-

+

FEEDBACK

NETWORK

VIN_DIFF

VIN

RIN

1

10

1000

FEEDBACK CAPACITANCE (pF)

0

26

52

78

104

130

156

100

RF

CF

+

-

20

LMP2021, LMP2022

SNOSAY9G – SEPTEMBER 2008 – REVISED FEBRUARY 2016

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Product Folder Links: LMP2021 LMP2022

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Copyright © 2008–2016, Texas Instruments Incorporated

Application Information (continued)

offset as the duty cycle and amplitude of the pulses vary from part to part, depending on the correction at the

time. The lowest input current will be obtained when the impedances, both resistive and capacitive, are matched

between the inputs. By balancing the input capacitances, the effect can be minimized. A simple way to balance

the input impedance is adding a capacitance in parallel to the feedback resistance. The addition of this feedback

capacitance reduces the bias current and increases the stability of the operational amplifier. Figure 42 shows the

input bias current of the LMP202x when RF is set to 1 GΩ. As it can be seen from Figure 42, choosing the

optimum value of CF will help reducing the input bias current.

Figure 42. Input Bias Current vs. CF with RF = 1 GΩ

The effect of bias current on a circuit can be estimated with the following:

AV*IBIAS+*ZS - IBIAS−*ZF

(2)

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 46, 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.

8.1.4 Sensor Impedance

The sensor resistance, or the resistance connected to the inputs of the LMP202x, contributes to the total

impedance seen by the auto correcting input stage.