Datasheet

LNA
R
S
R
S
R
F
C
F
To
PGA
Attenuator
V
CNTL
AFE5805
IN
AFE5805
www.ti.com SBOS421D –MARCH 2008–REVISED MARCH 2010
VCA—GAIN CONTROL When the AFE5805 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 AFE5805 is controlled by a single-ended
V
CNTL
voltage to +1.2V in order to minimize the
control signal input, the V
CNTL
pin. The control voltage
internal loading of the LNA outputs. Small
range spans from 0V to 1.2V, referenced to ground.
improvements in reduced power dissipation and
This control voltage varies the attenuation of the VCA
improved distortion performance may also be
based on its linear-in-dB characteristic with its
realized.
maximum attenuation (minimum gain) at V
CNTL
= 0V,
and minimum attenuation (maximum gain) at V
CNTL
=
1.2V. Table 17 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 17. Nominal Gain Control Ranges for Each
of the Four PGA Gain Settings
MIN GAIN AT MAX GAIN AT
PGA GAIN V
CNTL
= 0V V
CNTL
= 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 52. 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.
CW DOPPLER PROCESSING
The AFE5805 gain-control input has a –3dB
The AFE5805 integrates many of the elements
bandwidth of approximately 1.5MHz. This wide
necessary to allow for the implementation of a CW
bandwidth, although useful in many applications, can
doppler processing circuit, such as a V/I converter for
allow high-frequency noise to modulate the gain
each channel and a cross-point switch matrix with an
control input. In practice, this modulation can easily
8-input into 10-output (8×10) configuration.
be avoided by additional external filtering (R
F
and C
F
)
of the control input, as Figure 52 shows. Stepping the
In order to switch the AFE5805 from the default TGC
control voltage from 0V to 1.2V, the gain control
mode operation into CW mode, bit D5 of the VCA
response time is typically less than 500ns to settle
control register must be updated to low ('0'); see
within 10% of the final signal level of 1V
PP
(–6dBFS)
Table 5. This setting also enables access to all other
output.
registers that determine the switch matrix
configuration (see the Input Register Bit Map tables).
The control voltage input (VCNTL pin) represents a
In order to process CW signals, the LNA internally
high-impedance input. Multiple AFE5805 devices can
feeds into a differential V/I amplifier stage. The
be connected in parallel with no significant loading
transconductance of the V/I amplifier is typically
effects using the VCNTL pin of each device. Note that
15.6mA/V with a 100mV
PP
input signal. For proper
when the V
CNTL
pin is left unconnected, it floats up to
operation, the CW outputs must be connected to an
a potential of about +3.7V. For any voltage level
external bias voltage of +2.5V. Each CW output is
above 1.2V and up to 5.0V, the VCA continues to
designed to sink a small dc current of 0.9mA, and
operate at its minimum attenuation level; however, it
can deliver a signal current of up to 2.9mA
PP
.
is recommended to limit the voltage to approximately
1.5V or less.
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