LMH6505 datasheet(14/20 Pages) NSC

14 / 20 page

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

≤ 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

This plot shows the expected gain variation for the maximum

G DC current capability (±4.5 mA). For example, with gain

V) set to −60 dB, if the RG 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-

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): 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 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.

The input signal is applied through the R

G resistor. The VIN

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 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.

20171066

FIGURE 4. LMH6505 Gain Variation over R

G DC

Current Capability vs. Gain

20171054

FIGURE 5. Inverting Amplifier

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