LMH6517 Datasheet, PDF (26/34 Pages) National Semiconductor (TI) – Multi Standard, IF and Baseband, Dual, DVGA

LMH6517
SNOSB19K – NOVEMBER 2008 – REVISED MARCH 2013
www.ti.com
ADC Noise Filter
Below are filter schematics and a table of values for some common IF frequencies. The filter shown in Figure 58
offers a good compromise between bandwidth, noise rejection and cost. This filter topology is the same as is
used on the ADC14V155KDRB High IF Receiver reference design board. This filter topology works best with the
12 and 14 bit sub-sampling analog to digital converters shown in the Table 6 table.
Filter Component Values
Fc
BW
Components
R1, R2
L1, L2
C1, C2
C3
L5
R3, R4
Table 5. Filter Component Values
75 MHz
40 MHz
100Ω
390 nH
10 pF
22 pF
220 nH
100Ω
140 MHz
20 MHz
200Ω
39 0nH
3 pF
41 pF
27 nH
200Ω
170 MHz
25 MHz
100Ω
560 nH
1.4 pF
32 pF
30 nH
100Ω
250 MHz
Narrow Band
499Ω
—
47 pF
11 pF
22 nH
499Ω
R1
AMP V_OUT -
L1
C1
L5
ADC V_IN +
C2
L2
AMP V_OUT + _R2
Figure 58. Sample Filter
ADC V_IN -
ADC V_CM
While the filters shown above have excellent performance in most respects they have one very large drawback,
and that is voltage loss. There is a 6dB loss right up front from the matching resistors (R1 and R2 in Figure 58)
and there are additional losses in the filter, primarily due to the resistive losses of the inductors. One solution is
to use larger inductors with higher Q ratings. An even better solution is to use the filter as an impedance
transforming circuit. Designing a filter with a low impedance input and a high impedance output will result in a
voltage gain that can be used to offset the voltage losses. While this solution won't work with high impedance
amplifiers, the LMH6517's low impedance output stage is perfectly suited for it. In essence the additional power
gained from driving a given voltage into a lower value load impedance is used to offset the power lost in the filter
and matching resistors.
The filter shown in Figure 59 uses both an impedance transform as well as a slight input impedance mismatch to
reduce the voltage loss from the amplifier to the ADC input. This configuration makes use of the strengths of the
LMH6517 output stage to deliver the best linearity possible. Due to the low impedance output stage the
LMH6517 can drive a lot of current into a low impedance load and still deliver high linearity signals.
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