LMH6504 Datasheet, PDF (16/29 Pages) National Semiconductor (TI) – Wideband, Low Power, Variable Gain Amplifier

LMH6504
SNOSA96D – NOVEMBER 2003 – REVISED MARCH 2013
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LMH6504 GAIN CONTROL RANGE AND MINIMUM GAIN
Before discussing Gain Control Range, it is important to understand the issues which limit it. The minimum gain
of the LMH6504, theoretically, is zero, but in practical circuits 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 determine 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.
LMH6504 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 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
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 LMH6504 at half gain and below (V_G ≤ 1V).
AVOIDING OVERDRIVE OF THE LMH6504 GAIN CONTROL INPUT
There is an additional requirement for the LMH6504 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 resistive 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 LMH6504 LARGE SIGNAL PERFORMANCE
Figure 45 illustrates an inverting gain scheme for the LMH6504.
V_G
1
2
25:
V_IN
6
LMH6504
V_O
3
7
R_G
4
R_F
Figure 45. Inverting Amplifier
The input signal is applied through the R_G resistor. The V_IN pin should be grounded through a 25Ω resistor. The
maximum gain range of this configuration is given in the following equation:
A_VMAX = -
R_F
R_G
·K
(5)
The inverting slew rate of the LMH6504 is much higher than that of the non-inverting slew rate. This 2X
performance improvement comes about because in the non-inverting configuration, 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.
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