LMH6401_16 Datasheet, PDF (28/48 Pages) Texas Instruments – LMH6401 DC to 4.5 GHz, Fully-Differential, Digital Variable-Gain Amplifier

LMH6401
SBOS730A – APRIL 2015 – REVISED MAY 2015
www.ti.com
The LMH6401 can be either dc- or ac-coupled at the outputs. For dc-coupled applications, the device provides
an option to control the output common-mode voltage using the VOCM pin. Device performance is optimal when
the output common-mode voltage is within ±0.5 V of mid-supply (see Figure 21) and performance degrades
outside the range when the output swing approaches clipping levels. The LMH6401 can achieve a maximum
output swing of 6 V_PPD with the output common-mode voltage centered at mid-supply.
Note that by default, the output common-mode voltage is set to mid-supply before the two 10-Ω, on-chip
resistors; see the Functional Block Diagram section. On a single-supply operation when dc-coupling the device
outputs to an ADC using common-mode, level-shifting resistors, the output common-mode voltage and resistor
values being calculated must include the two internal 10-Ω resistors in the equation. When operating the
LMH6401 on split supplies and dc-coupling the outputs, TI recommends matching the output common-mode
voltage of the LMH6401 with the input common-mode voltage of the ADC. A simple design procedure is to select
the supply voltages (VS+ and VS–) such that the default output common-mode voltage being set is equal to the
input common-mode voltage of the ADC. As illustrated in Figure 66, the supplies of the LMH6401 are selected
such that the default output common-mode voltage is set to mid-supply or 1.23 V, which is within the input
common-mode voltage range of the ADC (1.185 V to 1.265 V).
10.1.2.1 Driving Capacitive Loads
With high-speed signal paths, capacitive loading at the output is highly detrimental to the signal path, as shown
in Figure 60. The device on-chip resistors are included in order to isolate the parasitic capacitance associated
with the package and the printed circuit board (PCB) pads that the device is soldered to. However, designers
must make every effort to reduce parasitic loading on the amplifier output pins. The LMH6401 is stable with most
capacitive loads up to 10 pF; however, bandwidth suffers with capacitive loading on the output.
10
0
-10
-20
-30
-40
10
No Cap
1 pF
2.4pF
4.7pF
10pF
100
1000
Frequency (MHz)
10000
D024
Figure 60. Frequency Response vs Capacitive Load
10.1.3 Thermal Feedback Control
The LMH6401 can be used to optimize long-term settling responses using thermal feedback gain and frequency
control registers. These registers are disabled on power-up and can be enabled by clearing the thermal SD bit;
see the Thermal Feedback Gain Control register. The thermal feedback gain control bits increase the low-
frequency gain and the thermal feedback frequency control bits shift the boost frequency. The thermal feedback
gain and frequency registers both have a range of 32 steps. When the function is enabled, there is a small initial
gain offset to optimize the control range. The thermal feedback off condition is illustrated in the gain control plot
(Figure 61), along with a sweep of gain settings of 0, 4, 8…28, and 31 with a 0 register value representing the
minimum gain setting. The frequency control is illustrated in Figure 62 with the optimal gain setting from the gain
sweep over the values of 0, 4, 8…28, and 31 with a 0 register value representing the minimum frequency boost
setting.
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