lmh6504mmx National Semiconductor Corporation, lmh6504mmx Datasheet - Page 14

• Manufacturer Part Number: lmh6504mmx
• Description: Wideband, Low Power, Variable Gain Amplifier
• Manufacturer: National Semiconductor Corporation

www.national.com
Application Information
LMH6504 GAIN CONTROL FUNCTION
In the plot, Gain vs. V
the control voltage. The “Gain (V/V)” plot, sometimes re-
ferred to as the S-curve, is the linear (V/V ) gain. This is a
hyperbolic tangent relationship and is given by Equation 3.
The “Gain (dB)” plots the gain in dB and is linear over a wide
range of gains. Because of this, the LMH6504 gain control is
referred to as “linear-in-dB.”
For applications where the LMH6504 will be used at the
heart of a closed loop AGC circuit, the S-curve control char-
acteristic provides a broad linear (in dB) control range with
soft limiting at the highest gains where large changes in
control voltage result in small changes in gain. For applica-
tions requiring a fully linear (in dB) control characteristic, use
the LMH6504 at half gain and below (V
AVOIDING OVERDRIVE OF THE LMH6504 GAIN
CONTROL INPUT
There is an additional requirement for the LMH6504 Gain
Control Input (V
supplies). The gain control circuitry may saturate and the
gain may actually be reduced. In applications where V
being driven from a DAC, this can easily be addressed in the
software. If there is a linear loop driving V
loop, other methods of limiting the input voltage should be
implemented. One simple solution is to place a 2.2:1 resis-
tive divider on the V
is operating off of
exceed 5V and through the divider V
IMPROVING THE LMH6504 LARGE SIGNAL
PERFORMANCE
Figure 4 illustrates an inverting gain scheme for the
LMH6504.
The input signal is applied through the R
pin should be grounded through a 25Ω resistor. The maxi-
mum gain range of this configuration is given in the following
equation:
Eq. 5
The inverting slew rate of the LMH6504 is much higher than
that of the non-inverting slew rate. This 2X performance
FIGURE 4. Inverting Amplifier
G
): V
±
G
5V supplies as well, its output will not
G
G
input. If the device driving this divider
, we can see the gain as a function of
must not exceed +2.3V (with
G
can not exceed 2.3V.
G
G
G
≤ 1V).
(Continued)
, such as an AGC
resistor. The V
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±
G
5V
IN
is
14
improvement comes about because in the non-inverting con-
figuration, the slew rate of the overall amplifier is limited by
the input buffer. In the inverting circuit, the input buffer re-
mains at a fixed voltage and does not affect slew rate.
TRANSMISSION LINE MATCHING
One method for matching the characteristic impedance of a
transmission line is to place the appropriate resistor at the
input or output of the amplifier. Figure 5 shows a typical
circuit configuration for matching transmission lines.
The resistors R
istic impedance, Z
C
frequency range. It compensates for the increase of the op
amp’s output impedance with frequency.
MINIMIZING PARASITIC EFFECTS ON SMALL SIGNAL
BANDWIDTH
The best way to minimize parasitic effects is to use surface
mount components and to minimize lead lengths and com-
ponent distance from the LMH6504. For designs utilizing
through-hole components, specifically axial resistors, resis-
tor self-capacitance should be considered. Example: the
average magnitude of parasitic capacitance of RN55D 1%
metal film resistors is about 0.15 pF with variations of as
much as 0.1 pF between lots. Given the LMH6504’s ex-
tended bandwidth, these small parasitic reactance variations
can cause measurable frequency response variations in the
highest octave. We therefore recommend the use of surface
mount resistors to minimize these parasitic reactance ef-
fects.
RECOMMENDATIONS
Here are some recommendations to avoid problems and to
get the best performance:
• Do not place a capacitor across R
• Keep traces connecting R
• Place a small resistor (20-50Ω) between the output and
• Cut away the ground plane, if any, under R
• Keep decoupling capacitors as close as possible to the
• Connect pin 2 through a minimum resistance of 25Ω.
ADJUSTING OFFSETS AND DC LEVEL SHIFTING
Offsets can be broken into two parts: an input-referred term
and an output-referred term. These errors can be trimmed
using the circuit in Figure 6 . First set V
O
priately chosen series RC combination could be used to
shape the frequency response.
possible
C
LMH6504.
to match the output transmission line over a greater
L
FIGURE 5. TRANSMISSION LINE MATCHING
S
, R
O
I
, R
, of the transmission line or cable. Use
O
, and R
F
T
separated and as short as
are equal to the character-
F
G
. However, an appro-
to 0V and adjust the
G
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