VCA8500_0803 Datasheet, PDF (28/40 Pages) Texas Instruments – 8-Channel, Ultralow-Power, Variable Gain Amplifier with Low-Noise Pre-Amp

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