LMP2021 datasheet(19/31 Pages) TI

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FEEDBACK

NETWORK

VIN_DIFF

VIN

RIN

CIN

INPUT

SWITCHES

VIN_DIFF

RIN

RON_SWITCH

COUT

VOUT

VIN

FEEDBACK

NETWORK

VIN_DIFF

VIN

RIN

LMP2021, LMP2022

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SNOSAY9E – SEPTEMBER 2008 – REVISED MARGH 2013

AV*IBIAS+*ZS - IBIAS−*ZF

(1)

Where AV is the closed loop gain of the system and IBIAS+ and IBIAS− denote the positive and negative bias

current, respectively. It is common to show the average of these bias currents in product datasheets. If IBIAS+ and

IBIAS− are not individually specified, use the IBIAS value provided in datasheet graphs or tables for this calculation.

For the application circuit shown in Figure 50, the LMP2022 amplifiers each have a gain of 18. With a sensor

impedance of 500

Ω for the bridge, and using the above equation, the total error due to the bias current on the

outputs of the LMP2022 amplifier will be less than 200 nV.

SENSOR IMPEDANCE

The sensor resistance, or the resistance connected to the inputs of the LMP2021/LMP2022, contributes to the

total impedance seen by the auto correcting input stage.

Figure 47. Auto Correcting Input Stage Model

As shown in Figure 47, the sum of RIN and RON-SWITCH will form a low pass filter with COUT during correction

cycles. As RIN increases, the time constant of this filter increases, resulting in a slower output signal which could

have the effect of reducing the open loop gain, AVOL, of the LMP2021/LMP2022. In order to prevent this

reduction in AVOL in presence of high impedance sensors or other high resistances connected to the input of the

LMP2021/LMP2022, a capacitor can be placed in parallel to this input resistance. This is shown in Figure 48

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