LMH6624_05 Datasheet, PDF (16/19 Pages) National Semiconductor (TI) – Single/Dual Ultra Low Noise Wideband Operational Amplifier

Application Section (Continued)
noise density (i_ni) equation for the basic transimpedance
configuration and is plotted against feedback resistance (R_f)
showing all contributing noise sources in Figure 8. This plot
indicates the expected total equivalent input current noise
density (i_ni) for a given feedback resistance (R_f). The total
equivalent output voltage noise density (e_no) is i_ni*R_f.
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FIGURE 7. Transimpedance Amplifier Configuration
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FIGURE 9. Low Noise Integrator
HIGH-GAIN SALLEN-KEY ACTIVE FILTERS
The LMH6624/LMH6626 are well suited for high gain Sallen-
Key type of active filters. Figure 10 shows the 2^nd order
Sallen-Key low pass filter topology. Using component predis-
tortion methods discussed in OA-21 enables the proper
selection of components for these high-frequency filters.
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FIGURE 8. Current Noise Density vs. Feedback
Resistance
(5)
LOW NOISE INTEGRATOR
The LMH6624/LMH6626 implement a deBoo integrator
shown in Figure 9. Positive feedback maintains integration
linearity. The LMH6624/LMH6626’s low input offset voltage
and matched inputs allow bias current cancellation and pro-
vide for very precise integration. Keeping R_G and R_S low
helps maintain dynamic stability.
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FIGURE 10. Sallen-Key Active Filter Topology
LOW NOISE MAGNETIC MEDIA EQUALIZER
The LMH6624/LMH6626 implement a high-performance low
noise equalizer for such application as magnetic tape chan-
nels as shown in Figure 11. The circuit combines an integra-
tor with a bandpass filter to produce the low noise equaliza-
tion. The circuit’s simulated frequency response is illustrated
in Figure 12.
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