0.1

100

100k

10

100

1000

10000

1k

10

1M

FREQUENCY (Hz)

10k

1

VS = 5V

LMP2012QML

SNOSAU5H – MARCH 2007 – REVISED APRIL 2013

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HOW THE LMP2012 WORKS

The LMP2012 uses new, patented techniques to achieve the high DC accuracy traditionally associated with

chopper-stabilized amplifiers without the major drawbacks produced by chopping. The LMP2012 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 incoming 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 in Figure 3, 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 4 of the LMP2012. This data was

taken under the exact same conditions. The auto-zero action is visible at about 30 kHz but note the absence of

mixing products at other frequencies. As a result, the LMP2012 has very low distortion of 0.02% and very low

mixing products.

Figure 3.

Figure 4.

INPUT CURRENTS

The LMP2012's input currents are different than standard bipolar or CMOS input currents in that it appears as a

current 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. At high temperatures, 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 k

Ω can provide some protection

against very large transients or overloads, and will not increase the offset significantly.

8

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