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AN-2195 Datasheet(PDF) 2 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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AN-2195 Datasheet(HTML) 2 Page - Texas Instruments

   
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½
LMH6521
0.01 PF
0.01 PF
0.01 PF
TC4-1W
1:4
1 PH
ADC16DV160
L4
180 nH
4 pF
4 pF
L4
180 nH
L3
160 nH
0.01 PF
C4
4 pF
L3
160nH
0.01 PF
L5
15 nH
50:
50:
C5
41 pF
40.2:
40.2:
1 PH
50:
VRM
Introduction
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1
Introduction
Sampled data systems can be categorized into two main types. The first and simplest is the baseband
system known as the “1st Nyquist-zone” system. The second is a more complex under-sampled system,
often referred to as the sub-sampled system or Intermediate frequency (IF)-sampled system. Baseband
system applications are generally DC-coupled while the IF-sample systems applications tend to be AC-
coupled. In this application reoprt, the LMH6521 is combined with Texas Instruments high-speed analog-
to-digital convertor (ADC), the ADC16DV160, that is optimized for an IF frequency of 192 MHz.
The LMH6521 contains two high-performance, digitally controlled variable-gain amplifiers (DVGA) with
exceptional gain and phase matching between channels over the entire attenuation range that mates
nicely with the dual channels of the ADC16DV160. The ADC16DV160 is a monolithic, dual-channel, high-
performance CMOS ADC capable of converting analog input signals into 16-bit digital words at rates up to
160 MSPS. The output noise density of the LMH6521 is typically 33 nV/Hz, which makes the LMH6521
suitable to drive 14-bit to 16-bit ADC’s.
2
Circuit Description
As shown in Figure 1, a low loss 1:4 (impedance ratio) input transformer TC4-1W is used to match the
LMH6521’s 200
Ω balanced input impedance to a 50Ω unbalanced signal source resulting in a low input
insertion loss of 0.8 dB. The LMH6521 provides variable gain, isolation, and source matching to the
ADC16DV160. The band-pass filter between the LMH6521 and ADC16DV160 provides attenuation of the
amplifier distortion products and noise outside the Nyquist zone helping to preserve the available SNR of
the ADC. The filter is a 3rd order 200
Ω matched tapped-L anti-aliasing filter designed for an intermediate
frequency of 192 MHz and a 20 MHz bandwidth.
Figure 1. Tapped-L Band-pass Filter For fc = 192 MHz with a
20 MHz Bandwidth Designed for 100
Ω Impedance
3
Tapped Inductor Band-pass Filter
The anti-aliasing band-pass filter is called a “Tapped-L” or “Tapped Inductor” filter because it uses series
inductors for the T-match impedance transform. Figure 2 shows L1, L2, and the combination of C11 and
C12 to make up the T-match structure of the filter. As shown in Figure 2, the filter is broken up into three
segments for analysis purposes: down impedance transform, up impedance transform, and band-pass
tank. The tank provides a 1st-order band-pass profile while the impedance transform matches the load
resistance, RL, to the source resistance, RS, at the designated center frequency and increases high
frequency roll-off to 4th order.
2
AN-2195 Driving High Speed ADCs With the LMH6521 DVGA for High IF
SNOA569A – November 2011 – Revised April 2013
AC-Coupled Applications
Submit Documentation Feedback
Copyright © 2011–2013, Texas Instruments Incorporated


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