LMH6401_16 Datasheet, PDF (27/48 Pages) Texas Instruments – LMH6401 DC to 4.5 GHz, Fully-Differential, Digital Variable-Gain Amplifier

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Application Information (continued)
LMH6401
SBOS730A – APRIL 2015 – REVISED MAY 2015
Figure 59. Smith Chart Showing Differential Input Impedance (z_O = 100 Ω)
10.1.2 Analog Output Characteristics
The LMH6401, as with most RF amplifiers, has two 10-Ω, on-chip resistors on each output leg to provide
isolation from board parasitics at the output pins; see the Functional Block Diagram section. When designing a
filter between the LMH6401 and the interfacing circuitry (ADC), the filter source impedance must be calculated by
taking into account the two 10-Ω, on-chip resistors. Table 11 shows the calculated external source impedance
values (R_O+ and R_O–) required for various matched filter loads (R_L). An important note is that the filter design
between the LMH6401 and the ADC is not limited to a matched filter, and source impedance values (R_O+ and
R_O–) can be reduced to achieve higher swing at the filter outputs. Achieving lower loss in the filter source
impedance resistors or higher swing at the filter outputs is often desirable because the amplifier must output
reduced swing to maintain the same full-scale input at the ADC and, thus, better linearity performance. An
example 370-MHz, un-matched, low-pass filter between the LMH6401 and ADS54J60 is illustrated in Figure 64,
with (R_O+ and R_O–) set to 20 Ω and R_L set to 100 Ω.
Table 11. Load Component Values⁽¹⁾
LOAD (R_L)
50 Ω
100 Ω
200 Ω
400 Ω
1 kΩ
R_O+ AND R_O– FOR A MATCHED TOTAL LOAD RESISTANCE AT
TERMINATION
AMPLIFIER OUTPUT
15 Ω
100 Ω
40 Ω
200 Ω
90 Ω
400 Ω
190 Ω
800 Ω
490 Ω
2000 Ω
TERMINATION LOSS
6 dB
6 dB
6 dB
6 dB
6 dB
(1) The total load includes termination resistors.
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