LMH6654 Datasheet, PDF (16/31 Pages) National Semiconductor (TI) – Single/Dual Low Power, 250 MHz, Low Noise Amplifiers

LMH6654, LMH6655
SNOS956E – JUNE 2001 – REVISED AUGUST 2014
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7 Application and Implementation
7.1 Application Information
The LMH6654 single and LMH6655 dual high speed, voltage feedback amplifiers are manufactured on TI’s new
VIP10™ (Vertically Integrated PNP) complementary bipolar process. These amplifiers can operate from ±2.5 V to
±6 V power supply. They offer low supply current, wide bandwidth, very low voltage noise and large output
swing. Many of the typical performance plots found in the datasheet can be reproduced if 50 Ω coax and 50 Ω
R_IN/R_OUT resistors are used.
7.2 Typical Application
7.2.1 Design Requirements
7.2.1.1 Components Selection and Feedback Resistor
It is important in high-speed applications to keep all component leads short since wires are inductive at high
frequency. For discrete components, choose carbon composition axially leaded resistors and micro type
capacitors. Surface mount components are preferred over discrete components for minimum inductive effect.
Never use wire wound type resistors in high frequency applications.
Large values of feedback resistors can couple with parasitic capacitance and cause undesired effects such as
ringing or oscillation in high-speed amplifiers. Keep resistors as low as possible consistent with output loading
consideration. For a gain of 2 and higher, 402 Ω feedback resistor used for the typical performance plots gives
optimal performance. For unity gain follower, a 25 Ω feedback resistor is recommended rather than a direct short.
This effectively reduces the Q of what would otherwise be a parasitic inductance (the feedback wire) into the
parasitic capacitance at the inverting input.
7.2.2 Detailed Design Procedure
7.2.2.1 Driving Capacitive Loads
Capacitive loads decrease the phase margin of all op amps. The output impedance of a feedback amplifier
becomes inductive at high frequencies, creating a resonant circuit when the load is capacitive. This can lead to
overshoot, ringing and oscillation. To eliminate oscillation or reduce ringing, an isolation resistor can be placed as
shown in Figure 41 below. At frequencies above
1
F=
2 5 R_ISO C_LOAD
(1)
the load impedance of the Amplifier approaches R_ISO. The desired performance depends on the value of the
isolation resistor. The isolation resistance vs. capacitance load graph in the typical performance characteristics
provides the means for selection of the value of R_S that provides ≤ 3 dB peaking in closed loop A_V = 1 response.
In general, the bigger the isolation resistor, the more damped the pulse response becomes. For initial evaluation,
a 50Ω isolation resistor is recommended.
25:
-
V_IN
+
R_ISO
V_OUT
Figure 41. Isolation Resistor Placement
16
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