Datasheet

10
1k 100k 10M
R
S
(:)
0.1
1
10
1000
1M10k
100
100
VOLTAGE NOISE DENSITY (nV/
Hz)
e
ni
e
n
e
i
e
t
e
ni
= e
n
+
2
e
i
+
2
e
t
2
LMP7707, LMP7708, LMP7709
SNOSAW5B –JUNE 2007–REVISED MARCH 2013 www.ti.com
where
• e
ni
is the total noise on the input
• 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
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 MΩ, which is an unrealistically high value.
As is evident in Figure 48, at lower R
S
values, the 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.
Figure 48. Total Input Noise
HIGH IMPEDANCE SENSOR INTERFACE
Many sensors have high source impedances that may range up to 10 MΩ. 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 V
IN
+ = V
S
– I
BIAS
*R
S
The last term, I
BIAS
*R
S
, shows the voltage drop across R
S
. 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 I
BIAS
*R
S
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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