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SNOSA71L –OCTOBER 2004–REVISED SEPTEMBER 2015
Feature Description (continued)
7.3.2 The Benefits of LMP201x: No 1/F Noise
Using patented methods, the LMP201x eliminates the 1/f noise present in other amplifiers. This noise, which
increases as frequency decreases, is a major source of measurement 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 10 nV/√Hz and a noise corner of 10 Hz, the RMS noise at 0.001 Hz is 1 µV/√Hz. 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 of 1000, this
produces a 0.50-mV peak-to-peak output error. This number of 0.001 Hz might appear unreasonably 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 LMP201x will only have a 0.21-mV output error. This is smaller by 2.4×. 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 LMP201x eliminates this source of error. The noise level is constant with frequency so that reducing the
bandwidth reduces the errors caused by noise.
7.3.3 No External Capacitors Required
The LMP201x does not need external capacitors. This eliminates the problems caused by capacitor leakage and
dielectric absorption, which can cause delays of several seconds from turn-on until the amplifier's error has
settled.
7.3.4 Copper Leadframe
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 35 μV/°C when soldered onto a copper trace. This can
result in thermocouple noise that is equal to the LMP201x 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 LMP201x 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.
7.3.5 More Benefits
The LMP201x offers the benefits mentioned above and more. It has a rail-to-rail output and consumes only 950
µA of supply current while providing excellent DC and AC electrical performance. In DC performance, the
LMP201x achieves 130 dB of CMRR, 120 dB of PSRR, and 130 dB of open loop gain. In AC performance, the
LMP201x provides 3 MHz of gain-bandwidth product and 4 V/µs of slew rate.
7.4 Device Functional Modes
7.4.1 Input Currents
The LMP201x input currents are different than standard bipolar or CMOS input currents. Due to the auto-zero
action of the input stage, the input current appears as a pulsating current at the chopping frequency (35 kHz)
flowing in one input and out the other. Under most operating conditions, these currents are in the picoamp level
and will have little or no effect in most circuits.
These currents tend to increase slightly when the common-mode voltage is near the minus supply. (See the
Typical Characteristics.) At high temperatures such as 85°C, the input currents become larger, 0.5 nA typical,
and are both positive except when the VCM is near V−. If operation is expected at low common-mode voltages
and high temperature, do not add resistance in series with the inputs to balance the impedances. Doing this can
cause an increase in offset voltage. A small resistance such as 1 kΩcan provide some protection against very
large transients or overloads, and will not increase the offset significantly.
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