LMH6629 datasheet(15/22 Pages) NSC

15 / 22 page

TOTAL INPUT NOISE vs. SOURCE RESISTANCE

To determine maximum signal-to-noise ratios from the

LMH6629, an understanding of the interaction between the

amplifier’s intrinsic noise sources and the noise arising from

its external resistors is necessary. Figure 6 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 (in = in

+ = i

−) source,

there is also thermal voltage noise (e

t = √(4KTR)) associated

with each of the external resistors.

30068020

FIGURE 6. Non-Inverting Amplifier Noise Model

Equation 1 provides the general form for total equivalent input

voltage noise density (e

ni).

Equation 1: General Noise Equation

(1)

Equation 2 is a simplification of Equation 1 that assumes R

f ||

g = Rseq for bias current cancellation:

Equation 2: Noise Equation with

f || Rg = Rseq

(2)

Figure 7 schematically shows e

ni alongside VIN (the portion of

S source which reaches the non-inverting input of Figure 4)

and external components affecting gain (A

v= 1 + Rf / Rg), all

connected to an ideal noiseless amplifier.

30068023

FIGURE 7. Non-Inverting Amplifier Equivalent Noise

Source Schematic

Figure 8 illustrates the equivalent noise model using this as-

sumption. Figure 9 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 Rf||Rg = Rseq for bias current

cancellation. The total equivalent output voltage noise (e

no) is

ni*AV.

30068021

FIGURE 8. Noise Model with R

f||Rg = Rseq

As seen in Figure 9, e

ni is dominated by the intrinsic voltage

noise (e

n) of the amplifier for equivalent source resistances

below 15

Ω. Between 15Ω and 2.5 kΩ, e

ni is dominated by the

thermal noise (e

t = √(4kT(2Rseq)) of the equivalent source re-

sistance R

seq; incidentally, this is the range of Rseq values

where the LMH6629 has the best (lowest) Noise Figure (NF)

for the case where R

seq = Rf || Rg.

Above 2.5 k

Ω, e

ni is dominated by the amplifier’s current noise

n = √(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 R

seq =

f||RG = 20Ω (eni = 1.07 nV√Hz from Figure 9), the LMH6629

produces a total equivalent output noise voltage (e

ni * 10 V/V

√(1.57 * 825 MHz)) of 385 μV

rms.

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