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LMV2011MFX Datasheet(PDF) 10 Page - National Semiconductor (TI) |
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LMV2011MFX Datasheet(HTML) 10 Page - National Semiconductor (TI) |
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10 / 15 page  Application Information THE BENEFITS OF LMV2011 NO 1/f NOISE Using patented methods, the LMV2011 eliminates the 1/f noise present in other amplifiers. That noise, which in- creases as frequency decreases, is a major source of mea- surement error in all DC-coupled measurements. Low- frequency noise appears as a constantly-changing signal in series with any measurement being made. As a result, even when the measurement is made rapidly, this constantly- changing noise signal will corrupt the result. The value of this noise signal can be surprisingly large. For example: If a conventional amplifier has a flat-band noise level of 10nV/ and a noise corner of 10Hz, the RMS noise at 0.001Hz is 1µV/ . This is equivalent to a 0.50µV peak-to-peak error, in the frequency range 0.001 Hz to 1.0 Hz. In a circuit with a gain of 1000, this produces a 0.50mV peak-to-peak output error. This number of 0.001 Hz might appear unrea- sonably low, but when a data acquisition system is operating for 17 minutes, it has been on long enough to include this error. In this same time, the LMV2011 will only have a 0.21mV output error. This is smaller by 2.4 x. Keep in mind that this 1/f error gets even larger at lower frequencies. At the extreme, many people try to reduce this error by integrating or taking several samples of the same signal. This is also doomed to failure because the 1/f nature of this noise means that taking longer samples just moves the measurement into lower frequencies where the noise level is even higher. The LMV2011 eliminates this source of error. The noise level is constant with frequency so that reducing the bandwidth reduces the errors caused by noise. Another source of error that is rarely mentioned is the error voltage caused by the inadvertent thermocouples created when the common "Kovar type" IC package lead materials are soldered to a copper printed circuit board. These steel- based leadframe materials can produce over 35µV/˚C when soldered onto a copper trace. This can result in thermo- couple noise that is equal to the LMV2011 noise when there is a temperature difference of only 0.0014˚C between the lead and the board! For this reason, the lead-frame of the LMV2011 is made of copper. This results in equal and opposite junctions which cancel this effect. The extremely small size of the SOT-23 package results in the leads being very close together. This further reduces the probability of temperature differences and hence decreases thermal noise. OVERLOAD RECOVERY The LMV2011 recovers from input overload much faster than most chopper-stabilized opamps. Recovery from driving the amplifier to 2X the full scale output, only requires about 40ms. Many chopper-stabilized amplifiers will take from 250ms to several seconds to recover from this same over- load. This is because large capacitors are used to store the unadjusted offset voltage. The wide bandwidth of the LMV2011 enhances performance when it is used as an amplifier to drive loads that inject transients back into the output. ADCs (Analog-to-Digital Con- verters) and multiplexers are examples of this type of load. To simulate this type of load, a pulse generator producing a 1V peak square wave was connected to the output through a 10pF capacitor. (Figure 1) The typical time for the output to recover to 1% of the applied pulse is 80ns. To recover to 0.1% requires 860ns. This rapid recovery is due to the wide bandwidth of the output stage and large total GBW. NO EXTERNAL CAPACITORS REQUIRED The LMV2011 does not need external capacitors. This elimi- nates the problems caused by capacitor leakage and dielec- tric absorption, which can cause delays of several seconds from turn-on until the amplifier’s error has settled. MORE BENEFITS The LMV2011 offers the benefits mentioned above and more. It has a rail-to-rail output and consumes only 950µA of supply current while providing excellent DC and AC electrical performance. In DC performance, the LMC2001 achieves 130dB of CMRR, 120dB of PSRR and 130dB of open loop gain. In AC performance, the LMV2011 provides 3MHz of gain-bandwidth product and 4V/µs of slew rate. HOW THE LMV2011 WORKS The LMV2011 uses new, patented techniques to achieve the high DC accuracy traditionally associated with chopper- stabilized amplifiers without the major drawbacks produced by chopping. The LMV2011 continuously monitors the input offset and corrects this error. The conventional chopping process produces many mixing products, both sums and differences, between the chopping frequency and the incom- ing signal frequency. This mixing causes large amounts of distortion, particularly when the signal frequency approaches the chopping frequency. Even without an incoming signal, the chopper harmonics mix with each other to produce even more trash. If this sounds unlikely or difficult to understand, look at the plot (Figure 2), of the output of a typical (MAX432) chopper-stabilized opamp. This is the output when there is no incoming signal, just the amplifier in a gain of -10 with the input grounded. The chopper is operating at about 150Hz; the rest is mixing products. Add an input signal and the noise gets much worse. Compare this plot with Figure 3 of the LMV2011. This data was taken under the exact same con- ditions. The auto-zero action is visible at about 30kHz but note the absence of mixing products at other frequencies. As a result, the LMV2011 has very low distortion of 0.02% and very low mixing products. 20051516 FIGURE 1. www.national.com 10 |
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