Application Information
THE BENEFITS OF LMP201X
NO 1/f NOISE
Using patented methods, the LMP201X 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 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 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 equiv-
alent 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 pro-
duces 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 LM-
P201X 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 LMP201X 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
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 sol-
dered 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.
OVERLOAD RECOVERY
The LMP201X 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.
20071516
FIGURE 1. Overload Recovery Test
The wide bandwidth of the LMP201X enhances performance
when it is used as an amplifier to drive loads that inject tran-
sients back into the output. ADCs (Analog-to-Digital Convert-
ers) 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 re-
cover to 1% of the applied pulse is 80 ns. To recover to 0.1%
requires 860ns. This rapid recovery is due to the wide band-
width of the output stage and large total GBW.
NO EXTERNAL CAPACITORS REQUIRED
The LMP201X does not need external capacitors. This elim-
inates 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 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 per-
formance. 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.
HOW THE LMP201X WORKS
The LMP201X 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 LMP201X continuously monitors the input off-
set and corrects this error. The conventional chopping pro-
cess 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) chop-
per-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 LMP201X.
This data was taken under the exact same conditions. The
auto-zero action is visible at about 30 kHz but note the ab-
sence of mixing products at other frequencies. As a result, the
LMP201X has very low distortion of 0.02% and very low mix-
ing products.
www.national.com 10
LMP2011 Single/LMP2012 Dual