Datasheet

0 10 20 30 40 50 60 70 80 90 100
-75
-50
-25
0
25
50
75
100
125
150
175
I
BIAS
(pA)
TIME (s)
LMP2021/LMP2022
10 20 30 40 50 60 70 80 90 100
I
BIAS
(
pA
)
TIME (s)
-75
-50
-25
0
25
50
75
100
125
150
175
C
G
= 20 pF
C
G
= 1000 pF
C
G
= 0 pF
Competitor A
C
G
= 0 pF
C
G
= 0, 1, 3, 5, 8, 10, 20, 50, 75, 100, 200, 500, 1000 pF
C
G
= 20 pF
C
G
= 1000 pF
V
S
= 5V, V
CM
= V
S
/2, R
G
= 1 G:
LMP2021, LMP2022
www.ti.com SNOSAY9E –SEPTEMBER 2008–REVISED MARGH 2013
Table 1. RMS Input Noise Performance
RMS Input Noise (nV)
Amplifier
System Bandwidth
Gain LMP2021/LMP2022 Competitor A
Requirement (Hz)
(V/V)
Figure 41 Circuit Figure 42 Circuit Figure 42 Figure 41 Circuit
100 229 See
(1)
300
10
1000 763 See
(1)
1030
100 229 196 300
100
1000 763 621 1030
10 71 46 95
1000 100 158 146 300
1000 608 462 1030
(1) No significant difference in Noise measurements at A
V
= 10V/V
INPUT BIAS CURRENT
The bias current of the LMP2021/LMP2022 behaves differently than a conventional amplifier due to the dynamic
transient currents created on the input of an auto-zero circuit. The input bias current is affected by the charge
and discharge current of the input auto-zero circuit. The amount of current sunk or sourced from that stage is
dependent on the combination of input impedance (resistance and capacitance), as well as the balance and
matching of these impedances across the two inputs. This current, integrated in the auto-zero circuit, causes a
shift in the apparent "bias current". Because of this, there is an apparent "bias current vs. input impedance"
interaction. In the LMP2021/LMP2022 for an input resistive impedance of 1 GΩ, the shift in input bias current can
be up to 40 pA. This input bias shift is caused by varying the input's capacitive impedance. Since the input bias
current is dependent on the input impedance, it is difficult to estimate what the actual bias current is without
knowing the end circuit and associated capacitive strays.
Figure 44 shows the input bias current of the LMP2021/LMP2022 and that of another commercially available
amplifier from a competitor. As it can be seen, the shift in LMP2021/LMP2022 bias current is much lower than
that of other chopper style or auto zero amplifiers available from other vendors.
Figure 44. Input Bias Current of LMP2021/LMP2022 is lower than Competitor A
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
LMP2021/LMP2022 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 45 shows the input bias current
vs. input impedance of the LMP2021/LMP2022. The value of R
G
or input resistance in this test is 1 GΩ. When
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