LMH6505 Datasheet, PDF (13/18 Pages) National Semiconductor (TI) – Wideband, Low Power, Linear-in-dB, Variable Gain Amplifier

LMH6505 GAIN CONTROL RANGE AND MINIMUM GAIN
Before discussing Gain Control Range, it is important to un-
derstand the issues which limit it. The minimum gain of the
LMH6505 is theoretically zero, but in practical circuits it is
limited by the amount of feedthrough, here defined as the gain
when V_G = 0V. Capacitive coupling through the board and
package, as well as coupling through the supplies, will deter-
mine the amount of feedthrough. Even at DC, the input signal
will not be completely rejected. At high frequencies
feedthrough will get worse because of its capacitive nature.
At frequencies below 10 MHz, the feed through will be less
than −60 dB and therefore, it can be said that with
A_VMAX = 20 dB, the gain control range is 80 dB.
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 referred
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 characteristic
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 applications 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 cutoff. 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 be-
low, shows this effect as a function of the gain set by V_G.
rent 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-
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 sup-
plies). 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 soft-
ware. If there is a linear loop driving V_G, such as an AGC loop,
other methods of limiting the input voltage should be imple-
mented. One simple solution is to place a 2.2:1 resistive
divider on the V_G input. If the device driving this divider is op-
erating 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 cur-
(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 remains 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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