LMP2014MTX datasheet(10/14 Pages) NSC

10 / 14 page

Application Information

THE BENEFITS OF LMP2014

NO 1/f NOISE

Using patented methods, the LMP2014 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 10 Hz, the RMS noise at 0.001

Hz is 1µV/

. 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 LMP2014 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 frequen-

cies. 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 LMP2014 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 35 µV/˚C when

soldered onto a copper trace. This can result in thermo-

couple noise that is equal to the LMP2014 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 LMP2014 is made of

copper. This results in equal and opposite junctions which

cancel this effect.

OVERLOAD RECOVERY

The LMP2014 recovers from input overload much faster

than most chopper-stabilized op amps. Recovery from driv-

ing 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.

The wide bandwidth of the LMP2014 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

10 pF capacitor. (Figure 1) 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 LMP2014 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 LMP2014 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 LMP2014 achieves

130 dB of CMRR, 120 dB of PSRR and 130 dB of open loop

gain. In AC performance, the LMP2014 provides 3 MHz of

gain-bandwidth product and 4 V/µs of slew rate.

HOW THE LMP2014 WORKS

The LMP2014 uses new, patented techniques to achieve the

high DC accuracy traditionally associated with chopper-

stabilized amplifiers without the major drawbacks produced

by chopping. The LMP2014 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 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 products. Add an input signal and the noise

gets much worse. Compare this plot with Figure 3 of the

LMP2014. This data was taken under the exact same con-

ditions. The auto-zero action is visible at about 30 kHz but

note the absence of mixing products at other frequencies. As

a result, the LMP2014 has very low distortion of 0.02% and

very low mixing products.

20132916

FIGURE 1.

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