LMH6629 datasheet(21/38 Pages) TI1

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100

10k

100k

RSEQ (:)

1000

100

0.1

eni

RSEQ = RF || RG

LMH6629

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SNOSB18E – APRIL 2010 – REVISED JUNE 2011

Figure 59 illustrates the equivalent noise model using this assumption. Figure 60 is a plot of eni against

equivalent source resistance (Rseq) with all of the contributing voltage noise source of Equation 4. This plot gives

the expected eni for a given (Rseq) which assumes Rf||Rg = Rseq for bias current cancellation. The total equivalent

output voltage noise (eno) is eni*AV.

Figure 59. Noise Model with Rf||Rg = Rseq

As seen in Figure 60, eni is dominated by the intrinsic voltage noise (en) of the amplifier for equivalent source

resistances below 15

Ω. Between 15Ω and 2.5 kΩ, eni is dominated by the thermal noise (et = √(4kT(2Rseq)) of

the equivalent source resistance Rseq; incidentally, this is the range of Rseq values where the LMH6629 has the

best (lowest) Noise Figure (NF) for the case where Rseq = Rf || Rg.

Above 2.5 k

Ω, eni is dominated by the amplifier’s current noise (in = √(2) inRseq). When Rseq = 190Ω (i.e., Rseq =

en/√(2) in), the contribution from voltage noise and current noise of LMH6629 is equal. For example, configured

with a gain of +10V/V giving a

−3dB of 825 MHz and driven from Rseq = Rf || Rg = 20Ω (eni = 1.07 nV√Hz from

Figure 60), the LMH6629 produces a total equivalent output noise voltage (eni * 10 V/V * √(1.57 * 825 MHz)) of

385

μVrms.

Figure 60. Voltage Noise Density vs. Source Resistance

If bias current cancellation is not a requirement, then Rf || Rg need not equal Rseq. In this case, according to

Equation 3, Rf || Rg should be as low as possible to minimize noise. Results similar to Equation 3 are obtained

for the inverting configuration of Figure 56 if Rseq is replaced by Rb and Rg is replaced by Rg + Rs. With these

substitutions, Equation 3 will yield an eni referred to the non-inverting input. Referring eni to the inverting input is

easily accomplished by multiplying eni by the ratio of non-inverting to inverting gains (1+Rg/ Rf).

NOISE FIGURE

Noise Figure (NF) is a measure of the noise degradation caused by an amplifier.

Equation 5: General Noise Figure Equation

(5)

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