LMH6505 Datasheet, PDF (14/20 Pages) National Semiconductor (TI) – Wideband, Low Power, Linear-in-dB, Variable Gain Amplifier

Application Information (Continued)
LMH6505 GAIN CONTROL FUNCTION
In the plot, Gain vs. V_G, we can see the gain as a function of
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 LMH6505 gain control is
referred to as “linear-in-dB.”
For applications where the LMH6505 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 LMH6505 at half gain and below (V_G ≤ 1V).
GAIN STABILITY
The LMH6505 architecture allows complete attenuation of
the output signal from full gain to complete cut-off. This is
achieved by having the gain control signal V_G “throttle” the
signal which gets through to the final stage and which results
in the output signal. As a consequence, the R_G pin’s (pin 3)
average current (DC current) influences the operating point
of this “throttle” circuit and affects the LMH6505’s gain
slightly. Figure 4 below, shows this effect as a function of the
gain set by V_G.
able limits, please refer to the LMH6502 (Differential Linear
in dB variable gain amplifier) datasheet instead at http://
www.national.com/ds/LM/LMH6502.pdf.
AVOIDING OVERDRIVE OF THE LMH6505 GAIN
CONTROL INPUT
There is an additional requirement for the LMH6505 Gain
Control Input (V_G): V_G must not exceed +2.3V (with ±5V
supplies). The gain control circuitry may saturate and the
gain may actually be reduced. In applications where V_G is
being driven from a DAC, this can easily be addressed in the
software. If there is a linear loop driving V_G, such as an AGC
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_G input. If the device driving this divider
is operating off of ±5V supplies as well, its output will not
exceed 5V and through the divider V_G can not exceed 2.3V.
IMPROVING THE LMH6505 LARGE SIGNAL
PERFORMANCE
Figure 5 illustrates an inverting gain scheme for the
LMH6505.
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FIGURE 5. Inverting Amplifier
The input signal is applied through the R_G resistor. The V_IN
pin should be grounded through a 25Ω resistor. The maxi-
mum gain range of this configuration is given in the following
equation:
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FIGURE 4. LMH6505 Gain Variation over R_G DC
Current Capability vs. Gain
This plot shows the expected gain variation for the maximum
R_G DC current capability (±4.5 mA). For example, with gain
(A_V) set to −60 dB, if the R_G pin DC current is increased to
4.5 mA sourcing, one would expect to see the gain increase
by about 3 dB (to −57 dB). Conversely, 4.5 mA DC sinking
current through R_G would increase gain by 1.75 dB (to
−58.25 dB). As you can see from Figure 4 above, the effect
is most pronounced with reduced gain and is limited to less
than 3.75 dB variation maximum.
If the application is expected to experience R_G DC current
variation and the LMH6505 gain variation is beyond accept-
Eq. 5
The inverting slew rate of the LMH6505 is much higher than
that of the non-inverting slew rate. This ≈ 2X performance
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 6 shows a typical
circuit configuration for matching transmission lines.
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