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LMP2021, LMP2022

SNOSAY9E – SEPTEMBER 2008 – REVISED MARGH 2013

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Figure 48. Sensor Impedance with Parallel Capacitance

LMP2021/LMP2022. An alternative circuit to achieve this is shown in Figure 49.

Figure 49. Alternative Sensor Impedance Circuit

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 ±2.5V 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.

0.32

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1

6.3

2.5

2.5

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.

DIFFERENTIAL BRIDGE SENSOR

Bridge sensors are used in a variety of applications such as pressure sensors and weigh scales. Bridge sensors

typically have a very small differential output signal. This very small signal needs to be accurately amplified

before it can be fed into an ADC. As discussed in the previous sections, the accuracy of the op amp used as the

ADC driver is essential to maintaining total system accuracy.

The high DC performance of the LMP2021/LMP2022 make these amplifiers ideal choices for use with a bridge

sensor. The LMP2021/LMP2022 have very low input offset voltage and very low input offset voltage drift. The

open loop gain of the LMP2021/LMP2022 is 160 dB.

The on chip EMI rejection filters available on the LMP2021/LMP2022 help remove the EMI interference

introduced to the signal and hence improve the overall system performance.

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