Datasheet

1
10 1000
FEEDBACK CAPACITANCE (pF)
0
26
52
78
104
130
156
NEGATIVE BIAS CURRENT (pA)
100
R
F
C
F
+
-
1
INPUT CAPACITANCE (pF)
-30
-18
-6
6
18
30
POSITIVE BIAS CURRENT (pA)
10 100 1000
R
G
C
G
+
-
LMP2021, LMP2022
SNOSAY9E –SEPTEMBER 2008–REVISED MARGH 2013 www.ti.com
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 R
G
, one can calculate the corresponding C
G
which
minimizes the non-inverting input bias current, positive bias current, value.
Figure 45. Input Bias Current vs. C
G
with R
G
= 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 R
F
is set to 1 GΩ. As it can be seen from Figure 46, choosing
the optimum value of C
F
will help reducing the input bias current.
Figure 46. Input Bias Current vs. C
F
with R
F
= 1 GΩ
The effect of bias current on a circuit can be estimated with the following:
18 Submit Documentation Feedback Copyright © 2008–2013, Texas Instruments Incorporated
Product Folder Links: LMP2021 LMP2022