LMH6624MA datasheet(15/20 Pages) NSC

15 / 20 page

Application Section (Continued)

(1)

f||Rg =Rseq for bias current cancellation. Figure 4 illus-

trates the equivalent noise model using this assumption.

Figure 5 is a plot of e

ni against equivalent source resistance

seq) with all of the contributing voltage noise source of

Equation 2. This plot gives the expected e

ni for a given (Rseq)

which assumes R

f||Rg =Rseq for bias current cancellation.

The total equivalent output voltage noise (e

no)iseni*AV.

(2)

As seen in Figure 5,e

ni is dominated by the intrinsic voltage

noise (e

n) of the amplifier for equivalent source resistances

below 33.5

Ω. Between 33.5Ω and 6.43kΩ,e

ni is dominated

by the thermal noise (e

t =

√(4kT(2R

seq)) of the external

resistor. Above 6.43k

Ω,e

ni is dominated by the amplifier’s

current noise (i

n = √(2) inRseq). When Rseq = 464

Ω (ie.,

n/√(2) in) the contribution from voltage noise and current

noise of LMH6624/LMH6626 is equal.. For example, config-

ured with a gain of +20V/V giving a −3dB of 90MHz and

driven from R

seq =25

Ω, the LMH6624 produces a total

equivalent input noise voltage (e

ni x

1.57*90MHz) of

16.5µV

rms.

If bias current cancellation is not a requirement, then R

f||Rg

need not equal R

seq. In this case, according to Equation 1,

f||Rg should be as low as possible to minimize noise.

Results similar to Equation 1 are obtained for the inverting

configuration of Figure 2 if R

seq is replaced by Rb and Rg is

replaced by R

g +Rs. With these substitutions, Equation 1 will

yield an e

ni referred to the non-inverting input. Referring eni

to the inverting input is easily accomplished by multiplying

ni 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.

(3)

The Noise Figure formula is shown in Equation 3. The addi-

tion of a terminating resistor R

T, reduces the external ther-

mal noise but increases the resulting NF. The NF is in-

creased because R

T reduces the input signal amplitude thus

reducing the input SNR.

(4)

The noise figure is related to the equivalent source resis-

tance (R

seq) and the parallel combination of Rf and Rg.To

minimize noise figure.

•

Minimize R

f||Rg

•

Choose the Optimum R

S (ROPT)

OPT is the point at which the NF curve reaches a minimum

and is approximated by:

OPT

≈ e

n/in

NON-INVERTING GAINS LESS THAN 10V/V

Using the LMH6624/LMH6626 at lower non-inverting gains

requires external compensation such as the shunt compen-

sation as shown in Figure 6. The compensation capacitors

are chosen to reduce frequency response peaking to less

than 1dB.

INVERTING GAINS LESS THAN 10V/V

The lag compensation of Figure 7 will achieve stability for

lower gains. It is best used for the inverting configuration

because of its affect on the non-inverting input impedance.

20058921

FIGURE 4. Noise Model with R

f||Rg =Rseq

20058922

FIGURE 5. Voltage Noise Density vs. Source

Resistance

20058924

FIGURE 6. External Shunt Compensation

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