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LMV751 Low Noise, Low Vos, Single Op Amp
May 1999
LMV751 Low Noise, Low Vos, Single Op Amp
General Description
The LMV751 is a high performance CMOS operational amplifier intended for applications requiring low noise and low input offset voltage. It offers modest bandwidth of 4.5MHz for very low supply current and is unity gain stable. The output stage is able to drive high capacitance, up to 1000pF and source or sink 8mA output current. It is supplied in the space saving SOT23-5 Tiny package. The LMV751 is designed to meet the demands of small size, low power, and high performance required by cellular phones and similar battery operated portable electronics.
Features
n n n n n n n n Low Noise 6.5nV Rt-Hz typ. Low Vos (0.05mV typ.) Wideband 4.5MHz GBP typ. Low Supply Current 500uA typ. Low Suppy Voltage 2.7V to 5.0V Ground-referenced Inputs Unity gain stable Small Package
Applications
n Cellular Phones n Portable Equipment n Radio Systems
Connection Diagrams
SOT23-5
DS101081-1
Top View
Ordering Information
Package 5-Pin SOT23-5 Ordering Info LMV751M5 LMV751M5X NSC Drawing MA05B MA05B Pkg Marking A32A A32A Supplied As 250 Units Tape and Reel 3k units Tape and Reel
(c) 1999 National Semiconductor Corporation
DS101081
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Absolute 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 3) Human Body Model Machine Model Differential Input Voltage Supply Voltage (V+ - V-) Lead Temperature (Soldering, 10 sec) 2000V 200V
Storage Temperature Range Junction Temperature (TJ) (Note 4)
-65C to 150C 150C
Recommended Operating Conditions
Supply Voltage Temperature Range Thermal resisance (JA) (Note 6) M5 Package, SOT23-5 274C/W 2.7V to 5.0V -40C TJ 85C
Supply Voltage
5.5V 260C
2.7V Electrical Characteristics
V+ = 2.7V, V- = 0V, VCM = 1.35V, TA = 25C unless otherwise stated. Boldface limits apply over theTemperature Range. Symbol VOS CMRR PSRR IS IIN IOS AVOL Parameter Input Offset Voltage Common Mode Rejection Ratio Power Supply Rejection Ratio Supply Current Input Current Input Offset Current Voltage Gain RL = 10k Connect to V+/2 VO = 0.2V to 2.2V RL = 2k Connect to V+/2 VO = 0.2V to 2.2V RL = 10k Connect to V+/2 RL = 2k Connect to V+/2 VO NegativeVoltage Swing RL = 10k Connect to V+/2 RL = 2k Connect to V+/2 IO Output Current Sourcing, VO = 0V VIN(diff) = 0.5V Sinking,VO = 2.7V VIN(diff) = 0.5V 0V < VCM < 1.3V V+ = 2.7V to 5.0V Condition Typ (Note 5) 0.05 100 107 0.5 1.5 0.2 120 120 2.62 2.62 78 78 12 11 15.5 7 7 10 110 95 100 85 2.54 2.52 2.54 2.52 140 160 140 160 6.0 1.5 6.0 1.5 Limit (Note 2) 1.0 1.5 85 70 85 70 0.7 0.75 100 Units mV max dB min dB min mA max pA max pA dB min
VO
Positive Voltage Swing
V min
mV max
mA min nV/ nV/ nV/ max
en (10Hz) en (1kHz) en (30kHz) IN(1kHz) GBW SR
Input Referred Voltage Noise Input Referred Voltage Noise Input Referred Voltage Noise
Input Referred Current Noise Gain-Bandwidth Product Slew Rate
0.01 4.5 2 2
pA/ MHZ min V/s
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5.0V Electrical Characteristics
V+ = 5.0V, V- = 0V, VCM = 2.5V, TA = 25C unless otherwise stated.Boldface limits apply over theTemperature Range. Symbol VOS CMRR PSRR IS IIN IOS AVOL Parameter Input Offset Voltage Common Mode Rejection Ratio Power Supply Rejection Ratio Supply Current Input Current Input offset Current Voltage Gain RL = 10k Connect to V+/2 VO = 0.2V to 4.5V RL = 2k Connect to V+/2 VO = 0.2V to 4.5V RL = 10k Connect to V+/2 RL = 2k Connect to V+/2 VO Negative Voltage Swing RL = 10k Connect to V+/2 RL = 2k Connect to V+/2 IO Output Current Sourcing, VO = 0V VIN (diff) = 0.5V Sinking, VO = 5V VIN (diff) = 0.5V 0V < VCM < 3.6V V+ = 2.7V to 5.0V Typ (Note 5) 0.05 103 107 0.6 1.5 0.2 120 120 4.89 4.89 86 86 15 20 15 6.5 6.5 10 110 95 100 85 4.82 4.80 4.82 4.80 160 180 160 180 8.0 2.5 8.0 2.5 V min Limit (Note 2) 1.0 1.5 85 70 85 70 0.8 0.85 100 Units mV max dB min dB min mA max pA max pA db min
VO
Positive Voltage Swing
mV max
mA min nV/ nV/ nV/ max
en (10Hz) en (1kHz) en (30kHz) IN (1kHz) GBW SR
Input Referred Voltage Noise Input Referred Voltage Noise Input Referred Voltage Noise
Input Referred Current Noise Gain-Bandwidth Product Slew Rate
0.01 5 2.3 2
pA/ MHz min V/s
Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Electrical specifications do not apply when operating the device beyond its rated operating conditions. Note 2: All limits are guaranteed by testing or statistical analysis Note 3: Human body model, 1.5k in series with 100pF. Machine model, 200 in series with 1000pF. Note 4: The maximum power dissipation is a function of TJ(max), JA, and TA. The maximum allowable power dissipation at any ambient temperature is PD = (TJ(max) - TA)/JA. All numbers apply for packages soldered directly into a PC board. Note 5: Typical values represent the most likely parametric norm. Note 6: All numbers are typical, and apply to packages soldered directly onto PC board in still air.
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Typical Performance Characteristics
Supply Curent vs. Voltage VOS vs. VCM V+ = 2.7V VOS vs VCM V+ = 5.0V
DS101081-35 DS101081-38 DS101081-37
Source Current vs Out V+ = 2.7V
Source Current vs VOUT V+ = 5.0V
Gain/Phase
DS101081-3 DS101081-28 DS101081-29
Sinking Current vs VOUT V+ = 2.7V
Sinking Current vs VOUT V+ = 5.0V
VOS vs V+
DS101081-36 DS101081-30 DS101081-31
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Typical Performance Characteristics
VIN vs VOUT V+ = 2.7V, RL = 2k
(Continued)
VIN vs VOUT V+ = 5.0V, RL = 2k
Input Bias vs VCM TA = 25C
DS101081-32
DS101081-33
DS101081-16
Input Bias vs VCM TA = 85C
PSRR +
PSRR -
DS101081-26 DS101081-5
DS101081-25
Voltage Noise
CMRR
DS101081-2
DS101081-39
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Application Hints
1.0 Noise There are many sources of noise in a system: thermal noise, shot noise, 1/f, popcorn noise, resistor noise, just to name a few. In addition to starting with a low noise op amp, such as the LMV751, careful attention to detail will result in the lowest overall noise for the system. 1.1 To invert or not invert? Both inverting and non-inverting amplifiers employ feedback to stabilize the closed loop gain of the block being designed. The loop gain (in decibels) equals the algebraic difference between the open loop and closed loop gains. Feedback improves the Total Harmonic Distortion (THD) and the output impedance. The various noise sources, when input referred, are amplified, not by the closed loop gain, but by the noise gain. For a non-inverting amplifier, the noise gain is equal to the closed loop gain, but for an inverting amplifier, the noise gain is equal to the closed loop gain plus one. For large gains, e.g., 100, the difference is negligible, but for small gains, such as one, the noise gain for the inverting amplifier would be two. This implies that non-inverting blocks are preferred at low gains. 1.2 Source impedance Because noise sources are uncorrelated, the system noise is calculated by taking the RMS sum of the various noise sources, that is, the square root of the sum of the squares. At very low source impedances, the voltage noise will dominate; at very high source impedances, the input noise current times the equivalent external resistance will dominate. For a detailed example calculation, refer to Note 1. 1.3 Bias current compensation resistor In CMOS input op amps, the input bias currents are very low, so there is no need to use RCOMP (Figure 1 and 2) for bias current compensation that would normally be used with early generation bipolar op amps. In fact, inclusion of the resistor would act as another thermal noise source in the system, increasing the overall noise.
Where T = temperature in K R = resistor value in ohms B = noise bandwidth in Hz K = Boltzmann's constant (1.38 x 10-23 W-sec/K) Actual resistor noise measurements may have more noise than the calculated value. This additional noise component is known as excess noise. Excess noise has a 1/f spectral response, and is proportional to the voltage drop across the resistor. It is convenient to define a noise index when referring to excess noise in resistors. The noise index is the RMS value in uV of noise in the resistor per volt of DC drop across the resistor in a decade of frequency. Noise index expressed in dB is: NI = 20 log ((EEX/VDC) x 106) db Where: EEX = resistor excess noise in uV per frequency decade. VDC = DC voltage drop across the resistor. Excess noise in carbon composition resistors corresponds to a large noise index of +10 dB to -20 dB. Carbon film resistors have a noise index of -10 dB to -25 dB. Metal film and wire wound resistors show the least amount of excess noise, with a noise index figure of -15 dB to -40 dB. 1.5 Other noise sources: As the op amp and resistor noise sources are decreased, other noise contributors will now be noticeable. Small air currents across thermocouples will result in low frequency variations. Any two dissimilar metals, such as the lead on the IC and the solder and copper foil of the pc board, will form a thermocouple. The source itself may also generate noise. An example would be a resistive bridge. All resistive sources generate thermal noise based on the same equation listed above under resistor types.(2)
DS101081-23
Figure 1
DS101081-24
Figure 2 1.4 Resistor types Thermal noise is generated by any passive resistive element. This noise is white; meaning it has a constant spectral density. Thermal noise can be represented by a meansquare voltage generator eR2 in series with a noiseless resistor, where eR2 is given by: Where: eR2 = 4K TRB (volts)2
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Application Hints
1.6 Putting it all together
(Continued)
To a first approximation, the total input referred noise of an op amp is: Et2 = en2 + ereq2 + (in*Req)2 where Req is the equivalent source resistance at the inputs. At low impedances, voltage noise dominates. At high impedances, current noise dominates. With a typical noise current on most CMOS input op amps of 0.01 pA/rt-Hz, the current noise contribution will be smaller than the voltage noise for Req less than one megohm. 2.0 Other Considerations 2.1 Comparator operation Occasionally operational amplifiers are used as comparators. This is not optimum for the LMV751 for several reasons. First, the LMV751 is compensated for unity gain stability, so the speed will be less than could be obtained on the same process with a circuit specifically designed for comparator operation. Second, op amp output stages are designed to be linear, and will not necessarily meet the logic levels required under all conditions. Lastly, the LMV751 has the newer PNP-NPN common emitter output stage, characteristic of many rail-to-rail output op amps. This means that when used in open loop applications, such as comparators, with very light loads, the output PNP will saturate, with the output current being diverted into the previous stage. As a result, the supply current will increase to the 20-30 mA. range. When used as a comparator, a resistive load between 2k and 10k should be used with a small amount of hysteresis to alleviate this problem. When used as an op amp, the closed loop gain will drive the inverting input to within a few millivolts of the non-inverting input. This will automatically reduce the output drive as the output settles to the correct value; thus it is only when used as a comparator that the current will increase to the tens of milliampere range.
2.2 Rail-to-Rail Because of the output stage discussed above, the LMV751 will swing "rail-to-rail" on the output. This normally means within a few hundred millivolts of each rail with a reasonable load. Referring to the Electrical Characteristics table for 2.7V to 5.0V, it can be seen that this is true for resistive loads of 2k and 10k. The input stage consists of cascoded P-channel MOSFETS, so the input common mode range includes ground, but typically requires 1.2V to 1.3V headroom from the positive rail. This is better than the industry standard LM324 and LM358 that have PNP input stages, and the LMV751 has the advantage of much lower input bias currents. 2.3 Loading The LMV751 is a low noise, high speed op amp with excellent phase margin and stablility. Capacitive loads up to 1000 pF can be handled, but larger capacitive loads should be isolated from the output. The most straightforward way to do this is to put a resistor in series with the output. This resistor will also prevent excess power dissapation if the output is accidentally shorted. 2.4 General Circuits With the low noise and low input bias current, the LMV751 would be useful in active filters, integrators, current to voltage converters, low frequency sine wave generators, and instrumentation amplifiers. (3)
Note: 1. Sherwin, Jim "Noise Specs Confusing?" AN-104, National Semiconductor. 2. Christensen, John, "Noise-figure curve ease the selection of low-noise op amps", EDN, pp 81-84, Aug. 4, 1994 3. "Op Amp Circuit Collection", AN-31, National Semiconductor.
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LMV751 Low Noise, Low Vos, Single Op Amp
Physical Dimensions
inches (millimeters) unless otherwise noted
SOT23-5 Order Number LMV751M5 NS Package Number MA05B
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