LMV1012-15 National Semiconductor Corporation, LMV1012-15 Datasheet - Page 10

• Manufacturer Part Number: LMV1012-15
• Description: LMV2011 - High Precision, Rail-to-rail Output Operational Amplifier, Package: Soic Narrow, Pin Nb=8
• Manufacturer: National Semiconductor Corporation

www.national.com
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/
is 1µV/
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.
and a noise corner of 10Hz, the RMS noise at 0.001Hz
. This is equivalent to a 0.50µV peak-to-peak
10
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.
FIGURE 1.
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