LMH6629_1011 Datasheet, PDF(20/28 Page) National Semiconductor (TI) – Ultra-Low Noise, High-Speed Operational Amplifier with Shutdown

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LMH6629_1011 Datasheet, PDF (20/28 Pages) National Semiconductor (TI) – Ultra-Low Noise, High-Speed Operational Amplifier with Shutdown

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and external components affecting gain (Av= 1 + Rf / Rg), all

connected to an ideal noiseless amplifier.

30068054

FIGURE 9. Non-Inverting Amplifier Equivalent Noise

Source Schematic

Figure 10 illustrates the equivalent noise model using this as-

sumption. Figure 11 is a plot of eni against equivalent source

resistance (Rseq) with all of the contributing voltage noise

source of Equation 3. 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.

30068022

FIGURE 11. 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 2,

Rf || Rg should be as low as possible to minimize noise. Re-

sults similar to Equation 2 are obtained for the inverting

configuration of Figure 7 if Rseq is replaced by Rb and Rg is

replaced by Rg + Rs. With these substitutions, Equation 2 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.

30068021

FIGURE 10. Noise Model with Rf||Rg = Rseq

As seen in Figure 11, eni is dominated by the intrinsic voltage

noise (en) of the amplifier for equivalent source resistances

below 15â¦. Between 15â¦ and 2.5 kâ¦, eni is dominated by the

thermal noise (et = â(4kT(2Rseq)) of the equivalent source re-

sistance 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 kâ¦, eni is dominated by the amplifierâs current noise

(in = â(2) inRseq). When Rseq = 190â¦ (i.e., Rseq = en/â(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 â3dB of 825 MHz and driven from Rseq = Rf

|| Rg = 20â¦ (eni = 1.07 nVâHz from Figure 11), the LMH6629

produces a total equivalent output noise voltage (eni * 10 V/V

* â(1.57 * 825 MHz)) of 385 Î¼Vrms.

Equation 4: General Noise Figure Equation

(4)

Looking at the two parts of the NF expression (inside the log

function) yields:

Si/ Soâ Inverse of the power gain provided by the amplifier

No/ Niâ Total output noise power, including the contribution

of RS, divided by the noise power at the input due to RS

To simplify this, consider Na as the noise power added by the

amplifier (reflected to its input port):

Si/ Soâ 1/ G

No/ Niâ G * (Ni+Na) / Ni (where G*(Ni +Na ) = No)

Substituting these two expressions into the NF expression:

Equation 5: Simplified Noise Figure Equation

(5)

The noise figure expression has simplified to depend only on

the ratio of the noise power added by the amplifier at its input

(considering the source resistor to be in place but noiseless

in getting Na) to the noise power delivered by the source re-

sistor (considering all amplifier elements to be in place but

noiseless in getting Ni).

For a given amplifier with a desired closed loop gain, to min-

imize noise figure:

â¢ Minimize Rf || Rg

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