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

G

≤ 1V).

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

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 25

Ω 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

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

priately chosen series RC combination could be used to

shape the frequency response.

•

Keep traces connecting R

possible

•

Place a small resistor (20-50

Ω) between the output and

C

L

•

Cut away the ground plane, if any, under R

G

•

Keep decoupling capacitors as close as possible to the

LMH6504.

•

Connect pin 2 through a minimum resistance of 25

Ω.

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

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FIGURE 4. Inverting Amplifier

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FIGURE 5. TRANSMISSION LINE MATCHING

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