LMH6624MF Datasheet, PDF (16/31 Pages) Texas Instruments – LMH6624/LMH6626 Single/Dual Ultra Low Noise Wideband Operational Amplifier

LMH6624
SNOSA42F – NOVEMBER 2002 – REVISED MARCH 2013
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Figure 51. Inverting Amplifier Configuration
TOTAL INPUT NOISE vs. SOURCE RESISTANCE
To determine maximum signal-to-noise ratios from the LMH6624/LMH6626, an understanding of the interaction
between the amplifier’s intrinsic noise sources and the noise arising from its external resistors is necessary.
Figure 52 describes the noise model for the non-inverting amplifier configuration showing all noise sources. In
addition to the intrinsic input voltage noise (e_n) and current noise (i_n = i_n+ = i_n^−) source, there is also thermal
voltage noise (e_t = √(4KTR)) associated with each of the external resistors. Equation 1 provides the general form
for total equivalent input voltage noise density (e_ni). Equation 2 is a simplification of Equation 1 that assumes
Figure 52. Non-Inverting Amplifier Noise Model
(2)
R_f||R_g = R_seq for bias current cancellation. Figure 53 illustrates the equivalent noise model using this assumption.
Figure 54 is a plot of e_ni against equivalent source resistance (R_seq) with all of the contributing voltage noise
source of Equation 2. This plot gives the expected e_ni for a given (R_seq) which assumes R_f||R_g = R_seq for bias
current cancellation. The total equivalent output voltage noise (e_no) is e_ni*A_V.
Figure 53. Noise Model with R_f||R_g = R_seq
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