop Output Impedance (Zout)
Capacitive Load Drive
52 of 78
Op Amp Model for Derivation of ROUT
+
-
RDIFF
xAol
RO-IN
+IN
-
+
VE
Op Amp Model
1A
VOUT
VO
RF
RI
IOUTVFB
ROUT = VOUT/IOUTFrom: Frederiksen, Thomas M. Intuitive Operational Amplifiers. McGraw-Hill Book Company. New York. Revised Edition. 1988.
Definition of Terms:
RO = Op Amp Open Loop Output Resistance
ROUT = Op Amp Closed Loop Output Resistance
ROUT = RO / (1+Aolβ)
53 of 78
ROUT vs RO
• RO does NOT change when Closed Loop feedback is used
• ROUT is the effect of RO, Aol, and β controlling VO
– Closed Loop feedback (β) forces VO to increase or decrease as needed to accommodate VO loading
– Closed Loop (β) increase or decrease in VO appears at VOUT as a reduction in RO
– ROUT increases as Loop Gain (Aolβ) decreases
Note: Some op amps have ZO characteristics other than pure resistance (RO) – consult data sheet / manufacturer.
54 of 78
RO & CL: Modified Aol Model
+
-
+
- RO
Data Sheet Aol CL
+
-
RFRI
100k 100k
1F
OPA452
VIN
VOUT
28.7
Extra Pole in Aol Plot due to RO & CL:
fpo1 = 1/(2∙П∙RO∙CL)
fpo1 = 1/(2∙П∙28.7Ω∙1μF)
fpo1 = 5.545kHz
Create a new “Modified Aol” Plot
55 of 78
RO & CL: OPA542 Modified Aol First Order
1 10 100 1K 10K 100k 1M 10M
Frequency (Hz)
Ga
in (
dB
)
-60
-40
-20
0
20
40
60
80
100
120OPA452
Aol
Modified Aoldue to CL
fpo1
1/ fcl
40dB/DecadeRate-Of-Closure
STABLE
56 of 78
Zo (Open Loop Output Impedance)Cap Load Drive
OPA376 and many other Single Supply Op Amps Open Loop Output Impedance is not Purely Resistive
As Cap Load increases Loop Gain Phase Margin decreases and we see the transient response for Cap Load increase in overshoot for OPA376
For about 500pF Load Capacitance Small-Signal Overshoot is 50%
57 of 78
2nd Order Transient Curves
Fro
m:
Do
rf,
Ric
ha
rd C
. M
od
ern
Co
ntr
ol
Sy
ste
ms
. A
dd
iso
n-W
es
ley
P
ub
lis
hin
g C
om
pa
ny
. R
ea
din
g,
Ma
ss
ac
hu
se
tts
. T
hir
d E
dit
ion
, 1
98
1.
Signal overshoot of 50% or normalized signal output of 1.5 yields a Damping ratio ( ) of 0.2
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2nd Order Damping Ratio vs Phase MarginF
rom
: D
orf
, R
ich
ard
C.
Mo
de
rn C
on
tro
l S
ys
tem
s.
Ad
dis
on
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sle
y
Pu
bli
sh
ing
Co
mp
an
y.
Re
ad
ing
, M
as
sa
ch
us
ett
s.
Th
ird
Ed
itio
n,
19
81
.
Damping ratio ( ) of 0.2 yields 23.5 degrees of phase margin for AC Loop Stability
23.5o
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Closed Loop Output Impedance
For Bipolar, Emitter-Follower Output Op amps like OPA177, open loop output impedance = RO (purely resistive inside UGBW)
Since ROUT = RO/(1+Aol) and RO is resistive ROUT looks opposite of Aol
and increase at higher frequencies
Closed Loop Output impedance gives an indication of what source impedance the closed loop op amp will have to drive loads over frequency
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Power Supply Specifications
Specified Voltage Range (VS)
Operating Voltage Range (VS)
Quiescent Current (IQ)
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Specified and Operating Voltage Range (VS)
For 2.2V < VS < 5.5V data sheet specifications will be met
For 2 < VS < 2.2V the op amp will still function but all data sheet
specifications may not be met i.e. Output Swing to Rail, Aol, etc may be
degraded
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Quiescent Current (IQ)
Quiescent Current:Supply Current to operate the op ampDoes NOT include load current
-
+ +Real Op Amp
IQ
IQ
-Vs
+Vs
Vout
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Temperature Range Specifications
Specified Range
Operating Range
Thermal Resistance (JA)
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Specified and Operating Temperature Range
For -40C < TA < +125C data sheet specifications will be met
For +125C < TA < +150C the op amp will still function but all data sheet
specifications may not be met i.e. Output Swing to Rail, Aol, etc may be
degraded
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Thermal Resistance (JA)
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Thermal Resistance (JA)
JA will be used with ambient temperature TA and internal
total power dissipation PD to compute maximum op amp junction temperature TJ
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Thermal Model
PD
RθJA
TA
TJ
TA
Thermal model with no heat sink
Analogous to an electrical circuit
TJ= PD( RθJA) + TA
T – is analogous to voltage
R – is analogous to resistance
P – is analogous to current
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)
Vout
+Vs
-Vs
IQ
IQ
-
+ +Real Op Amp
PIQ = [+Vs - (-Vs) ] IQ
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DC Normal Maximum Power Dissipation in Output Stage (POUT)
-
+ +Real Op Amp
IQ
IQ
-Vs
+Vs
VoutVin
RFRI
RL
Iout_DC
POUT_DC = Vs2
4 RL
Vout = 1
2 Vs
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DC Short Circuit Maximum Power Dissipation in Output Stage (POUT)
-
+ +Real Op Amp
IQ
IQ
-Vs
+Vs
VF1
RFRI
+
VG1
Isc
POUT_SHORT = Vs Isc
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-
+ +Real Op Amp
IQ
IQ
-Vs
+Vs
Vout
RFRI
RL
+
Vin
Iout_AC
Pc(Push-Pull) vs Vload for an AC Sinusoidal Signal
0
0.05
0.1
0.15
0.2
0.25
0.3
0 1 2 3 4 5
V(load) peak AC Sinusoidal Voltage
P(P
us
h P
ull
Ou
tput
Tra
ns
isto
rs)
AC Normal Maximum Power Dissipation in Output Stage (POUT)
For AC Sinusoidal Signals
POUT_AC = 2 Vs2
2 RL
Vout peak = 2 Vs
POUT_AC = 2 Vs2
2 RL
Vout peak = 2 Vs
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-
+ +Real Op Amp
IQ
IQ
Vcc 5
Vout
RFRI
RL
+
Vin
Iout_AC
AC Normal Maximum Power Dissipation in Output Stage (POUT)
For AC Sinusoidal Signals
-
+ +Real Op Amp
IQ
IQ
-Vs 2.5
+Vs 2.5
Vout
RFRI
RL
+
Vin
Iout_AC
AC Maximum Power Dissipation Formula based on symmetrical dual supplies
To use formula for single supply circuits set +Vs = +(Vcc/2) and -Vs = -(Vcc/2)
as shown.
Vcc
-Vs = -(Vcc/2)
+Vs = (Vcc/2)
POUT_AC = 2 Vs2
2 RL
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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 MaxFor 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 SearchVoltage? Current? Speed?
+
-
Current?
Voltage?
Sp
eed
?
Supply Voltage?Input Offset Voltage?Output Swing Voltage?
Supply Current?Output Current?Input Bias Current?
SSBW @ G=?Slew Rate? SR(V/us)=2pifVOP1e-6
where: f=Hz
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References
78 of 78
References
Jim Karki, Senior Applications Engineer, Texas Instruments
“Understanding Operational Amplifier Specifications” White Paper: SLOA011
John Brown, Strategic Marketing Engineer (Retired), Texas Instruments
“How to Use TI/BB Data Sheet Specs for Op Amps and IAs” Internal White Paper
Art Kay, Senior Applications Engineer, Texas Instruments
“Analysis and Measurement of Intrinsic Noise in Op Amp Circuits: Parts 1-7”
http://www.en-genius.net/site/zones/audiovideoZONE/technical_notes/avt_022508
Tim Green, Senior Applications Engineer, Texas Instruments
“Operational Amplifier Stability: Parts 1-9 of 15”
http://www.en-genius.net/site/zones/acquisitionZONE/technical_notes/acqt_121106