Datasheet

AMP Z
OUT
L2
L1
C1
C2
C3
L5
ADC Z
IN
R4
ADC V
IN
+
ADC V
IN
-
ADC V
CM
AMP V
OUT
-
AMP V
OUT
+
R3
R1
R2
PHASE (°)
V
OUT
(V)
2.5
2.0
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
-2.0
-2.5
0 45 90 135 180 225 270 315 360
OUT +
OUT -
COMMON MODE
= 0V
1.25 V
P
2.5 V
PP
5 V
PPD
(DIFFERENTIAL)
LMH6521
www.ti.com SNOSB47D –MAY 2011–REVISED MARCH 2013
Figure 40. Output Voltage with Respect to Output Common Mode
Figure 41. Wideband Bandpass Filter
Table Table 5 show values for some common IF frequencies for Figure 41. The filter shown in Figure 41 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 Table 7.
Table 5. 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 90 Ω 90 Ω 90 Ω 90 Ω
L1, L2 390 nH 370 nH 300 nH 225 nH
C1, C2 10 pF 3 pF 2.7 pF 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 Figure 42. 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 42 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 42 shows the LMH6521 driving the ADC16DV160 (16–bit ADC). The band-pass filter is a 3rd order 100Ω
matched tapped-L configured for a center frequency 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.
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