LMV793/LMV794 88 MHz, Low Noise, 1.8V CMOS Input, Decompensated Operational AmplifiersAugust 2007
LMV793/LMV794
88 MHz, Low Noise, 1.8V CMOS Input, DecompensatedOperational Amplifiers
General Description
The LMV793 (single) and the LMV794 (dual) CMOS inputoperational amplifiers offer a low input voltage noise density
while consuming only 1.15 mA (LMV793) ofof 5.8 nV/
quiescent current. The LMV793/LMV794 are stable at a gainof 10 and have a gain bandwidth product (GBW) of 88 MHz.The LMV793/LMV794 have a supply voltage range of 1.8V to5.5V and can operate from a single supply. The LMV793/LMV794 each feature a rail-to-rail output stage capable ofdriving a 600Ω load and sourcing as much as 60 mA of cur-rent.
The LMV793/LMV794 provide optimal performance in lowvoltage and low noise systems. A CMOS input stage, withtypical input bias currents in the range of a few femto-Am-peres, and an input common mode voltage range, whichincludes ground, make the LMV793/LMV794 ideal for lowpower sensor applications where high speeds are needed.The LMV793/LMV794 are manufactured using National’s ad-vanced VIP50 process. The LMV793 is offered in either a 5-Pin SOT23 or an 8-Pin SOIC package. The LMV794 is offeredin either the 8-Pin SOIC or the 8-Pin MSOP.
Features
(Typical 5V supply, unless otherwise noted)
5.8 nV/√Hz■Input referred voltage noise
100 fA■Input bias current
88 MHz■Gain bandwidth product
■Supply current per channel
1.15 mA—LMV793
1.30 mA—LMV794
■Rail-to-rail output swing
25 mV from rail—@ 10 kΩ load
45 mV from rail—@ 2 kΩ load
■Guaranteed 2.5V and 5.0V performance
0.04% @1 kHz, 600Ω■Total harmonic distortion
−40°C to 125°C■Temperature range
Applications
■
■■■■■
ADC interface
Photodiode amplifiersActive filters and buffersLow noise signal processingMedical instrumentation
Sensor interface applications
Typical Application
Photodiode Transimpedance Amplifier
20216369
Input Referred Voltage Noise vs. Frequency
20216339
© 2007 National Semiconductor Corporation202163www.national.com
元器件交易网www.cecb2b.com
LMV793/LMV794Absolute Maximum Ratings (Note 1)
If Military/Aerospace specified devices are required,please contact the National Semiconductor Sales Office/Distributors for availability and specifications.ESD Tolerance (Note 2)
Soldering Information
Infrared or Convection (20 sec)
Wave Soldering Lead Temp (10 sec)
235°C260°C
Operating Ratings
2000V
200V±0.3V6.0V
V+ +0.3V, V− −0.3V
−65°C to 150°C
+150°C
(Note 4)
Temperature Range (Note 3)Supply Voltage (V+ – V−) −40°C ≤ TA ≤ 125°C 0°C ≤ TA ≤ 125°C
(Note 1)
−40°C to 125°C2.0V to 5.5V1.8V to 5.5V180°C/W190°C/W236°C/W
Human Body Model Machine Model
VIN Differential
Supply Voltage (V+ – V−)Input/Output Pin VoltageStorage Temperature RangeJunction Temperature (Note 3)
Package Thermal Resistance (θJA (Note 3)) 5-Pin SOT23 8-Pin SOIC 8-Pin MSOP
2.5V Electrical Characteristics
SymbolVOSTC VOSIB
Parameter
Input Offset VoltageInput Offset Average Drift(Note 7)
Input Bias Current
LMV793LMV794VCM = 1.0V(Notes 8, 9)
Unless otherwise specified, all limits are guaranteed for TA = 25°C, V+ = 2.5V, V− = 0V, VCM = V+/2 = VO. Boldface limits apply atthe temperature extremes.
Conditions
Min(Note 6)
−40°C ≤ TA ≤ 85°C−40°C ≤ TA ≤ 125°C
IOSCMRRPSRR
Input Offset Current
(Note 9)
8075807580−0.3-0.3
LMV793LMV794
VOUT = 0.15V to 2.2V,RL = 10 kΩ to V+/2
VOUT
Output Swing High
RL = 2 kΩ to V+/2RL = 10 kΩ to V+/2
Output Swing Low
RL = 2 kΩ to V+/2RL = 10 kΩ to V+/2
IOUT
Output Short Circuit Current
Sourcing to V−
VIN = 200 mV (Note 10)Sinking to V+
VIN = –200 mV (Note 10)
www.national.com
2
Typ(Note 5)0.1−1.0−1.80.050.05109410098 9892110252030154715
Max(Note 6)±1.35±1.65 1251100
Units
mVμV/°C
pA
fAdB
Common Mode Rejection Ratio0V ≤ VCM ≤ 1.4VPower Supply Rejection Ratio
2.0V ≤ V+ ≤ 5.5V, VCM = 0V1.8V ≤ V+ ≤ 5.5V, VCM = 0V
dB
1.51.5 7582657175786567
mAmV fromrail
V
CMVRAVOL
Input Common-Mode VoltageRange
Open Loop Gain
CMRR ≥ 60 dBCMRR ≥ 55 dBVOUT = 0.15V to 2.2V,RL = 2 kΩ to V+/2
858082788884 352875
dB
元器件交易网www.cecb2b.com
LMV793/LMV794IS
Supply Current Per AmplifierLMV793LMV794
0.951.13224886.20.010.01
1.301.651.501.85
mA
SRGBWPeninTHD+N
Slew Rate
Gain Bandwidth ProductInput-Referred Voltage NoiseInput-Referred Current NoiseTotal Harmonic Distortion +Noise
AV = +10, Rising (10% to 90%)AV = +10, Falling (90% to 10%)AV = +10, RL = 10 kΩf = 1 kHzf = 1 kHz
f = 1 kHz, AV = 1, RL = 600Ω
V/μsMHznV/pA/%
5V Electrical Characteristics
SymbolVOSTC VOSIB
Parameter
Input Offset VoltageInput Offset Average Drift(Note 7)
Input Bias Current
LMV793LMV794
(Note 4)
Unless otherwise specified, all limits are guaranteed for TA = 25°C, V+ = 5V, V− = 0V, VCM = V+/2 = VO. Boldface limits apply atthe temperature extremes.
Conditions
Min(Note 6)
−40°C ≤ TA ≤ 85°C−40°C ≤ TA ≤ 125°C
IOSCMRRPSRR
Input Offset Current
(Note 9)
8075807580−0.3-0.3
LMV793LMV794
VOUT = 0.3V to 4.7V,RL = 10 kΩ to V+/2
VOUT
Output Swing High
RL = 2 kΩ to V+/2
LMV793LMV794
RL = 10 kΩ to V+/2
Output Swing Low
RL = 2 kΩ to V+/2
LMV793LMV794
RL = 10 kΩ to V+/2
858082788884
Typ(Note 5)0.1−1.0−1.80.10.11010010098 9789110353525424520
Max(Note 6)±1.35±1.65 1251100
dB
44 758275826571757880836567
mV fromrail
VUnits
mVμV/°C
VCM = 2.0V(Notes 8, 9)
pA
fAdB
Common Mode Rejection Ratio0V ≤ VCM ≤ 3.7VPower Supply Rejection Ratio
2.0V ≤ V+ ≤ 5.5V, VCM = 0V1.8V ≤ V+ ≤ 5.5V, VCM = 0V
CMVRAVOL
Input Common-Mode VoltageRange
Open Loop Gain
CMRR ≥ 60 dBCMRR ≥ 55 dBVOUT = 0.3V to 4.7V,RL = 2 kΩ to V+/2
dB
3www.national.com
元器件交易网www.cecb2b.com
LMV793/LMV794IOUT
Output Short Circuit Current
Sourcing to V−
VIN = 200 mV (Note 10)Sinking to V+
VIN = –200 mV (Note 10)
4537106
60211.151.303528885.80.010.01
1.401.751.702.05
mA
IS
Supply Current per AmplifierLMV793
LMV794 per Channel
mA
SRGBWPeninTHD+N
Slew Rate
Gain Bandwidth ProductInput-Referred Voltage NoiseInput-Referred Current NoiseTotal Harmonic Distortion +Noise
AV = +10, Rising (10% to 90%)AV = +10, Falling (90% to 10%)AV = +10, RL = 10 kΩf = 1 kHzf = 1 kHz
f = 1 kHz, AV = 1, RL = 600Ω
V/μsMHznV/pA/%
Note 1:Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device isintended to be functional, but specific performance is not guaranteed. For guaranteed specifications and the test conditions, see the Electrical CharacteristicsTables.
Note 2:Human Body Model, applicable std. MIL-STD-883, Method 3015.7. Machine Model, applicable std. JESD22-A115-A (ESD MM std. of JEDEC)Field-Induced Charge-Device Model, applicable std. JESD22-C101-C (ESD FICDM std. of JEDEC).
Note 3:The maximum power dissipation is a function of TJ(MAX), θJA. The maximum allowable power dissipation at any ambient temperature isPD = (TJ(MAX) - TA)/θJA. All numbers apply for packages soldered directly onto a PC Board.
Note 4:Electrical Table values apply only for factory testing conditions at the temperature indicated. Factory testing conditions result in very limited self-heatingof the device such that TJ = TA. No guarantee of parametric performance is indicated in the electrical tables under conditions of internal self-heating where TJ >TA.
Note 5:Typical values represent the most likely parametric norm as determined at the time of characterization. Actual typical values may vary over time and willalso depend on the application and configuration. The typical values are not tested and are not guaranteed on shipped production material.
Note 6:Limits are 100% production tested at 25°C. Limits over the operating temperature range are guaranteed through correlations using the statistical qualitycontrol (SQC) method.
Note 7:Offset voltage average drift is determined by dividing the change in VOS by temperature change.Note 8:Positive current corresponds to current flowing into the device.Note 9:Input bias current and input offset current are guaranteed by design
Note 10:The short circuit test is a momentary test, the short circuit duration is 1.5 ms.
www.national.com4
元器件交易网www.cecb2b.com
LMV793/LMV794Connection Diagrams
5-Pin SOT23 (LMV793)
8-Pin SOIC (LMV793)
8-Pin SOIC/MSOP (LMV794)
Top View
20216301
Top View
20216385
Top View
20216302
Ordering Information
Package5-Pin SOT23
Part NumberLMV793MFLMV793MFXLMV793MA
8-Pin SOIC
LMV793MAXLMV794MALMV794MAX
8-Pin MSOP
LMV794MMLMV794MMX
Package Marking
AS4ALMV793MALMV794MAAN4A
Transport Media1k Units Tape and Reel3k Units Tape and Reel
95 Units/Rail2.5k Units Tape and Reel
95 Units/Rail2.5k Units Tape and Reel1k Units Tape and Reel3.5k Units Tape and Reel
MUA08AM08ANSC Drawing
MF05A
5www.national.com
元器件交易网www.cecb2b.com
LMV793/LMV794Typical Performance Characteristics
= 5V, VCM = V+/2.
Supply Current vs. Supply Voltage (LMV793)
Unless otherwise specified, TA = 25°C, V– = 0, V+ = Supply Voltage
Supply Current vs. Supply Voltage (LMV794)
20216305
20216381
VOS vs. VCMVOS vs. VCM
2021630920216351
VOS vs. VCMVOS vs. Supply Voltage
20216311
20216312
www.national.com6
元器件交易网www.cecb2b.com
LMV793/LMV794Slew Rate vs. Supply Voltage
Input Bias Current vs. VCM
20216352
20216362
Input Bias Current vs. VCM
Sourcing Current vs. Supply Voltage
2021638720216320
Sinking Current vs. Supply VoltageSourcing Current vs. Output Voltage
2021631920216350
7www.national.com
元器件交易网www.cecb2b.com
LMV793/LMV794Sinking Current vs. Output VoltagePositive Output Swing vs. Supply Voltage
20216354
20216317
Negative Output Swing vs. Supply VoltagePositive Output Swing vs. Supply Voltage
2021631520216316
Negative Output Swing vs. Supply VoltagePositive Output Swing vs. Supply Voltage
2021631420216318
www.national.com8
元器件交易网www.cecb2b.com
LMV793/LMV794Negative Output Swing vs. Supply VoltageInput Referred Voltage Noise vs. Frequency
2021631320216339
Overshoot and Undershoot vs. CLOAD
THD+N vs. Frequency
20216330
20216326
THD+N vs. Frequency
THD+N vs. Peak-to-Peak Output Voltage (VOUT)
20216304
20216374
9www.national.com
元器件交易网www.cecb2b.com
LMV793/LMV794THD+N vs. Peak-to-Peak Output Voltage (VOUT)
Open Loop Gain and Phase
20216306
20216375
Closed Loop Output Impedance vs. Frequency
Small Signal Transient Response, AV = +10
20216353
20216332
Large Signal Transient Response, AV = +10Small Signal Transient Response, AV = +10
2021635520216357
www.national.com10
元器件交易网www.cecb2b.com
LMV793/LMV794Large Signal Transient Response, AV = +10
PSRR vs. Frequency
20216363
20216370
CMRR vs. Frequency
Input Common Mode Capacitance vs. VCM
20216356
20216376
11www.national.com
元器件交易网www.cecb2b.com
LMV793/LMV794Application Information
ADVANTAGES OF THE LMV793/LMV794
Wide Bandwidth at Low Supply Current
The LMV793/LMV794 are high performance op amps thatprovide a GBW of 88 MHz with a gain of 10 while drawing alow supply current of 1.15 mA. This makes them ideal for pro-viding wideband amplification in data acquisition applications.With the proper external compensation the LMV793/LMV794can be operated at gains of ±1 and still maintain much fasterslew rates than comparable unity gain stable amplifiers. Theincrease in bandwidth and slew rate is obtained without anyadditional power consumption over the LMV796.
Low Input Referred Noise and Low Input Bias CurrentThe LMV793/LMV794 have a very low input referred voltage
at 1 kHz). A CMOS input stage en-noise density (5.8 nV/
sures a small input bias current (100 fA) and low input referredcurrent noise (0.01 pA/). This is very helpful in maintain-ing signal integrity, and makes the LMV793/LMV794 ideal foraudio and sensor based applications.
Low Supply Voltage
The LMV793 and LMV794 have performance guaranteed at2.5V and 5V supply. These parts are guaranteed to be oper-ational at all supply voltages between 2.0V and 5.5V, forambient temperatures ranging from −40°C to 125°C, thus uti-lizing the entire battery lifetime. The LMV793/LMV794 arealso guaranteed to be operational at 1.8V supply voltage, fortemperatures between 0°C and 125°C optimizing their usagein low-voltage applications.
RRO and Ground Sensing
Rail-to-rail output swing provides the maximum possible dy-namic range. This is particularly important when operating atlow supply voltages. An innovative positive feedback schemeis used to boost the current drive capability of the outputstage. This allows the LMV793/LMV794 to source more than40 mA of current at 1.8V supply. This also limits the perfor-mance of these parts as comparators, and hence the usageof the LMV793 and the LMV794 in an open-loop configurationis not recommended. The input common-mode range in-cludes the negative supply rail which allows direct sensing atground in single supply operation.
Small Size
The small footprint of the LMV793 and the LMV794 packagesaves space on printed circuit boards, and enables the designof smaller electronic products, such as cellular phones,pagers, or other portable systems. Long traces between thesignal source and the op amp make the signal path moresusceptible to noise pick up.
The physically smaller LMV793/LMV794 packages, allow theop amp to be placed closer to the signal source, thus reducingnoise pickup and maintaining signal integrity.USING THE DECOMPENSATED LMV793
Advantages of Decompensated Op Amps
A unity gain stable op amp, which is fully compensated, isdesigned to operate with good stability down to gains of ±1.The large amount of compensation does provide an op ampthat is relatively easy to use; however, a decompensated opamp is designed to maximize the bandwidth and slew ratewithout any additional power consumption. This can be veryadvantageous.
www.national.com
12
The LMV793/LMV794 require a gain of ±10 to be stable.However, with an external compensation network (a simpleRC network) these parts can be stable with gains of ±1 andstill maintain the higher slew rate. Looking at the Bode plotsfor the LMV793 and its closest equivalent unity gain stable opamp, the LMV796, one can clearly see the increased band-width of the LMV793. Both plots are taken with a parallelcombination of 20 pF and 10 kΩ for the output load.
20216322
FIGURE 1. LMV793 AVOL vs. Frequency
20216323
FIGURE 2. LMV796 AVOL vs. Frequency
Figure 1 shows the much larger 88 MHz bandwidth of theLMV793 as compared to the 17 MHz bandwidth of theLMV796 shown in Figure 2. The decompensated LMV793has five times the bandwidth of the LMV796.
What is a Decompensated Op Amp?
The differences between the unity gain stable op amp and thedecompensated op amp are shown in Figure 3. This Bode plotassumes an ideal two pole system. The dominant pole of thedecompensated op amp is at a higher frequency, f1, as com-pared to the unity-gain stable op amp which is at fd as shownin Figure 3. This is done in order to increase the speed capa-bility of the op amp while maintaining the same power dissi-pation of the unity gain stable op amp. The LMV793/LMV794have a dominant pole at 8.6 Hz. The unity gain stableLMV796/LMV797 have their dominant pole at 1.6 Hz.
元器件交易网www.cecb2b.com
LMV793/LMV79420216325
FIGURE 4. LMV793 with Lead-Lag Compensation for
Inverting Configuration
20216324
FIGURE 3. Open Loop Gain for Unity-Gain Stable Op Amp
and Decompensated Op AmpHaving a higher frequency for the dominate pole will result in:1.The DC open-loop gain (AVOL) extending to a higher
frequency.
2.A wider closed loop bandwidth.
3.Better slew rate due to reduced compensation
capacitance within the op amp.
The second open loop pole (f2) for the LMV793/LMV794 oc-curs at 45 MHz. The unity gain (fu’) occurs after the secondpole at 51 MHz. An ideal two pole system would give a phasemargin of 45° at the location of the second pole. The LMV793/LMV794 have parasitic poles close to the second pole, givinga phase margin closer to 0°. Therefore it is necessary to op-erate the LMV793/LMV794 at a closed loop gain of 10 orhigher, or to add external compensation in order to assurestability.
For the LMV796, the gain bandwidth product occurs at 17MHz. The curve is constant from fd to fu which occurs beforethe second pole.
For the LMV793/LMV794, the GBW = 88 MHz and is constantbetween f1 and f2. The second pole at f2 occurs before AVOL= 1. Therefore fu’ occurs at 51 MHz, well before the GBWfrequency of 88 MHz. For decompensated op amps the unitygain frequency and the GBW are no longer equal. Gmin is theminimum gain for stability and for the LMV793/LMV794 thisis a gain of 10 or 20 dB.
Input Lead-Lag Compensation
The recommended technique which allows the user to com-pensate the LMV793/LMV794 for stable operation at any gainis lead-lag compensation. The compensation componentsadded to the circuit allow the user to shape the feedbackfunction to make sure there is sufficient phase margin whenthe loop gain is as low as 0 dB and still maintain the advan-tages over the unity gain op amp. Figure 4 shows the lead-lag configuration. Only RC and C are added for the necessarycompensation.
To cover how to calculate the compensation network valuesit is necessary to introduce the term called the feedback factoror F. The feedback factor F is the feedback voltage VA-VBacross the op amp input terminals relative to the op amp out-put voltage VOUT.
From feedback theory the classic form of the feedback equa-tion for op amps is:
A is the open loop gain of the amplifier and AF is the loop gain.Both are highly important in analyzing op amps. Normally AF>>1 and so the above equation reduces to:
Deriving the equations for the lead-lag compensation is be-yond the scope of this datasheet. The derivation is based onthe feedback equations that have just been covered. The in-verse of feedback factor for the circuit in Figure 4 is:
(1)
where 1/F's pole is located at
(2)
1/F's zero is located at
(3)
(4)
13www.national.com
元器件交易网www.cecb2b.com
LMV793/LMV794The circuit gain for Figure 4 at low frequencies is −RF/RIN, butF, the feedback factor is not equal to the circuit gain. Thefeedback factor is derived from feedback theory and is thesame for both inverting and non-inverting configurations. Yes,the feedback factor at low frequencies is equal to the gain forthe non-inverting configuration.
(5)
From this formula, we can see that
•1/F's zero is located at a lower frequency compared with1/F's pole.
•1/F's value at low frequency is 1 + RF/RIN.
•This method creates one additional pole and oneadditional zero.
•This pole-zero pair will serve two purposes:
—To raise the 1/F value at higher frequencies prior to itsintercept with A, the open loop gain curve, in order tomeet the Gmin = 10 requirement. For the LMV793/LMV794 some overcompensation will be necessary forgood stability.
—To achieve the previous purpose above with noadditional loop phase delay.
Please note the constraint 1/F ≥ Gmin needs to be satisfiedonly in the vicinity where the open loop gain A and 1/F inter-sect; 1/F can be shaped elsewhere as needed. The 1/F polemust occur before the intersection with the open loop gain A.In order to have adequate phase margin, it is desirable to fol-low these two rules:
Rule 11/F and the open loop gain A should intersect at the
frequency where there is a minimum of 45° of phasemargin. When over-compensation is required the in-tersection point of A and 1/F is set at a frequencywhere the phase margin is above 45°, therefore in-creasing the stability of the circuit.
Rule 21/F’s pole should be set at least one decade below
the intersection with the open loop gain A in order totake advantage of the full 90° of phase lead broughtby 1/F’s pole which is F’s zero. This ensures that theeffect of the zero is fully neutralized when the 1/F andA plots intersect each other.Calculating Lead-Lag Compensation for LMV793/LMV794
Figure 5 is the same plot as Figure 1, but the AVOL and phasecurves have been redrawn as smooth lines to more readilyshow the concepts covered, and to clearly show the key pa-rameters used in the calculations for lead-lag compensation.
20216348
FIGURE 5. LMV793/LMV794 Simplified Bode PlotTo obtain stable operation with gains under 10 V/V the openloop gain margin must be reduced at high frequencies towhere there is a 45° phase margin when the gain margin ofthe circuit with the external compensation is 0 dB. The poleand zero in F, the feedback factor, control the gain margin atthe higher frequencies. The distance between F and AVOL isthe gain margin; therefore, the unity gain point (0 dB) is whereF crosses the AVOL curve.
For the example being used RIN = RF for a gain of −1. There-fore F = 6 dB at low frequencies. At the higher frequenciesthe minimum value for F is 18 dB for 45° phase margin. FromEquation 5 we have the following relationship:
Now set RF = RIN = R. With these values and solving for RCwe have RC = R/5.9. Note that the value of C does not affectthe ratio between the resistors. Once the value of the resistorsare set, then the position of the pole in F must be set. A2 kΩ resistor is used for RF and RIN in this design. Thereforethe value for RC is set at 330Ω, the closest standard value for2 kΩ/5.9.
Rewriting Equation 2 to solve for the minimum capacitor valuegives the following equation:
C = 1/(2πfpRC)
The feedback factor curve, F, intersects the AVOL curve atabout 12 MHz. Therefore the pole of F should not be anylarger than 1.2 MHz. Using this value and Rc = 330Ω the min-imum value for C is 390 pF. Figure 6 shows that there is toomuch overshoot, but the part is stable. Increasing C to 2.2 nFdid not improve the ringing, as shown in Figure 7.
www.national.com14
元器件交易网www.cecb2b.com
LMV793/LMV7942021630320216310
FIGURE 6. First Try at Compensation, Gain = −1
FIGURE 9. RC = 240Ω and C = 2.2 nF, Gain = −1To summarize, the following steps were taken to compensatethe LMV793 for a gain of −1:
1.Values for Rc and C were calculated from the Bodie plot
to give an expected phase margin of 45°. The valueswere based on RIN = RF = 2 kΩ. These calculations gaveRc = 330Ω and C = 390 pF.
2.To reduce the ringing C was increased to 2.2 nF which
moved the pole of F, the feedback factor, farther awayfrom the AVOL curve.
3.There was still too much ringing so 2 dB of over-compensation was added to F. This was done bydecreasing RC to 240Ω.
The LMV796 is the fully compensated part which is compa-rable to the LMV793. Using the LMV796 in the same setup,but removing the compensation network, provide the re-sponse shown in Figure 10 .
20216307
FIGURE 7. C Increased to 2.2 nF, Gain = −1
Some over-compensation appears to be needed for the de-sired overshoot characteristics. Instead of intersecting theAVOL curve at 18 dB, 2 dB of over-compensation will be used,and the AVOL curve will be intersected at 20 dB. Using Equa-tion 5 for 20 dB, or 10 V/V, the closest standard value of RCis 240Ω. The following two waveforms show the new resistorvalue with C = 390 pF and 2.2 nF. Figure 9 shows the finalcompensation and a very good response for the 1 MHzsquare wave.
20216321
FIGURE 10. LMV796 Response
For large signal response the rise and fall times are dominat-ed by the slew rate of the op amps. Even though both partsare quite similar the LMV793 will give rise and fall times about2.5 times faster than the LMV796. This is possible becausethe LMV793 is a decompensated op amp and even though itis being used at a gain of −1, the speed is preserved by usinga good technique for external compensation.
20216308
FIGURE 8. RC = 240Ω and C = 390 pF, Gain = −1
15www.national.com
元器件交易网www.cecb2b.com
LMV793/LMV794Non-Inverting Compensation
For the non-inverting amp the same theory applies for estab-lishing the needed compensation. When setting the invertingconfiguration for a gain of −1, F has a value of 2. For the non-inverting configuration both F and the actual gain are thesame, making the non-inverting configuration more difficult tocompensate. Using the same circuit as shown in Figure 4, butsetting up the circuit for non-inverting operation (gain of +2)results in similar performance as the inverting configurationwith the inputs set to half the amplitude to compensate for theadditional gain. Figure 11 below shows the results.
The most difficult op amp configuration to stabilize is the gainof +1. With proper compensation the LMV793/LMV794 canbe used in this configuration and still maintain higher speedsthan the fully compensated parts. Figure 13 shows the gain =1, or the buffer configuration, for these parts.
20216384
FIGURE 13. LMV793 with Lead-Lag Compensation for
Non-Inverting ConfigurationFigure 13 is the result of using Equation 5 and additional ex-perimentation in the lab. RP is not part of Equation 5, but it isnecessary to introduce another pole at the input stage forgood performance at gain = +1. Equation 5 is shown belowwith RIN = ∞.
20216382
FIGURE 11. RC = 240Ω and C = 2.2 nF, Gain = +2
Using 2 kΩ for RF and solving for RC gives RC = 2000/6.9 =290Ω. The closest standard value for RC is 300Ω. After somefine tuning in the lab RC = 330Ω and RP = 1.5 kΩ werechoosen as the optimum values. RP together with the inputcapacitance at the non-inverting pin inserts another pole intothe compensation for the LMV793/LMV794. Adding this poleand slightly reducing the compensation for 1/F (using a slight-ly higher resistor value for RC) gives the optimum responsefor a gain of +1. Figure 14 is the response of the circuit shownin Figure 13. Figure 15 shows the response of the LMV796 inthe buffer configuration with no compensation and RP = RF =0.
20216383
FIGURE 12. LMV796 Response Gain = +2
The response shown in Figure 11 is close to the responseshown in Figure 9. The part is actually slightly faster in thenon-inverting configuration. Decreasing the value of RC toaround 200Ω can decrease the negative overshoot but willhave slightly longer rise and fall times. The other option is toadd a small resistor in series with the input signal. Figure 12shows the performance of the LMV796 with no compensation.Again the decompensated parts are almost 2.5 times fasterthan the fully compensated op amp.
20216388
FIGURE 14. RC = 330Ω and C = 10 nF, Gain = +1
www.national.com16
元器件交易网www.cecb2b.com
LMV793/LMV79420216361
FIGURE 16. Transimpedance Amplifier
20216389
FIGURE 15. LMV796 Response Gain = +1
With no increase in power consumption the decompensatedop amp offers faster speed over the compensated equivalentpart. These examples used RF = 2 kΩ. This value is highenough to be easily driven by the LMV793/LMV794, yet smallenough to minimize the effects from the parasitic capacitanceof both the PCB and the op amp.
Note: When using the LMV793/LMV794, proper high fre-quency PCB layout must be followed. The GBW of these partsis 88 MHz, making the PCB layout significantly more criticalthan when using the compensated counterparts which havea GBW of 17 MHz.
TRANSIMPEDANCE AMPLIFIER
An excellent application for either the LMV793 or the LMV794is as a transimpedance amplifier. With a GBW product of 88MHz these parts are ideal for high speed data transmissionby light. The circuit shown on the front page of the datasheetis the circuit used to test the LMV793/LMV794 as tran-simpedance amplifiers. The only change is that VB is tied tothe VCC of the part, thus the direction of the diode is reversedfrom the circuit shown on the front page.
Very high speed components were used in testing to checkthe limits of the LMV793/LMV794 in a transimpedance con-figuration. The photo diode part number is PIN-HR040 fromOSI Optoelectronics. The diode capacitance for this part isonly about 7 pF for the 2.5V bias used (VCC to virtual ground).The rise time for this diode is 1 nsec. A laser diode was usedfor the light source. Laser diodes have on and off times under5 nsec. The speed of the selected optical components al-lowed an accurate evaluation of the LMV793 as a tran-simpedance amplifier. Nationals Evaluation Board for decom-pensated op amps, PN 551013271-001 A, was used and onlyminor modifications were necessary and no traces had to becut.
Figure 16 is the complete schematic for a transimpedanceamplifier. Only the supply bypass capacitors are not shown.CD represents the photo diode capacitance which is given onits datasheet. CCM is the input common mode capacitance ofthe op amp and, for the LMV793 it is shown in the last drawingof the Typical Performance Characteristics section of thisdatasheet. In Figure 16 the inverting input pin of the LMV793is kept at virtual ground. Even though the diode is connectedto the 2.5V line, a power supply line is AC ground, thus CD isconnected to ground.
Figure 17 shows the schematic needed to derive F, the feed-back factor, for a transimpedance amplifier. In this figure CD+ CCM = CIN. Therefore it is critical that the designer knowsthe diode capacitance and the op amp input capacitance. Thephoto diode is close to an ideal current source once its ca-pacitance is included in the model. What kind of circuit is this?Without CF there is only an input capacitor and a feedbackresistor. This circuit is a differentiator! Remember, differen-tiator circuits are inherently unstable and must be compen-sated. In this case CF compensates the circuit.
20216364
FIGURE 17. Transimpedance Feedback Model
17www.national.com
元器件交易网www.cecb2b.com
LMV793/LMV794Using feedback theory, F = VA/VOUT, this becomes a voltagedivider giving the following equation:After a bit of algebraic manipulation the above equation re-duces to:
The noise gain is 1/F. Because this is a differentiator circuit,a zero must be inserted. The location of the zero is given by:
CF has been added for stability. The addition of this part addsa pole to the circuit. The pole is located at:
In the above equation the only unknown is CF. In trying tosolve this equation the fourth power of CF must be dealt with.An excel spread sheet with this equation can be used and allthe known values entered. Then through iteration, the valueof CF when both sides are equal will be found. That is thecorrect value for CF, and of course the closest standard valueis used for CF.
Before moving the lab, the transfer function of the tran-simpedance amplifier must be found and the units must be inOhms.
To attain maximum bandwidth and still have good stability thepole is to be located on the open loop gain curve which is A.If additional compensation is required one can always in-crease the value of CF, but this will also reduce the bandwidthof the circuit. Therefore A = 1/F, or AF = 1. For A the equationis:
The expression fGBW is the gain bandwidth product of the part.For a unity gain stable part this is the frequency where A = 1.For the LMV793 fGBW = 88 MHz. Multiplying A and F resultsin the following equation:
The LMV793 was evaluated for RF = 10 kΩ and 100 kΩ, rep-resenting a somewhat lower gain configuration and with the100 kΩ feedback resistor a fairly high gain configuration. TheRF = 10 kΩ is covered first. Looking at the Input CommonMode Capacitance vs. VCM chart for CCM for the operatingpoint selected CCM = 15 pF. Note that for split supplies VCM =2.5V, CIN = 22 pF and fGBW = 88 MHz. Solving for CF the cal-culated value is 1.75 pF, so 1.8 pF is selected for use.Checking the frequency of the pole finds that it is at 8.8 MHz,which is right at the minimum gain recommended for this part.Some over compensation was necessary for stability and thefinal selected value for CF is 2.7 pF. This moves the pole to5.9 MHz. Figure 18 and Figure 19 show the rise and fall timesobtained in the lab with a 1V output swing. The laser diodewas difficult to drive due to thermal effects making the startingand ending point of the pulse quite different, therefore the twoseparate scope pictures.
For the above equation s = jω. To find the actual amplitude ofthe equation the square root of the square of the real andimaginary parts are calculated. At the intersection of F and A,we have:
20216390
FIGURE 18. Fall Time
www.national.com18
元器件交易网www.cecb2b.com
LMV793/LMV7942021639120216392
FIGURE 19. Rise Time
In Figure 18 the ringing and the hump during the on time isfrom the laser. The higher drive levels for the laser gave ring-ing in the light source as well as light changing from thethermal characteristics. The hump is due to the thermal char-acteristics.
Solving for CF using a 100 kΩ feedback resistor, the calcu-lated value is 0.54 pF. One of the problems with more gain isthe very small value for CF. A 0.5 pF capacitor was used, itsmeasured value being 0.64 pF. For the 0.64 pF location thepole is at 2.5 MHz. Figure 20 shows the response for a 1Voutput.
FIGURE 20. High Gain Response
A transimpedance amplifier is an excellent application for theLMV793. Even with the high gain using a 100 kΩ feedbackresistor, the bandwidth is still well over 1 MHz. Other than alittle over compensation for the 10 kΩ feedback resistor con-figuration using the LMV793 was quite easy. Of course a verygood board layout was also used for this test. For informationon photo diodes please contact OSI Optoelectronics, (310)978-0516. For further information on transimpedance ampli-fiers please contact your National Semiconductor represen-tative.
19www.national.com
元器件交易网www.cecb2b.com
LMV793/LMV794Physical Dimensions inches (millimeters) unless otherwise noted
5-Pin SOT23
NS Package Number MF05A
8-Pin SOIC
NS Package Number M08A
www.national.com20
元器件交易网www.cecb2b.com
LMV793/LMV7948-Pin MSOP
NS Package Number MUA08A
21www.national.com
元器件交易网www.cecb2b.com
LMV793/LMV794 88 MHz, Low Noise, 1.8V CMOS Input, Decompensated Operational AmplifiersNotes
THE CONTENTS OF THIS DOCUMENT ARE PROVIDED IN CONNECTION WITH NATIONAL SEMICONDUCTOR CORPORATION(“NATIONAL”) PRODUCTS. NATIONAL MAKES NO REPRESENTATIONS OR WARRANTIES WITH RESPECT TO THE ACCURACYOR COMPLETENESS OF THE CONTENTS OF THIS PUBLICATION AND RESERVES THE RIGHT TO MAKE CHANGES TOSPECIFICATIONS AND PRODUCT DESCRIPTIONS AT ANY TIME WITHOUT NOTICE. NO LICENSE, WHETHER EXPRESS,IMPLIED, ARISING BY ESTOPPEL OR OTHERWISE, TO ANY INTELLECTUAL PROPERTY RIGHTS IS GRANTED BY THISDOCUMENT.
TESTING AND OTHER QUALITY CONTROLS ARE USED TO THE EXTENT NATIONAL DEEMS NECESSARY TO SUPPORTNATIONAL’S PRODUCT WARRANTY. EXCEPT WHERE MANDATED BY GOVERNMENT REQUIREMENTS, TESTING OF ALLPARAMETERS OF EACH PRODUCT IS NOT NECESSARILY PERFORMED. NATIONAL ASSUMES NO LIABILITY FORAPPLICATIONS ASSISTANCE OR BUYER PRODUCT DESIGN. BUYERS ARE RESPONSIBLE FOR THEIR PRODUCTS ANDAPPLICATIONS USING NATIONAL COMPONENTS. PRIOR TO USING OR DISTRIBUTING ANY PRODUCTS THAT INCLUDENATIONAL COMPONENTS, BUYERS SHOULD PROVIDE ADEQUATE DESIGN, TESTING AND OPERATING SAFEGUARDS.
EXCEPT AS PROVIDED IN NATIONAL’S TERMS AND CONDITIONS OF SALE FOR SUCH PRODUCTS, NATIONAL ASSUMES NOLIABILITY WHATSOEVER, AND NATIONAL DISCLAIMS ANY EXPRESS OR IMPLIED WARRANTY RELATING TO THE SALEAND/OR USE OF NATIONAL PRODUCTS INCLUDING LIABILITY OR WARRANTIES RELATING TO FITNESS FOR A PARTICULARPURPOSE, MERCHANTABILITY, OR INFRINGEMENT OF ANY PATENT, COPYRIGHT OR OTHER INTELLECTUAL PROPERTYRIGHT.
LIFE SUPPORT POLICY
NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES ORSYSTEMS WITHOUT THE EXPRESS PRIOR WRITTEN APPROVAL OF THE CHIEF EXECUTIVE OFFICER AND GENERALCOUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein:
Life support devices or systems are devices which (a) are intended for surgical implant into the body, or (b) support or sustain life andwhose failure to perform when properly used in accordance with instructions for use provided in the labeling can be reasonably expectedto result in a significant injury to the user. A critical component is any component in a life support device or system whose failure to performcan be reasonably expected to cause the failure of the life support device or system or to affect its safety or effectiveness.
National Semiconductor and the National Semiconductor logo are registered trademarks of National Semiconductor Corporation. All otherbrand or product names may be trademarks or registered trademarks of their respective holders.
Copyright© 2007 National Semiconductor Corporation
For the most current product information visit us at www.national.com
National SemiconductorAmericas CustomerSupport CenterEmail:
new.feedback@nsc.comTel: 1-800-272-9959
National Semiconductor EuropeCustomer Support CenterFax: +49 (0) 180-530-85-86
Email: europe.support@nsc.comDeutsch Tel: +49 (0) 69 9508 6208English Tel: +49 (0) 870 24 0 2171Français Tel: +33 (0) 1 41 91 8790
National Semiconductor AsiaPacific Customer Support CenterEmail: ap.support@nsc.com
National Semiconductor JapanCustomer Support CenterFax: 81-3-5639-7507
Email: jpn.feedback@nsc.comTel: 81-3-5639-7560
www.national.com
因篇幅问题不能全部显示，请点此查看更多更全内容