2

2

+

-

LMV841, LMV842, LMV844

www.ti.com

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

equation:

(1)

The voltage noise of the equivalent resistance can be calculated using the following equation:

(2)

where:

)

T = absolute temperature (K)

The total equivalent input voltage noise is given by the equation:

(3)

where:

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)

Copyright © 2006–2013, Texas Instruments Incorporated

Submit Documentation Feedback

19

Product Folder Links: LMV841 LMV842 LMV844