LMH6624MA datasheet(17/31 Pages) TI

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NF = 10 LOG

en + in (RSeq + (Rf||Rg) ) + 4KT (RSeq + (Rf||Rg))

4KT (RSeq + (Rf||Rg))

100

10k

100k

RSEQ (:)

0.1

100

eni

LMH6624

www.ti.com

SNOSA42F – NOVEMBER 2002 – REVISED MARCH 2013

(3)

As seen in Figure 54, eni is dominated by the intrinsic voltage noise (en) of the amplifier for equivalent source

resistances below 33.5

Ω. Between 33.5Ω and 6.43kΩ, eni is dominated by the thermal noise (et = √(4kT(2Rseq))

of the external resistor. Above 6.43k

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

Rseq = 464Ω (ie., en/√(2) in) the contribution from voltage noise and current noise of LMH6624/LMH6626 is equal.

For example, configured with a gain of +20V/V giving a

−3dB of 90MHz and driven from Rseq = 25Ω, the

LMH6624 produces a total equivalent input noise voltage (eni × √Hz1.57*90MHz) of 16.5μVrms.

Figure 54. 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 1, Rf||Rg should be as low as possible to minimize noise. Results similar to Equation 1 are obtained for

the inverting configuration of Figure 51 if Rseq is replaced by Rb and Rg is replaced by Rg + Rs. With these

substitutions, Equation 1 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.

NOISE FIGURE

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

(4)

The Noise Figure formula is shown in Equation 4. The addition of a terminating resistor RT, reduces the external

thermal noise but increases the resulting NF. The NF is increased because RT reduces the input signal amplitude

thus reducing the input SNR.

(5)

The noise figure is related to the equivalent source resistance (Rseq) and the parallel combination of Rf and Rg.

To minimize noise figure.

•

Minimize Rf||Rg

•

Choose the Optimum RS (ROPT)

ROPT is the point at which the NF curve reaches a minimum and is approximated by:

ROPT ≈ en/in

(6)

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