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

Competitor A

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

300

10

1000

763

1030

100

229

196

300

100

1000

763

621

1030

10

71

46

95

1000

100

158

146

300

1000

608

462

1030

(1)

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 G

Ω, 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

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