Datasheet

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VCA — GAIN CONTROL
LNA
R
S
R
S
R
F
C
F
To
PGA
Attenuator
V
CNTRL
VCA8500
OUTPUT
VCA8500
SBOS390A – JANUARY 2008 – REVISED MARCH 2008
When the VCA8500 operates in CW mode, the
attenuator stage remains connected to the LNA
The attenuator (VCA) for each of the eight channels
outputs. Therefore, it is recommended to set the
of the VCA8500 is controlled by a single-ended
V
CNTRL
voltage to +1.2V in order to minimize the
control signal input, the V
CNTRL
pin. The control
internal loading of the LNA outputs. Small
voltage range spans from 0V to 1.2V, referenced to
improvements in reduced power dissipation and
ground. This control voltage varies the attenuation of
improved distortion performance may also be
the VCA based on its linear-in-dB characteristic with
realized.
its maximum attenuation (minimum gain) at V
CNTRL
=
0V, and minimum attenuation (maximum gain) at
V
CNTRL
= 1.2V. Table 12 shows the nominal gains for
each of the four PGA gain settings. The total gain
range is typically 46dB and remains constant
independent of the PGA selected; the Max Gain
column reflects the absolute gain of the full signal
path comprised of the fixed LNA gain of 20dB and the
programmable PGA gain.
Table 12. Nominal Gain Control Ranges for Each
of the Four PGA Gain Settings
MIN GAIN AT MAX GAIN AT
PGA GAIN V
CNTRL
= 0V V
CNTRL
= 1.2V
20dB – 4.5dB 41.5dB
25dB – 0.5dB 45.5dB
27dB 1.5dB 47.5dB
30dB 3.5dB 49.5dB
As previously discussed, the VCA architecture uses
eight attenuator segments that are equally spaced in
Figure 66. External Filtering of the V
CNTL
Input
order to approximate the linear-in-dB gain-control
slope. This approximation results in a monotonic
slope; gain ripple is typically less than ± 0.5dB.
The VCA8500 gain-control input has a – 3dB
The output stage of the VCA8500 delivers a
bandwidth of approximately 1.5MHz. This wide
differential output signal that swings symmetrically
bandwidth, although useful in many applications, can
around a fixed common-mode output voltage of
allow high-frequency noise to modulate the gain
+1.65V. The design of the output stage includes a
control input. In practice, this modulation can easily
common-mode control loop to hold the output
be avoided by additional external filtering (R
F
and C
F
)
common-mode voltage stable over a wide range of
of the control input, as Figure 66 shows. Stepping the
operating conditions. At the same time the output
control voltage from 0V to 1.2V, the gain control
offset and drift are kept to a minimum, allowing the
response time is typically less than 500ns to settle
VCA8500 to be dc-coupled directly to other devices
within 10% of the final signal level of a 1V
PP
output.
(such as an ADC). In cases where the output of the
The control voltage input (V
CNTRL
pin) represents a
VCA8500 drives devices with a non-matching input
high-impedance input. Multiple VCA8500 devices can
common-mode level, small ac-coupling capacitors
be connected in parallel with no significant loading
(for instance, 0.1 µ F) should be used.
effects using the V
CNTRL
pin of each device. Note that
It should be noted, however, that unlike many other
when the V
CNTRL
pin is left unconnected, it floats up
high-speed operational amplifiers, the VCA8500 is
to a potential of about +3.7V. For any voltage level
designed to drive a typical output load of 1k Ω
above 1.2V and up to 5.0V, the VCA continues to
single-ended (from each output to ground) or 2k Ω
operate at its minimum attenuation level; however, it
differentially. For most applications, this consideration
is recommended to limit the voltage to approximately
should not represent a limitation; many high-speed
1.5V or less.
ADCs have input impedances in the k Ω range. For
the VCA8500 to maintain the ability to provide the full
2V
PP
output swing, however, it is recommended to
keep the output loading to 800 Ω , single-ended (1.6k Ω
differential), or higher. In addition, care should be
taken to keep the capacitive loading of the outputs to
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