LMH6654 Datasheet, PDF (18/31 Pages) National Semiconductor (TI) – Single/Dual Low Power, 250 MHz, Low Noise Amplifiers

LMH6654, LMH6655
SNOS956E – JUNE 2001 – REVISED AUGUST 2014
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Typical Application (continued)
7.2.2.3 Total Input Noise vs. Source Resistance
The noise model for the non-inverting amplifier configuration showing all noise sources is described in Figure 44.
In addition to the intrinsic input voltage noise (e_n) and current noise (i_n = i_n+ = i_n−) sources, there also exits
thermal voltage noise et = 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 45 illustrates the equivalent noise model using this
assumption. The total equivalent output voltage noise (e_no) is e_ni * A_V.
Figure 44. Non-Inverting Amplifier Noise Model
e_ni = e_n2 + (i_n+ · R_Seq)² + 4kTR_Seq + (i_n- · (R_f || R_g))² + 4kT(R_f || R_g)
(3)
Figure 45. Noise Model with R_f || R_g = R_seq
e_ni = _{en2 + 2 (in · RSeq)}² _{+ 4kT (2RSeq)}
(4)
If bias current cancellation is not a requirement, then R_f || R_g does not need to equal R_seq. In this case, according
to Equation 3, R_f and R_g should be as low as possible in order to minimize noise. Results similar to Equation 3
are obtained for the inverting configuration on if R_seq is replaced by R_b || R_g is replaced by R_g + R_s. With these
substitutions, Equation 3 will yield an e_ni referred to the non-inverting input. Referring e_ni to the inverting input is
easily accomplished by multiplying e_ni by the ratio of non-inverting to inverting gains.
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