1

INPUT CAPACITANCE (pF)

-30

-18

-6

6

18

30

10

100

1000

RG

CG

+

-

0

10 20 30 40 50 60 70 80 90 100

-75

-50

-25

0

25

50

75

100

125

150

175

TIME (s)

LMP2021/LMP2022

10

20

30

40

50

60

70

80

90 100

TIME (s)

-75

-50

-25

0

25

50

75

100

125

150

175

CG = 20 pF

CG = 1000 pF

CG = 0 pF

Competitor A

CG = 0 pF

CG = 0, 1, 3, 5, 8, 10, 20, 50, 75, 100, 200, 500, 1000 pF

CG = 20 pF

CG = 1000 pF

VS = 5V, VCM = VS/2, RG = 1 G:

19

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 40. Input Bias Current of LMP202x is lower than Competitor A

8.1.3 Lowering the Input Bias Current

As mentioned in the Input Bias Current section, the input bias current of an auto zero amplifier such as the

LMP202x varies with input impedance and feedback impedance. Once the value of a certain input resistance, i.e.

sensor resistance, is known, it is possible to optimize the input bias current for this fixed input resistance by

choosing the capacitance value that minimizes that current. Figure 41 shows the input bias current vs. input

impedance of the LMP202x. The value of RG or input resistance in this test is 1 GΩ. When this value of input

resistance is used, and when a parallel capacitance of 22 pF is placed on the circuit, the resulting input bias

current is nearly 0 pA. Figure 41 can be used to extrapolate capacitor values for other sensor resistances. For

this purpose, the total impedance seen by the input of the LMP202x needs to be calculated based on Figure 41.

By knowing the value of RG, one can calculate the corresponding CG which minimizes the non-inverting input

bias current, positive bias current, value.

Figure 41. Input Bias Current vs. CG with RG = 1 GΩ

In a typical I-V converter, the output voltage will be the sum of DC offset plus bias current and the applied signal

through the feedback resistor. In a conventional input stage, the inverting input's capacitance has very little effect

on the circuit. This effect is generally on settling time and the dielectric soakage time and can be ignored. In auto

zero amplifiers, the input capacitance effect will add another term to the output. This additional term means that

the baseline reading on the output will be dependent on the input capacitance. The term input capacitance for

this purpose includes circuit strays and any input cable capacitances. There is a slight variation in the capacitive