RF

RG

· K

-

AVMAX =

2

VO

VG

RG

25:

LMH6504

3

1

6

7

4

RF

VIN

LMH6504

SNOSA96D – NOVEMBER 2003 – REVISED MARCH 2013

www.ti.com

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 VG = 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

AVMAX = 20 dB, the gain control range is 80 dB.

LMH6504 GAIN CONTROL FUNCTION

In the plot, Gain vs. VG, 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 (VG ≤ 1V).

AVOIDING OVERDRIVE OF THE LMH6504 GAIN CONTROL INPUT

There is an additional requirement for the LMH6504 Gain Control Input (VG): VG must not exceed +2.3V (with

±5V supplies). The gain control circuitry may saturate and the gain may actually be reduced. In applications

where VG is being driven from a DAC, this can easily be addressed in the software. If there is a linear loop

driving VG, 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 VG 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 VG can not exceed 2.3V.

IMPROVING THE LMH6504 LARGE SIGNAL PERFORMANCE

Figure 45 illustrates an inverting gain scheme for the LMH6504.

Figure 45. Inverting Amplifier

The input signal is applied through the RG resistor. The VIN pin should be grounded through a 25Ω resistor. The

maximum gain range of this configuration is given in the following equation:

(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.

16

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