LMH6629 datasheet(18/22 Pages) NSC

18 / 22 page

30068012

FIGURE 15. Transimpedance Amplifier Equivalent Input

Source Model

From Figure 14, 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

F threshold, LMH6629’s input noise current (in) 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

The LMH6629 implement a deBoo integrator shown in Figure

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

G and RS low helps maintain dynamic

stability.

30068029

FIGURE 16. Noise Integrator

HIGH-GAIN SALLEN-KEY ACTIVE FILTERS

The LMH6629 is well suited for high-gain Sallen-Key type of

active filters. Figure 17 shows the 2nd 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.

30068036

FIGURE 17. Low Pass Sallen-Key Active Filter Topology

LOW-NOISE MAGNETIC MEDIA EQUALIZER

The LMH6629 implement a high-performance low-noise

equalizer for such application as magnetic tape channels as

shown in Figure 18. The circuit combines an integrator (used

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 Figure 19.

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

c intact. The integrator’s -3dB roll-off is set

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.

LMH6629 Datasheet(PDF) 18 Page - National Semiconductor (TI)

LMH6629 Datasheet(HTML) 18 Page - National Semiconductor (TI)

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

Datasheet Download

Link URL