-

+

FEEDBACK

NETWORK

VIN_DIFF

VIN

RIN

CIN

INPUT

SWITCHES

VIN_DIFF

RIN

RON_SWITCH

COUT

VOUT

+

-

CIN

VIN

-

+

FEEDBACK

NETWORK

VIN_DIFF

VIN

RIN

CIN

INPUT

SWITCHES

VIN_DIFF

RIN

RON_SWITCH

COUT

VOUT

+

-

VIN

21

LMP2021, LMP2022

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SNOSAY9G – SEPTEMBER 2008 – REVISED FEBRUARY 2016

Product Folder Links: LMP2021 LMP2022

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Copyright © 2008–2016, Texas Instruments Incorporated

Application Information (continued)

Figure 43. Auto Correcting Input Stage Model

As shown in Figure 43, 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 LMP202x. In order to prevent this reduction in AVOL in

presence of high impedance sensors or other high resistances connected to the input of the LMP202x, a

capacitor can be placed in parallel to this input resistance. This is shown in Figure 44.

Figure 44. Sensor Impedance with Parallel Capacitance

CIN in Figure 44 adds a zero to the low pass filter and hence eliminating the reduction in AVOL of the LMP202x.

An alternative circuit to achieve this is shown in Figure 45.

Figure 45. Alternative Sensor Impedance Circuit