LMH6518_15 Datasheet, PDF (24/41 Pages) Texas Instruments – LMH6518 900 MHz, Digitally Controlled, Variable Gain Amplifier

LMH6518
SNOSB21C – MAY 2008 – REVISED JULY 2013
J1
Input (from 50: source)
(2.5V CM)
+5V
R₅
200:
C₁
1 nF
R₁
R₂ 100:
49.9:
R₄
200:
C₂
1 nF
R₃
24.9:
+IN
LMH6518
-IN
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Figure 59. Recommended Single-Ended Bench-Test Input Drive from 50Ω Source
With the schematic of Figure 59, each LMH6518 input sees 25Ω to ground at the higher frequencies when the
capacitors look like shorts. This impedance increases to 125Ω at DC for both inputs, thereby preserving the
required matching at any frequency. This configuration, using properly selected R’s and C’s, allows four times
less biasing power dissipation than when the undriven input is biased with an effective 25Ω from the LMH6518
input to ground.
It is possible to drive the LMH6518 input from a ground referenced 50Ω source by providing level shift circuitry
on the driven input. Figure 60 shows a circuit where ½ the input signal reaches the LMH6518 input while the
negative supply voltage (V_EE) ensures that the 50Ω source at J1 does not experience any biasing current while
providing 50Ω termination to the source. The driven input (+IN) is biased to 2.5V (V_CC/2):
R₁
63.4W
+5V
J1
Input (from 50W source)
(Ground Referenced)
R₂
63.4W
R₃
82.5W
V_EE
(-3.3V)
R₅
76.8W
R₄
76.8W
+IN
LMH6518
-IN
Figure 60. LMH6518 Driven by a Ground Referenced Source
In the schematic of Figure 60, the equivalent impedance from each LMH6518 input to ground is around 38Ω.
This configuration’s power consumption of ∼0.5W (in R₁ - R₅) is higher than that of Figure 59 because of
additional power dissipated to perform the level shifting. Additional 50Ω attenuators can be placed between J1
and R₂/R₃ junction in Figure 60 in order to accommodate higher input voltages.
It is also possible to shift the LMH6518 output common mode level using a level shift approach similar to that of
Figure 60. The circuit in Figure 61 shows an implementation where the LMH6518’s nominal 1.2V CM output, set
by a 1.2V on V_CM input from the Gsample/s ADC, is shifted lower for proper interface to different ADC's which
require V_CM = 0V and have high input impedance:
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