Datasheet

e
ni
= e
n
+
2
e
i
+
2
e
t
2
V
-
V
+
V
OUT
R
O
R
G
V
IN
R
F
V
-
V
+
R
O
R
G
R
F
V
-
V
+
R
O
R
G
R
F
V
-
V
+
R
O
R
G
R
F
+
-
+
-
+
-
+
-
LMP7701, LMP7702, LMP7704
SNOSAI9H –SEPTEMBER 2005–REVISED MARCH 2013 www.ti.com
Figure 51. Noise Reduction Circuit
TOTAL NOISE CONTRIBUTION
The LMP7701/LMP7702/LMP7704 have very low input bias current, very low input current noise, and very low
input voltage noise. As a result, these amplifiers are ideal choices for circuits with high impedance sensor
applications.
Figure 52 shows the typical input noise of the LMP7701/LMP7702/LMP7704 as a function of source resistance
where:
e
n
denotes the input referred voltage noise
e
i
is the voltage drop across source resistance due to input referred current noise or e
i
= R
S
* i
n
e
t
shows the thermal noise of the source resistance
e
ni
shows the total noise on the input.
Where:
The input current noise of the LMP7701/LMP7702/LMP7704 is so low that it will not become the dominant factor
in the total noise unless source resistance exceeds 300 MΩ, which is an unrealistically high value.
As is evident in Figure 52, at lower R
S
values, total noise is dominated by the amplifier's input voltage noise.
Once R
S
is larger than a few kilo-Ohms, then the dominant noise factor becomes the thermal noise of R
S
. As
mentioned before, the current noise will not be the dominant noise factor for any practical application.
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