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| LMH6624_05 Datasheet, PDF (15/19 Pages) National Semiconductor (TI) – Single/Dual Ultra Low Noise Wideband Operational Amplifier | |||
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Application Section (Continued)
(1)
Rf||Rg = Rseq for bias current cancellation. Figure 4 illustrates
the equivalent noise model using this assumption. Figure 5
is a plot of eni against equivalent source resistance (Rseq)
with all of the contributing voltage noise source of Equation
2. This plot gives the expected eni for a given (Rseq) which
assumes Rf||Rg = Rseq for bias current cancellation. The total
equivalent output voltage noise (eno) is eni*AV.
Rf||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 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.
20058921
FIGURE 4. Noise Model with Rf||Rg = Rseq
(2)
As seen in Figure 5, 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, config-
ured 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 x
1.57*90MHz) of
16.5µVrms.
(3)
The Noise Figure formula is shown in Equation 3. The addi-
tion of a terminating resistor RT, reduces the external ther-
mal noise but increases the resulting NF. The NF is in-
creased because RT reduces the input signal amplitude thus
reducing the input SNR.
(4)
The noise figure is related to the equivalent source resis-
tance (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
SINGLE SUPPLY OPERATION
The LMH6624/LMH6626 can be operated with single power
supply as shown in Figure 6. Both the input and output are
capacitively coupled to set the DC operating point.
20058922
FIGURE 5. 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,
FIGURE 6. Single Supply Operation
20058926
LOW NOISE TRANSIMPEDANCE AMPLIFIER
Figure 7 implements a low-noise transimpedance amplifier
commonly used with photo-diodes. The transimpedance
gain is set by Rf. Equation 4 provides the total input current
15
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