Datasheet

= 20 nV/ Hz
e
n out
= e
n in
× A
noise
× 101 = 2 PV/ Hz
e
n in
= e
nv
2
+ e
nr
2
= 20 nV/ Hz
1 nV/ Hz
( )
2
20 nV/ Hz
( )
2
= +
e
nr
= 4kTR
eq
= 1 nV/ Hz
= 4 × 1.38 × 10
-23
J/K × 298K × 99:
R
F
+ R
G
R
F
× R
G
R
eq
=
10 k: × 100:
= 99:
=
10 k: + 100:
= 45 nV/ Hz
e
n out
= e
n in
× A
noise
× 101 = 4.5 PV/ Hz
e
n in
= e
nv
2
+ e
nr
2
= 45 nV/ Hz
40 nV/ Hz
( )
2
20 nV/ Hz
( )
2
= +
e
nr
= 4kTR
eq
= 40 nV/ Hz
= 4 × 1.38 × 10
-23
J/K × 298K × 99 k:
R
F
+ R
G
R
F
× R
G
R
eq
=
10 M: × 100 k:
= 99 k:
=
10 M: + 100 k:
LMV841, LMV842, LMV844
SNOSAT1G –OCTOBER 2006–REVISED FEBRUARY 2013 www.ti.com
The equivalent resistance for the first example with a resistor R
F
of 10MΩ and a resistor R
G
of 100kΩ at 25°C
(298 K) equals:
(5)
Now the noise of the resistors can be calculated, yielding:
(6)
The total noise at the input of the op amp is:
(7)
For the first example, this input noise will, multiplied with the noise gain, give a total output noise of:
(8)
In the second example, with a resistor R
F
of 10kΩ and a resistor R
G
of 100Ω at 25°C (298K), the equivalent
resistance equals:
(9)
The resistor noise for the second example is:
(10)
The total noise at the input of the op amp is:
(11)
For the second example the input noise will, multiplied with the noise gain, give an output noise of
(12)
In the first example the noise is dominated by the resistor noise due to the very high resistor values, in the
second example the very low resistor values add only a negligible contribution to the noise and now the
dominating factor is the op amp itself. When selecting the resistor values, it is important to choose values that
don't add extra noise to the application. Choosing values above 100kΩ may increase the noise too much. Low
values will keep the noise within acceptable levels; choosing very low values however, will not make the noise
even lower, but will increase the current of the circuit.
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