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| LMH6624_16 Datasheet, PDF (19/38 Pages) Texas Instruments – LMH6624 and LMH6626 Single/Dual Ultra Low Noise Wideband Operational Amplifier | |||
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Feature Description (continued)
LMH6624, LMH6626
SNOSA42G â NOVEMBER 2002 â REVISED DECEMBER 2014
Figure 50. Noise Model with Rf||Rg = Rseq
(4)
As seen in Figure 51, eni is dominated by the intrinsic voltage noise (en) of the amplifier for equivalent source
resistances below 26 Ω. Between 26 ⦠and 3.1 kâ¦, eni is dominated by the thermal noise (et = â(4kT(2Rseq)) of
the equivalent source resistance Rseq. Above 3.1 kâ¦, eni is dominated by the amplifierâs current noise (in = â2
inRseq). When Rseq = 283 ⦠(that is, Rseq = en/â2 in) the contribution from voltage noise and current noise of
LMH6624 and LMH6626 is equal. For example, configured with a gain of +20V/V giving a â3 dB of 90 MHz and
driven from Rseq = Rf || Rg = 25 Ω (eni = 1.3 nVâHz from Figure 51), the LMH6624 produces a total output noise
voltage (eni à 20 V/V à â(1.57 à 90 MHz)) of 309 μVrms.
100
10
et
eni
en
1
0.1
10
in
100
1k
10k
RSEQ (:)
100k
Figure 51. 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 3, Rf || Rg should be as low as possible to minimize noise. Results similar to Equation 3 are obtained
for the inverting configuration of Figure 48 if Rseq is replaced by Rb and Rg is replaced by Rg + Rs. With these
substitutions, Equation 3 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.
Copyright © 2002â2014, Texas Instruments Incorporated
Product Folder Links: LMH6624 LMH6626
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