lmh6629sdx National Semiconductor Corporation, lmh6629sdx Datasheet - Page 18

• Manufacturer Part Number: lmh6629sdx
• Description: Lmh6629 Ultra-low Noise, High Speed Operational Amplifier
• Manufacturer: National Semiconductor Corporation

www.national.com
From
noise characteristics, for R
is entirely dominated by R
R
a factor and at no R
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
The LMH6629 implement a deBoo integrator shown in
16. Positive feedback maintains integration linearity. The
LMH6629’s low input offset voltage and matched inputs allow
bias current cancellation and provide for very precise inte-
gration. Keeping R
stability.
HIGH-GAIN SALLEN-KEY ACTIVE FILTERS
The LMH6629 is well suited for high-gain Sallen-Key type of
active filters.
FIGURE 15. Transimpedance Amplifier Equivalent Input
F
threshold, LMH6629’s input noise current (i
Figure
14, it is clear that with LMH6629’s extremely low
Figure 17
FIGURE 16. Noise Integrator
G
F
setting does the LMH6629 input noise
and R
Source Model
shows the 2
F
F
< 2.5kΩ, the noise performance
S
thermal noise. Only above this
low helps maintain dynamic
nd
order Sallen-Key low-
n
) starts being
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Figure
18
pass filter topology. Using component predistortion methods
discussed in OA-21 enables the proper selection of compo-
nents for these high-frequency filters.
LOW-NOISE MAGNETIC MEDIA EQUALIZER
The LMH6629 implement a high-performance low-noise
equalizer for such application as magnetic tape channels as
shown in
to limit noise) with a bandpass filter (used to boost the re-
sponse centered at a frequency or over a band of interest) to
produce the low noise equalization. The circuit’s simulated
frequency response is illustrated in
In this circuit, the bandpass filter center frequency is set by
For higher selectivity, use high C values; for wider bandwidth,
use high L values, while keeping the product of L and C values
the same to keep f
by
If
the integrator and the bandpass filter frequency interaction is
minimized so that the operating frequencies of each can be
set independently. Lowering the value of R2 increases the
bandpass gain (boost) without affecting the integrator fre-
quencies. With the LMH6629’s wide Gain Bandwidth (4GHz),
the center frequency could be adjusted higher without worries
about loop gain limitation. This increases flexibility in tuning
the circuit.
FIGURE 17. Low Pass Sallen-Key Active Filter Topology
Figure
18. The circuit combines an integrator (used
c
intact. The integrator’s -3dB roll-off is set
Figure
19.
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