LMH6521 Datasheet, PDF (20/24 Pages) Texas Instruments – LMH6521 High Performance Dual DVGA

Table IF Frequency Bandpass Filter Component Values show
values for some common IF frequencies for Figure 17. The
filter shown in Figure 17 offers a good compromise between
bandwidth, noise rejection and cost. This filter topology works
best with the 12 to 16 bit analog to digital converters shown
in the COMPATIBLE HIGH SPEED ANALOG TO DIGITAL
CONVERTERS table.
IF Frequency Bandpass Filter Component Values
Center
Frequency
75 MHz 150 MHz 180 MHz 250 MHz
Bandwidth
40 MHz 60 MHz 75 MHz 100 MHz
R1, R2
L1, L2
C1, C2
90 Ω 90 Ω
390 nH 370 nH
10 pF 3 pF
90 Ω
300 nH
2.7 pF
90 Ω
225 nH
1.9 pF
C3
22 pF 19 pF 15 pF 11 pF
L5
220 nH 62 nH 54 nH 36 nH
R3, R4
100 Ω 100 Ω 100 Ω 100 Ω
An alternate narrowband filter approach is presented in Fig-
ure 18. The narrow band-pass antialiasing filter between the
LMH6521 and ADC16DV160 attenuates the output noise of
the LMH6521 outside the Nyquist zone helping to preserve
the available SNR of the ADC. Figure 18 shows a 1:4 input
transformer used to match the 200Ω balanced input of the
LMH6521 to the 50 unbalanced source to minimize insertion
lost at the input. Figure 18 shows the LMH6521 driving the
ADC16DV160 (16–bit ADC). The band-pass filter is a 3rd or-
der 100Ω matched tapped-L configured for a center frequen-
cy of 192MHz with a 20MHz bandwidth across the differential
inputs of the ADC16DV160. The ADC16DV160 is a dual
channel 16–bit ADC with maximum sampling rate of 160
MSPS. Using a 2–tone large input signal with the LMH6521
set to maximum gain (26dB) to drive an input signal level at
the ADC of –1dBFS, the SNR and SFDR results are shown
in table below.
LMH6521+BPF+ADC16DV160 vs Typical ADC16DV160
Specifications
Configuraton
ADC
Input
SNR
(dBFS)
SFDR
(dBFS)
LMH6521+BPF
+ADC16DV160
–1dBFS
75.5
82
ADC16DV160 only
–1dBFS
76
89
POWER SUPPLIES
The LMH6521 was designed primarily to be operated on 5V
power supplies. The voltage range for V_CC is 4.75V to 5.25V.
When operated on a board with high speed digital signals it
is important to provide isolation between digital signal noise
and the LMH6521 inputs. The SP16160CH1RB reference
board provides an example of good board layout.
FIGURE 18. Narrowband Tapped-L Bandpass Filter
Center Frequency is 192MHz with a 20MHz Bandwidth
Designed for 200Ω Impedance
30120125
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