LMP2021 datasheet(18/31 Pages) TI

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1000

FEEDBACK CAPACITANCE (pF)

104

130

156

100

INPUT CAPACITANCE (pF)

-30

-18

-6

100

1000

LMP2021, LMP2022

SNOSAY9E – SEPTEMBER 2008 – REVISED MARGH 2013

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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 45 can be used to extrapolate capacitor values for other sensor

resistances. For this purpose, the total impedance seen by the input of the LMP2021/LMP2022 needs to be

calculated based on Figure 45. 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 45. 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

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 46 shows the

input bias current of the LMP2021/LMP2022 when RF is set to 1 GΩ. As it can be seen from Figure 46, choosing

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

Figure 46. Input Bias Current vs. CF with RF = 1 GΩ

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

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