LMH6629_1011 Datasheet, PDF (22/28 Pages) National Semiconductor (TI) – Ultra-Low Noise, High-Speed Operational Amplifier with Shutdown

The optimum value of C_F is given by Equation 6 resulting in
the I-V -3dB bandwidth shown in Equation 7, or around 200
MHz in this case (assuming GBWP= 4GHz with COMP pin =
HI for LLP-8 package). This C_F value is a “starting point” and
C_F needs to be tuned for the particular application as it is often
less than 1pF and thus is easily affected by board parasitics,
etc. For maximum speed, the LMH6629 COMP pin should be
HI (for LLP-8 package). This CF value is a “starting point” and
CF needs to be tuned for the particular application as it is often
less than 1pF and thus is easily affected by board parasitics,
etc. For maximum speed, the LMH6629 COMP pin should be
HI (or use the SOT23 package).
Equation 6: Optimum C_F Value
(6)
Equation 7: Resulting -3dB Bandwidth
(7)
Equation 8 provides the total input current noise density (i_ni)
equation for the basic Transimpedance configuration and is
plotted against feedback resistance (R_F) showing all con-
tributing noise sources in Figure 16. The plot indicates the
expected total equivalent input current noise density (i_ni) for a
given feedback resistance (R_F). This is depicted in the
schematic of Figure 17 where total equivalent current noise
density (i_ni) is shown at the input of a noiseless amplifier and
noiseless feedback resistor (R_F). The total equivalent output
voltage noise density (e_no) is i_ni*R_F.
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FIGURE 17. Transimpedance Amplifier Equivalent Input
Source Model
From Figure 16, it is clear that with LMH6629’s extremely low-
noise characteristics, for R_F < 2.5kΩ, the noise performance
is entirely dominated by R_F thermal noise. Only above this
R_F threshold, LMH6629’s input noise current (i_n) starts being
a factor and at no R_F setting does the LMH6629 input noise
voltage play a significant role. This noise analysis has ignored
the possible noise gain increase, due to photo-diode capaci-
tance, at higher frequencies.
LOW-NOISE INTEGRATOR
Figure 18 shows a deBoo integrator implemented with the
LMH6629. Positive feedback maintains integration linearity.
The LMH6629’s low input offset voltage and matched inputs
allow bias current cancellation and provide for very precise
integration. Keeping R_G and R_S low helps maintain dynamic
stability.
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FIGURE 16. Current Noise Density vs. Feedback
Resistance
Equation 7: Noise Equation for Transimpedance
Amplifier
(8)
FIGURE 18. Low-Noise Integrator
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HIGH-GAIN SALLEN-KEY ACTIVE FILTERS
The LMH6629 is well suited for high-gain Sallen-Key type of
active filters. Figure 19 shows the 2^nd order Sallen-Key low-
pass filter topology. Using component predistortion methods
discussed in OA-21 enables the proper selection of compo-
nents for these high-frequency filters.
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