LMP7707MASLASHNOPB datasheet(18/41 Pages) TI1

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100k

10M

RS (:)

0.1

1000

10k

100

eni

eni =

en +

ei +

LMP7707, LMP7708, LMP7709

SNOSAW5B – JUNE 2007 – REVISED MARCH 2013

www.ti.com

where

•

eni is the total noise on the input

•

en denotes the input referred voltage noise

•

ei is the voltage drop across source resistance due to input referred current noise or ei = RS * in

•

et is the thermal noise of the source resistance

(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.

As is evident in Figure 48, at lower RS values, the total noise is dominated by the amplifier’s input voltage noise.

Once RS is larger than a few kilo-Ohms, then the dominant noise factor becomes the thermal noise of RS. As

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,

where VIN + = VS – IBIAS*RS

The last term, IBIAS*RS, shows the voltage drop across RS. To prevent errors introduced to the system due to this

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

the error contribution by IBIAS*RS less than the input voltage noise of the amplifier, so that it will not become the

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

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Product Folder Links: LMP7707 LMP7708 LMP7709

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