LMH6624_16 Datasheet, PDF (18/38 Pages) Texas Instruments – LMH6624 and LMH6626 Single/Dual Ultra Low Noise Wideband Operational Amplifier

LMH6624, LMH6626
SNOSA42G – NOVEMBER 2002 – REVISED DECEMBER 2014
Feature Description (continued)
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Figure 48. Inverting Amplifier Configuration
7.2.2 Total Input Noise vs. Source Resistance
To determine maximum signal-to-noise ratios from the LMH6624 and LMH6626, an understanding of the
interaction between the amplifier’s intrinsic noise sources and the noise arising from its external resistors is
necessary.
Figure 49 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 3 provides the general form
for total equivalent input voltage noise density (e_ni). Equation 4 is a simplification of Equation 3 that assumes
R_f||R_g = R_seq for bias current cancellation. Figure 50 illustrates the equivalent noise model using this assumption.
Figure 51 is a plot of e_ni against equivalent source resistance (R_seq) with all of the contributing voltage noise
sources of Equation 4. 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 49. Non-Inverting Amplifier Noise Model
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