10

100

1k

10k

3 dB FILTER BANDWIDTH (Hz)

0.01

0.1

1

10

AV = 60 dB

AV = 40 dB

2-Stage Filter

18

LMP2021, LMP2022

SNOSAY9G – SEPTEMBER 2008 – REVISED FEBRUARY 2016

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Product Folder Links: LMP2021 LMP2022

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Copyright © 2008–2016, Texas Instruments Incorporated

Application Information (continued)

Using the circuit in Figure 38 has the advantage of removing the non-linear filter bandwidth dependency which is

seen when the circuit in Figure 37 is used. The difference in noise performance of the circuits in Figure 37 and

Figure 38 becomes apparent only at higher gains. At voltage gains of 10 V/V or less, there is no difference

between the noise performance of the two circuits.

Figure 39. RMS Input Referred Noise vs. Frequency

Figure 39 shows the total input referred noise vs. 3 dB corner of both filters of Figure 37 and Figure 38 at gains

of 100V/V and 1000V/V. For these measurements and using Figure 37's circuit, RF = 49.7 kΩ and RIN = 497Ω.

Value of CF has been changed to achieve the desired 3 dB filter corner frequency. In the case of Figure 38's

circuit, RF = 49.7 kΩ and RIN = 497Ω, RFILT = 49.7 kΩ, and CFILT has been changed to achieve the desired 3 dB

filter corner frequency. Figure 39 compares the RMS noise of these two circuits. As Figure 39 shows, the RMS

noise measured the circuit in Figure 38 has lower values and also depicts a more linear shape.

8.1.2 Input Bias Current

The bias current of the LMP202x 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. This effectivly creates a repetitive impulse current noise of 100's

of pA. For this reason, the LMP202x is not recommeded for source impedances of 1 M

Ω or greater.

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 by the input capacitence, causes a shift in the apparent "bias current". Because of this,

there is an apparent "bias current vs. input impedance" interaction. In the LMP202x 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 40 shows the input bias current of the LMP202x and that of another commercially available amplifier from

a competitor. As it can be seen, the shift in LMP202x bias current is much lower than that of other chopper style

or auto zero amplifiers available from other vendors.