LMH6554_15 Datasheet, PDF (14/29 Pages) Texas Instruments – LMH6554 2.8-GHz Ultra Linear Fully Differential Amplifier

LMH6554
SNOSB30P – OCTOBER 2008 – REVISED JANUARY 2015
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Typical Applications (continued)
9.2.1.2 Detailed Design Procedure
To match the input impedance of the circuit in Figure 27 to a specified source resistance, RS, requries that RT ||
RIN = RS. The equations governing RIN and AV for single-to-differential operation are also provided in
Figure 27. These equations, along with the source matching condition, must be solved iteratively to achieve the
desired gain with the proper input termination. Component values for several common gain configuration in a 50
Ω environment are given in Table 1.
9.2.1.2.1 Enable / Disable Operation
The LMH6554 is equipped with an enable pin (V_EN) to reduce power consumption when not in use. The V_EN pin,
when not driven, floats high (on). When the V_EN pin is pulled low, the amplifier is disabled and the amplifier
output stage goes into a high impedance state so the feedback and gain set resistors determine the output
impedance of the circuit. For this reason input to output isolation will be poor in the disabled state and the part is
not recommended in multiplexed applications where outputs are all tied together.
With a 5V difference between V⁺ and V^-, the V_EN threshold is ½ way between the supplies (e.g. 2.5V with 5V
single supply) as shown in Figure 26. R2 ensures active (enable) mode with V_EN floating, and R1 provides input
current limiting. V_EN also has ESD diodes to either supply.
V⁺
R
Supply
Mid-Point
LMH6554
Bias
Circuitry
Q2 Q1
R2
20k
R1
10k
V_EN
R
I Tail
V^-
Figure 26. Enable Block Diagram
9.2.1.2.2 Single-Ended Input to Differential Output Operation
In many applications, it is required to drive a differential input ADC from a single ended source. Traditionally,
transformers have been used to provide single to differential conversion, but these are inherently bandpass by
nature and cannot be used for DC coupled applications. The LMH6554 provides excellent performance as a
single-ended input to differential output converter down to DC. Figure 27 shows a typical application circuit where
an LMH6554 is used to produce a balanced differential output signal from a single ended source.
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