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AN-2195 Datasheet(PDF) 3 Page - Texas Instruments

Part No. AN-2195
Description  AN-2195 Driving High Speed ADCs with the LMH6521 DVGA for High IF AC-Coupled Applications
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Maker  TI [Texas Instruments]
Homepage  http://www.ti.com
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AN-2195 Datasheet(HTML) 3 Page - Texas Instruments

   
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FREQUENCY (MHz)
Down
Impedance
Transform
RL
RS
L1
C11
L2
CT
LT
Bandpass Tank
C12
Up
Impedance
Transform
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Tapped Inductor Band-pass Filter
Figure 2. Single-Ended Tapped-L Band-pass Filter Segments
Frequencies above the high frequency corner of 202 MHz have greater than 4th-order roll-off
(>24dB/octave) whereas lower frequencies below 182 MHz will have only a 1st order roll-off. At lower
frequencies there is less total bandwidth for aliasing. This filter scheme can provide > 40 dB harmonic
attenuation with minimal filter complexity and nearly 0 dB insertion loss to allow the LMH6521 to drive the
ADC input to full scale without compressing at the supply rails. Ripple in the pass-band is easily kept
below 1 dB. The equivalent noise bandwidth (ENBW) of this filter is approximately 44 MHz. Figure 3
shows the filter profile over frequency.
Figure 3. 192 MHz Tapped-L Filter Profile
Inductor L5 (Figure 1) in parallel with the ADC input capacitance and C5 to form a resonant tank to help
ensure the ADC input looks like a real resistance at the target IF center frequencies. Inductor L5 shorts
the ADC inputs at dc which introduces a zero into the transfer function. The value of C5 should be
adjusted with respect to the ADC input capacitance. Since C5 is parallel to C
IN(ADC), the equivalent value for
C5 is equal to the calculated value (C5
calculated) minus the ADC input capacitance, CIN(ADC).
3
SNOA569A – November 2011 – Revised April 2013
AN-2195 Driving High Speed ADCs With the LMH6521 DVGA for High IF
AC-Coupled Applications
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Copyright © 2011–2013, Texas Instruments Incorporated


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