LMH6521_16 Datasheet, PDF (14/29 Pages) Texas Instruments – High Performance Dual DVGA

LMH6521
SNOSB47D – MAY 2011 – REVISED MARCH 2013
www.ti.com
1P
+
LMH6521
-
1P
R_T
0.01P
R_T.
0.01P
FILTER
+IN
R_T
V_RM ^ADC16DV160
R_T
-IN
Figure 30. Example Output Configuration
The outputs of the LMH6521 need to be biased to near ground potential. On the evaluation board, 1µH inductors
are installed to provide proper output biasing. The bias current is approximately 36mA per output pin and is not a
function of the load condition, which makes the LMH6521 robust to handle various output load conditions while
maintaining superior linearity as shown in Figure 31. With large inductors and high operating frequencies the
inductor will present a very high impedance and will have minimal AC current. If the inductor is chosen to have a
smaller value, or if the operating frequency is very low there could be enough AC current flowing in the inductor
to become significant. Make sure to check the inductor datasheet to not exceed the maximum current limit.
60
40
50
VOUT= 4 VPP
36
40
32
30
28
Power Gain
20
24
10
20
Maximum Gain
f = 200 MHz
0
16
50 100 150 200 250 300
FILTER DIFFERENTIAL INPUT RESISTANCE ( )
Figure 31. OIP3 vs Amplifier Load Resistance
DIGITAL CONTROL
The LMH6521 will support three modes of gain control, parallel mode, serial mode (SPI compatible) and pulse
mode. Parallel mode is fastest and requires the most board space for logic line routing. Serial mode is
compatible with existing SPI compatible systems. The pulse mode is both fast and compact, but must step
through intermediate gain steps when making large gain changes.
Pins MOD0 and MOD1 are used to configure the LMH6521 for the three gain control modes. MOD0 and MOD1
have weak pull-up resistors to an internal 2.5V reference but is designed for 2.5V-5V CMOS logic levels. MOD0
and MOD1 can be externally driven (LOGIC HIGH) to voltages between 2.5V to 5V to configure the LMH6521
into one of the three digital control modes. Some pins on the LMH6521 have different functions depending on the
digital control mode. These functions are shown in Digital Control Modes Pin Functions.
14
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