LMP2012QML

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SNOSAU5H – MARCH 2007 – REVISED APRIL 2013

APPLICATION INFORMATION

THE BENEFITS OF LMP2012 NO 1/f NOISE

Using patented methods, the LMP2012 eliminates the 1/f noise present in other amplifiers. That 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 10nV/

√Hz and a noise corner of 10 Hz, the RMS noise at 0.001 Hz is 1µV/√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 LMP2012 will only have a 0.21 mV 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 LMP2012 eliminates this source of error. The noise level is constant with frequency so that reducing the

bandwidth reduces the errors caused by noise.

OVERLOAD RECOVERY

The LMP2012 recovers from input overload much faster than most chopper-stabilized op amps. Recovery from

driving the amplifier to 2X the full scale output, only requires about 40 ms. Many chopper-stabilized amplifiers will

take from 250 ms to several seconds to recover from this same overload. This is because large capacitors are

used to store the unadjusted offset voltage.

Figure 2.

The wide bandwidth of the LMP2012 enhances performance when it is used as an amplifier to drive loads that

inject transients back into the output. ADCs (Analog-to-Digital Converters) 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 10 pF capacitor. See Figure 2. The typical time for the output to recover to 1% of the

applied pulse is 80 ns. 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 LMP2012 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.

MORE BENEFITS

The LMP2012 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

LMP2012 achieves 130 dB of CMRR, 120 dB of PSRR and 130 dB of open loop gain. In AC performance, the

LMP2012 provides 3 MHz of gain-bandwidth product and 4 V/µs of slew rate.

Copyright © 2007–2013, Texas Instruments Incorporated

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