Datasheet
e
n out
= e
n in
× A
noise
e
n in
= e
nv
2
+ e
nr
2
e
nr
= 4kTR
eq
R
F
+ R
G
R
F
× R
G
R
eq
=
+
-
R
F
R
G
e
n in
LMV841, LMV842, LMV844
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SNOSAT1G –OCTOBER 2006–REVISED FEBRUARY 2013
The input referred voltage noise of the op amp is already located at the input, we can use the input referred
voltage noise without further transferring. The input referred current noise needs to be converted to an input
referred voltage noise. The current noise is negligibly small, as long as the equivalent resistance is not
unrealistically large, so we can leave the current noise out for these examples. That leaves us with the noise
sources of the resistors, being the thermal noise voltage. The influence of the resistors on the total noise can be
seen in the following examples, one with high resistor values and one with low resistor values. Both examples
describe an op amp configuration with a gain of 101 which will give the circuit a bandwidth of 44.5kHz. The op
amp noise is the same for both cases, i.e. an input referred noise voltage of 20nV/ and a negligibly small input
referred noise current.
Figure 39. Noise Circuit
To calculate the noise of the resistors in the feedback network, the equivalent input referred noise resistance is
needed. For the example in Figure 39, this equivalent resistance R
eq
can be calculated using the following
equation:
(1)
The voltage noise of the equivalent resistance can be calculated using the following equation:
(2)
where:
e
nr
= thermal noise voltage of the equivalent resistor
R
eq
(V/ )
k = Boltzmann constant (1.38 x 10
–23
J/K)
T = absolute temperature (K)
R
eq
= resistance (Ω)
The total equivalent input voltage noise is given by the equation:
(3)
where:
e
n in
= total input equivalent voltage noise of the circuit
e
nv
= input voltage noise of the op amp
The final step is multiplying the total input voltage noise by the noise gain, which is in this case the gain of the op
amp configuration:
(4)
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