LMH6624_05 Datasheet, PDF (14/19 Pages) National Semiconductor (TI) – Single/Dual Ultra Low Noise Wideband Operational Amplifier

Application Section
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FIGURE 1. Non-Inverting Amplifier Configuration
INTRODUCTION
The LMH6624/LMH6626 are very wide gain bandwidth, ultra
low noise voltage feedback operational amplifiers. Their ex-
cellent performances enable applications such as medical
diagnostic ultrasound, magnetic tape & disk storage and
fiber-optics to achieve maximum high frequency signal-to-
noise ratios. The set of characteristic plots in the "Typical
Performance" section illustrates many of the performance
trade offs. The following discussion will enable the proper
selection of external components to achieve optimum sys-
tem performance.
BIAS CURRENT CANCELLATION
To cancel the bias current errors of the non-inverting con-
figuration, the parallel combination of the gain setting (R_g)
and feedback (R_f) resistors should equal the equivalent
source resistance (R_seq) as defined in Figure 1. Combining
this constraint with the non-inverting gain equation also seen
in Figure 1, allows both R_f and R_g to be determined explicitly
from the following equations:
R_f = A_VR_seq and R_g = R_f/(A_V-1)
When driven from a 0Ω source, such as the output of an op
amp, the non-inverting input of the LMH6624/LMH6626
should be isolated with at least a 25Ω series resistor.
As seen in Figure 2, bias current cancellation is accom-
plished for the inverting configuration by placing a resistor
(R_b) on the non-inverting input equal in value to the resis-
tance seen by the inverting input (R_f||(R_g+R_s)). R_b should to
be no less than 25Ω for optimum LMH6624/LMH6626 per-
formance. A shunt capacitor can minimize the additional
noise of R_b.
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FIGURE 2. Inverting Amplifier Configuration
TOTAL INPUT NOISE vs. SOURCE RESISTANCE
To determine maximum signal-to-noise ratios from the
LMH6624/LMH6626, an understanding of the interaction be-
tween the amplifier’s intrinsic noise sources and the noise
arising from its external resistors is necessary.
Figure 3 describes the noise model for the non-inverting
amplifier configuration showing all noise sources. In addition
to the intrinsic input voltage noise (e_n) and current noise
(i_n = i_n+ = i_n−) source, there is also thermal voltage noise
(e_t = √(4KTR)) associated with each of the external resistors.
Equation 1 provides the general form for total equivalent
input voltage noise density (e_ni). Equation 2 is a simplifica-
tion of Equation 1 that assumes
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FIGURE 3. Non-Inverting Amplifier Noise Model
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