LMH6505 Datasheet, PDF (16/18 Pages) National Semiconductor (TI) – Wideband, Low Power, Linear-in-dB, Variable Gain Amplifier

Signal frequencies must not reach the gain control port of the
LMH6505, or the output signal will be distorted (modulated by
itself). A fast settling AGC needs additional filtering beyond
the integrator stage to block signal frequencies. This is pro-
vided in Figure 9 by a simple R-C filter (R₁₀ and C₃); better
distortion performance can be achieved with a more complex
filter. These filters should be scaled with the input signal fre-
quency. Loops with slower response time, which means
longer integration time constants, may not need the R₁₀ –
C₃ filter.
Checking the loop stability can be done by monitoring the
V_G voltage while applying a step change in input signal am-
plitude. Changing the input signal amplitude can be easily
done with an arbitrary waveform generator.
CIRCUIT LAYOUT CONSIDERATIONS & EVALUATION
BOARDS
A good high frequency PCB layout including ground plane
construction and power supply bypassing close to the pack-
age is critical to achieving full performance. The amplifier is
sensitive to stray capacitance to ground at the I^- input (pin 7)
so it is best to keep the node trace area small. Shunt capac-
itance across the feedback resistor should not be used to
compensate for this effect. Capacitance to ground should be
minimized by removing the ground plane from under the body
of R_G. Parasitic or load capacitance directly on the output (pin
6) degrades phase margin leading to frequency response
peaking.
The LMH6505 is fully stable when driving a 100Ω load. With
reduced load (e.g. 1k.) there is a possibility of instability at
very high frequencies beyond 400 MHz especially with a ca-
pacitive load. When the LMH6505 is connected to a light load
as such, it is recommended to add a snubber network to the
output (e.g. 100Ω and 39 pF in series tied between the
LMH6505 output and ground). C_L can also be isolated from
the output by placing a small resistor in series with the output
(pin 6).
Component parasitics also influence high frequency results.
Therefore it is recommended to use metal film resistors such
as RN55D or leadless components such as surface mount
devices. High profile sockets are not recommended.
National Semiconductor suggests the following evaluation
board as a guide for high frequency layout and as an aid in
device testing and characterization:
Device
LMH6505
Package
SOIC
Evaluation Board
Part Number
LMH730066
The evaluation board can be shipped when a device sample
request is placed with National Semiconductor. Evaluation
board documentation can be found in the LMH6505 product
folder at www.National.com.
www.national.com
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