AD9874-EB Datasheet, PDF (28/40 Pages) Analog Devices – IF Digitizing Subsystem

AD9874
⌺-⌬ ADC
FS
DEC1
،12
GCP
VGA
DAC
C_DAC
DEC2
AND
DEC3
I+Q
I+Q
AGCR
REF LEVEL
SELECT
LARGER
+
K
1
(1 – Z^–1)
AGCA /AGCD
SCALING
AGCV
SETTING
RSSI DATA
DVGA
I/Q DATA
TO SSI
TO SSI
Figure 18. Functional Block Diagram of VGA and AGC
VARIABLE GAIN AMPLIFIER OPERATION WITH
AUTOMATIC GAIN CONTROL
The AD9874 contains both a variable gain amplifier (VGA) and
a digital VGA (DVGA) along with all of the necessary signal
estimation and control circuitry required to implement auto-
matic gain control (AGC), as shown in Figure 18. The AGC
control circuitry provides a high degree of programmability,
allowing users to optimize the AGC response as well as the
AD9874’s dynamic range for a given application. The VGA is
programmable over a 12 dB range and implemented within the
ADC by adjusting its full-scale reference level. Increasing the
ADC’s full scale is equivalent to attenuating the signal. An
additional 12 dB of digital gain range is achieved by scaling the
output of the decimation filter in the DVGA. Note that a slight
increase in the supply current (i.e., 0.67 mA) is drawn from
VDDI and VDDF as the VGA changes from 0 dB to 12 dB
attenuation.
The purpose of the VGA is to extend the usable dynamic range
of the AD9874 by allowing the ADC to digitize a desired signal
over a large input power range as well as recover a low level
signal in the presence of larger unfiltered interferers without
saturating or clipping the ADC. The DVGA is most useful in
extending the dynamic range in narrow-band applications
requiring a 16-bit I and Q data format. In these applications,
quantization noise resulting from internal truncation to 16 bits
as well as external 16-bit fixed point post-processing can
degrade the AD9874’s effective noise figure by 1 dB or more.
The DVGA is enabled by writing a 1 to the AGCV field. The
VGA (and the DVGA) can operate in either a user controlled
Variable Gain Mode or Automatic Gain Control (AGC) Mode.
It is worth noting that the VGA imparts negligible phase error
upon the desired signal as its gain is varied over a 12 dB range.
This is due to the bandwidth of the VGA being far greater than
the downconverted desired signal (centered about f_CLK/8) and
remaining relatively independent of gain setting. As a result,
phase modulated signals should experience minimal phase error
as the AGC varies the VGA gain while tracking an interferer or
the desired signal under fading conditions. Note that the enve-
lope of the signal will still be affected by the AGC settings.
Variable Gain Control
The variable gain control is enabled by setting the AGCR field
of Register 0x06 to 0. In this mode, the gain of the VGA (and
the DVGA) can be adjusted by writing to the 16-bit AGCG
register. The maximum update rate of the AGCG register via
the SPI port is f_CLK/240. The MSB of this register is the bit that
enables 16 dB of attenuation in the mixer. This feature allows
the AD9874 to cope with large level signals beyond the VGA’s
range (i.e., > –18 dBm at LNA input) to prevent overloading
of the ADC.
The lower 15 bits specify the attenuation in the remainder of
the signal path. If the DVGA is enabled, the attenuation range
is from –12 dB to +12 dB since the DVGA provides 12 dB of
digital gain. In this case, all 15 bits are significant. However,
with the DVGA disabled, the attenuation range extends from
0 dB to 12 dB and only the lower 14 bits are useful. Figure 19
shows the relationship between the amount of attenuation and
the AGC register setting for both cases.
12
ONLY
VGA ENABLED
6
DVGA AND
VGA ENABLED
0
VGA
RANGE
DVGA
–6
RANGE
–12
0000
1FFF
3FFF
5FFF
AGCG SETTING – HEX
7FFF
Figure 19. AGC Gain Range Characteristics vs.
AGCG Register Setting with and without DVGA
Enabled
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REV. A