LMC2001AIMX datasheet(6/11 Pages) NSC

6 / 11 page

Application Notes

The Benefits of LMC2001

No 1/f Noise

Using patented methods, the LMC2001 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 mea-

surement is made rapidly, this constantly changing noise sig-

nal will corrupt the result. The value of this noise signal can

be surprisingly large. For example: If a conventional ampli-

fier has a high frequency 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 6µV peak-to-peak error. In a circuit

with a gain of 1000, this produces a 6mV peak-to-peak out-

put error. This number of 0.001 Hz might appear unreason-

ably 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 LMC2001 will only have a 0.51mV out-

put error. This is more than 13.3 times less error.

Keep in mind that this 1/f error gets even larger at lower fre-

quencies.

At the extreme, many people try to reduce this error by inte-

grating 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 measure-

ment into lower frequencies where the noise level is even

higher.

The LMC2001 eliminates this source of error. The noise level

is constant with frequency so that reducing the bandwidth re-

duces the errors caused by noise.

Another source of error that is rarely mentioned is the error

voltages caused by the inadvertent thermocouples created

when the common “Kovar type” package lead materials are

soldered to a copper printed circuit board. These steel based

leadframe materials can produce over 35uV/˚C when sol-

dered onto a copper trace. This can result in thermocouple

noise that is equal to the LMC2001 noise when there is a

temperature difference of only 0.0014˚C between the lead

and the board!

For this reason, the leadframe of the LMC2001 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 LMC2001 recovers from input overload much faster

than most chopper stabilized opamps. Recovery, from driv-

ing the amplifier to 2X the full scale output, only requires

about 50ms. Most chopper stabilized amplifiers will take

from 250ms 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 LMC2001 enhances performance

when it is used as an amplifier to drive loads that inject tran-

sients back into the output. A to Ds and multiplexers are ex-

amples of this type of load. To simulate this type of load, a

pulse generator producing a 1V peak square wave was con-

nected 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 LMC2001 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 is settled.

More Benefits

The LMC2001 offers the benefits mentioned above and

more. It is rail-to-rail output and consumes only 750µA of

supply current while providing excellent DC and AC electrical

performance. In DC performance, the LMC2001 achieves

120dB of CMRR, 120dB of PSRR and 137dB of open loop

gain. In AC performance, the LMC2001 provides 6MHz of

gain-bandwidth product and 5V/µs of slew rate.

How the LMC2001 Works

The LMC2001 uses new, patented techniques to achieve the

high DC accuracy traditionally associated with chopper sta-

bilized amplifiers without the major drawbacks produced by

chopping. The LMC2001 continuously monitors the input off-

set and corrects this error. The conventional chopping pro-

cess produces many mixing products, both sums and differ-

ences, between the chopping frequency and the incoming

signal frequency. This mixing causes large amounts of dis-

tortion, 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 mess

gets much worse. Compare this plot with

Figure 3 of the

LMC2001. This data was taken under the exact same condi-

tions. The auto zero action is visible at about 11kHz but note

the absence of mixing products at other frequencies. As a re-

sult, the LMC2001 has very low distortion of 0.02% and very

low mixing products.

Input Currents

The LMC2001 input current is different than standard bipolar

or CMOS input currents in that it appears as a current flow-

ing in one input and out the other. Under most operating con-

ditions, these currents are in the picoamp level and will have

little or no effect in most circuits. These currents increase to

the nA level when the common-mode voltage is near the mi-

nus supply. (see the typical curves) At high temperatures

such as 85˚C, the input currents become larger, 0.5nA typi-

cal, 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 in-

puts to balance the impedances. Doing this can cause an in-

crease in offset voltage.

DS100058-B0

FIGURE 1.

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