10

1k

100k

10M

0.1

1

10

1000

1M

10k

100

100

2

2

2

LMP7707, LMP7708, LMP7709

SNOSAW5B – JUNE 2007 – REVISED MARCH 2013

www.ti.com

where

•

•

•

•

(1)

The input current noise of the LMP7707/LMP7708/LMP7709 is so low that it will not become the dominant factor

in the total noise unless source resistance exceeds 300 M

Ω, which is an unrealistically high value.

mentioned before, the current noise will not be the dominant noise factor for any practical application.

Figure 48. Total Input Noise

HIGH IMPEDANCE SENSOR INTERFACE

Many sensors have high source impedances that may range up to 10 M

Ω. The output signal of sensors often

needs to be amplified or otherwise conditioned by means of an amplifier. The input bias current of this amplifier

can load the sensor’s output and cause a voltage drop across the source resistance as shown in Figure 49,

voltage, an op amp with very low input bias current must be used with high impedance sensors. This is to keep

dominant noise factor. The LMP7707/LMP7708/LMP7709 have very low input bias current, typically 200 fA.

18

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