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| LMH6624MF Datasheet, PDF (17/31 Pages) Texas Instruments – LMH6624/LMH6626 Single/Dual Ultra Low Noise Wideband Operational Amplifier | |||
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www.ti.com
LMH6624
SNOSA42F â NOVEMBER 2002 â 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 33.5â¦. Between 33.5⦠and 6.43kâ¦, eni is dominated by the thermal noise (et = â(4kT(2Rseq))
of the external resistor. Above 6.43kâ¦, eni is dominated by the amplifierâs current noise (in = â(2) inRseq). When
Rseq = 464⦠(ie., en/â(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 â3dB of 90MHz and driven from Rseq = 25â¦, the
LMH6624 produces a total equivalent input noise voltage (eni à âHz1.57*90MHz) of 16.5μVrms.
100
10
et
eni
en
1
0.1
10
in
100
1k
10k
RSEQ (:)
100k
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.
2 22
2
en + in (RSeq + (Rf||Rg) ) + 4KT (RSeq + (Rf||Rg))
NF = 10 LOG
4KT (RSeq + (Rf||Rg))
(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 â en/in
(6)
Copyright © 2002â2013, Texas Instruments Incorporated
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