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LMP2021, LMP2022

SNOSAY9G – SEPTEMBER 2008 – REVISED FEBRUARY 2016

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Application Information (continued)

8.1.5 Transient Response to Fast Inputs

On chip continuous auto zero correction circuitry eliminates the 1/f noise and significantly reduces the offset

voltage and offset voltage drift; all of which are very low frequency events. For slow changing sensor signals this

correction is transparent. For excitations which may otherwise cause the output to swing faster than 40 mV/µs,

there are additional considerations which can be viewed two perspectives: for sine waves and for steps.

For sinusoidal inputs, when the output is swinging rail-to-rail on ±2.5-V supplies, the auto zero circuitry will

introduce distortions above 2.55 kHz. For smaller output swings, higher frequencies can be amplified without the

auto zero slew limitation as shown in table below. Signals above 20 kHz, are not affected, though normally,

closed loop bandwidth should be kept below 20 kHz so as to avoid aliasing from the auto zero circuit.

VOUT-PEAK (V)

fMAX-SINE WAVE (kHz)

0.32

20

1

6.3

2.5

2.5

(1)

No significant difference in Noise measurements at AV = 10V/V

For step-like inputs, such as those arising from disturbances to a sensing system, the auto zero slew rate

limitation manifests itself as an extended ramping and settling time, lasting ~100 µs.

8.1.6 Digital Acquisition Systems

High resolution ADC’s with 16-bits to 24-bits of resolution can be limited by the noise of the amplifier driving

them. The circuit configuration, the value of the resistors used and the source impedance seen by the amplifier

can affect the noise of the amplifier. The total noise at the output of the amplifier can be dominated by one of

several sources of noises such as: white noise or broad band noise, 1/f noise, thermal noise, and current noise.

In low frequency applications such as medical instrumentation, the source impedance is generally low enough

that the current noise coupled into it does not impact the total noise significantly. However, as the 1/f or flicker

noise is paramount to many application, the use of an auto correcting stabilized amplifier like the LMP202x

reduces the total noise.

Table 1 summarizes the input and output referred RMS noise values for the LMP202x compared to that of

Competitor A. As described in previous sections, the outstanding noise performance of the LMP202x can be

even further improved by adding a simple low pass filter following the amplification stage.

The use of an additional filter, as shown in Figure 38 benefits applications with higher gain. For this reason, at a

gain of 10, only the results of circuit in Figure 37 are shown. The RMS input noise of the LMP202x are compared

with Competitor A's input noise performance. Competitor A's RMS input noise behaves the same with or without

an additional filter.

Table 1. RMS Input Noise Performance

Amplifier

Gain

(V/V)

System Bandwidth Requirement

(Hz)

RMS Input Noise (nV)

LMP202x

Competitor A

Figure 37 Circuit

Figure 38 Circuit

Figure 37, Figure 38 Circuit

10

100

229

See(1)

300

1000

763

See(1)

1030

100

100

229

196

300

1000

763

621

1030

1000

10

71

46

95

100

158

146

300

1000

608

462

1030