link to page 10 link to page 11 link to page 11 Application Information THE BENEFITS OF THE LMP2015/LMP2016's NO 1/f NOISE Using patented methods, the LMP2015/LMP2016 eliminate the 1/f noise present in other amplifiers. That noise, which increases as frequency decreases, is a major source of mea- surement error in all DC-coupled measurements. Low fre- quency 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 con- 20212516 ventional amplifier has a flat-band noise level of 10 nV/ LMP2015 Single/LMP2016 Dual and a noise corner of 10 Hz, the RMS noise at 0.001 Hz is 1 FIGURE 1. Overload Recovery Test µV/ . This is equivalent to a 0.50 µV peak-to-peak error, in the frequency range 0.001 Hz to 1.0 Hz. In a circuit with a gain The wide bandwidth of the LMP2015/LMP2016 enhance per- of 1000, this produces a 0.50 mV peak-to-peak output error. formance when it is used as an amplifier to drive loads that This number of 0.001 Hz might appear unreasonably low, but inject transients back into the output. ADCs (Analog-to-Digital when a data acquisition system is operating for 17 minutes, it Converters) and multiplexers are examples of this type of has been on long enough to include this error. In this same load. To simulate this type of load, a pulse generator produc- time, the LMP2015/LMP2016 will have only a 0.21 mV output ing a 1V peak square wave was connected to the output error. This is smaller by 2.4 x. Keep in mind that this 1/f error through a 10 pF capacitor. (Figure 1) The typical time for the gets even larger at lower frequencies. At the extreme, many output to recover to 1% of the applied pulse is 80 ns. To re- people try to reduce this error by integrating or taking several cover to 0.1% requires 860 ns. This rapid recovery is due to samples of the same signal. This is also doomed to failure the wide bandwidth of the output stage and large total GBWP. because the 1/f nature of this noise means that taking longer NO EXTERNAL CAPACITORS REQUIRED samples just moves the measurement into lower frequencies where the noise level is even higher. The LMP2015/LMP2016 do not need external capacitors. This eliminates the problems caused by capacitor leakage The LMP2015/LMP2016 eliminate this source of error. The and dielectric absorption, which can cause delays of several noise level is constant with frequency so that reducing the seconds from turn-on until the amplifier's error has settled. bandwidth reduces the errors caused by noise. Another source of error that is rarely mentioned is the error MORE BENEFITS voltage caused by the inadvertent thermocouples created The LMP2015/LMP2016 offer the benefits mentioned above when the common "Kovar type" IC package lead materials are and more. These parts have rail-to-rail outputs and consume soldered to a copper printed circuit board. These steel based only 950 µA of supply current while providing excellent DC leadframe materials can produce over 35 μV/°C when sol- and AC electrical performance. In DC performance, the dered onto a copper trace. This can result in thermocouple LMP2015/LMP2016 achieve 130 dB of CMRR, 120 dB of noise that is equal to the LMP2015/LMP2016 noise when PSRR and 130 dB of open loop gain. In AC performance, the there is a temperature difference of only 0.0014°C between LMP2015/LMP2016 provide 3 MHz of gain bandwidth product the lead and the board! and 4 V/µs of slew rate. For this reason, the lead frame of the LMP2015/LMP2016 is made of copper. This results in equal and opposite junctions HOW THE LMP2015/LMP2016 WORK which cancel this effect. The extremely small size of the The LMP2015/LMP2016 use new, patented techniques to SOT23 package results in the leads being very close togeth- achieve the high DC accuracy traditionally associated with er. This further reduces the probability of temperature differ- chopper-stabilized amplifiers without the major drawbacks ences and hence decreases thermal noise. produced by chopping. The LMP2015/LMP2016 continuously monitor the input offset and correct this error. The conven- OVERLOAD RECOVERY tional chopping process produces many mixing products, The LMP2015/LMP2016 recover from input overload much both sums and differences, between the chopping frequency faster than most chopper-stabilized op amps. Recovery from and the incoming signal frequency. This mixing causes a driving the amplifier to 2X the full scale output, only requires large amount of distortion, particularly when the signal fre- about 40 ms. Many chopper-stabilized amplifiers will take quency approaches the chopping frequency. Even without an from 250 ms to several seconds to recover from this same incoming signal, the chopper harmonics mix with each other overload. This is because large capacitors are used to store to produce even more trash. To explain this Figure 2 shows the unadjusted offset voltage. a plot, of the output of a typical (MAX432) chopper-stabilized op amp. 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 150 Hz; the rest is mixing prod- ucts. Add an input signal and the noise gets much worse. Compare this plot with Figure 3 of the LMP2015/LMP2016. This data was taken under the exact same conditions. The auto-zero action is visible at about 30 kHz but note the ab- sence of mixing products at other frequencies. As a result, the LMP2015/LMP2016 have very low distortion of 0.02% and very low mixing products. www.national.com 10 Document Outline LMP2015 Single/LMP2016 Dual General Description Features Applications Connection Diagrams Ordering Information Absolute Maximum Ratings Operating Ratings 2.7V DC Electrical Characteristics 2.7V AC Electrical Characteristics 5V DC Electrical Characteristics 5V AC Electrical Characteristics Typical Performance Characteristics Application Information THE BENEFITS OF THE LMP2015/LMP2016'sNO 1/f NOISE OVERLOAD RECOVERY FIGURE 1. Overload Recovery Test NO EXTERNAL CAPACITORS REQUIRED MORE BENEFITS HOW THE LMP2015/LMP2016 WORK FIGURE 2. The Output of a Chopper Stabilized Op Amp (MAX432) FIGURE 3. The Output of the LMP2015/LMP2016 INPUT CURRENTS PRECISION STRAIN GAUGE AMPLIFIER FIGURE 4. Precision Strain Gauge Amplifier FIGURE 5. Composite Amplifier Configuration TABLE 1. Composite Amplifier Measured Performance FIGURE 6. Composite Amplifier Configuration FIGURE 7. AC Coupled ADC Driver LMP2015 AS AN ADC DRIVER FIGURE 8. DC Coupled ADC Driver Physical Dimensions
