Datasheet

-80 -60 -40 -20 0 20
-0.5
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
DELTA A
V
(dB)
A
V
(dB)
4.5 mA SOURCING
4.5 mA SINKING
LMH6505
SNOSAT4E –DECEMBER 2005–REVISED APRIL 2013 www.ti.com
LMH6505 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 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 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.
LMH6505 GAIN CONTROL FUNCTION
In the plot, Gain vs. V
G
(Figure 19), 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 44 below, shows this
effect as a function of the gain set by V
G
.
Figure 44. 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 current through R
G
would increase
gain by 1.75 dB (to −58.25 dB). As you can see from Figure 44 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
acceptable limits, please refer to the LMH6502 (Differential Linear in dB variable gain amplifier) datasheet
instead at http://www.ti.com/lit/gpn/LMH6502.
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