AD9864 Analog Devices, AD9864 Datasheet - Page 32

• Manufacturer Part Number: AD9864
• Description: IF Digitizing Subsystem
• Manufacturer: Analog Devices

Specifications of AD9864

• Resolution (bits): 24bit
• # Chan: 1
• Sample Rate: 18MSPS
• Interface: Ser,SPI
• Analog Input Type: Diff-Uni,SE-Uni
• Adc Architecture: Sigma-Delta
• Pkg Type: CSP

AD9864
AUTOMATIC GAIN CONTROL (AGC)
The gain of the VGA (and DVGA) is automatically adjusted
when the AGC is enabled via the AGCR field of Register 0x06.
In this mode, the gain of the VGA is continuously updated at
f
level into the ADC does not exceed the ADC clip level and that
the rms output level of the ADC is equal to a programmable
reference level. With the DVGA enabled, the AGC control loop
also attempts to minimize the effects of 16-bit truncation noise
prior to the SSI output by continuously adjusting the DVGA’s
gain to ensure maximum digital gain while not exceeding the
programmable reference level.
This programmable level can be set at 3 dB, 6 dB, 9 dB, 12 dB,
and 15 dB below the ADC saturation (clip) level by writing
values from 1 to 5 to the 3-bit AGCR field. Note that the ADC
clip level is defined to be 2 dB below its full scale (i.e., –18 dBm
at the LNA input for a matched input and maximum attenua-
tion). If AGCR is 0, automatic gain control is disabled. Since
clipping of the ADC input will degrade the SNR performance,
the reference level should also take into consideration the peak-
to-rms characteristics of the target (or interferer) signals.
Referring again to Figure 58, the majority of the AGC loop
operates in the discrete time domain. The sample rate of the
loop is f
algorithm are updated at this rate. The number of overload and
ADC reset occurrences within the final I/Q update rate of the
AD9864, as well as the AGC value (8 MSB), can be read from
the SSI data upon proper configuration.
The AGC performs digital signal estimation at the output of the
first decimation stage (DEC1) as well as the DVGA output that
follows the last decimation stage (DEC3). The rms power of the
I and Q signal is estimated by the equation
CLK
/60 in an attempt to ensure that the maximum analog signal
Figure 60. Normalized RSSI Error vs. Normalized IF Frequency Offset
Xest
–12
–15
–18
–3
–6
–9
0
CLK
0
[ ]
n
/60; therefore, registers associated with the AGC
=
NORMALIZED FREQUENCY OFFSET ((
Abs
0.01
(
I
[ ]
n
)
+
Abs
0.02
(
Q
[ ]
n
)
0.03
f
IN
–
f
IF
0.04
)
f
CLK
)
0.05
Rev. 0 | Page 32 of 44
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Signal estimation after the first decimation stage allows the
AGC to cope with out-of-band interferers and in-band signals
that could otherwise overload the ADC. Signal estimation after
the DVGA allows the AGC to minimize the effects of the 16-bit
truncation noise.
When the estimated signal level falls within the range of the
AGC, the AGC loop adjusts the VGA (or DVGA) attenuation
setting so that the estimated signal level is equal to the pro-
grammed level specified in the AGCR field. The absolute signal
strength can be determined from the contents of the ATTN and
RSSI field that is available in the SSI data frame when properly
configured. Within this AGC tracking range, the 6-bit value in
the RSSI field remains constant while the 8-bit ATTN field var-
ies according to the VGA/DVGA setting. Note that the ATTN
value is based on the 8 MSB contained in the AGCG field of
Registers 0x03 and 0x04.
A description of the AGC control algorithm and the user
adjustable parameters follows. First, consider the case in which
the in-band target signal is bigger than all out-of-band interfer-
ers and the DVGA is disabled. With the DVGA disabled, a
control loop based only on the target signal power measured
after DEC1 is used to control the VGA gain, and the target
signal will be tracked to the programmed reference level. If the
signal is too large, the attenuation is increased with a propor-
tionality constant determined by the AGCA setting. Large
AGCA values result in large gain changes, thus rapid tracking
of changes in signal strength. If the target signal is too small
relative to the reference level, the attenuation is reduced; but
now the proportionality constant is determined by both the
AGCA and AGCD settings. The AGCD value is effectively
subtracted from AGCA, so a large AGCD results in smaller
gain changes and thus slower tracking of fading signals.
The 4-bit code in the AGCA field sets the raw bandwidth of the
AGC loop. With AGCA = 0, the AGC loop bandwidth is at its
minimum of 50 Hz, assuming f
of AGCA increases the loop bandwidth by a factor of √2; thus
the maximum bandwidth is 9 kHz. A general expression for the
attack bandwidth is
and the corresponding attack time is
assuming that the loop dynamics are essentially those of a
single-pole system.
The 4-bit code in the AGCD field sets the ratio of the attack
time to the decay time in the amplitude estimation circuitry.
When AGCD is zero, this ratio is one. Incrementing AGCD
multiplies the decay time constant by 21/2, allowing a 180:1
t
BW
ATTACK
A
=
50
=
×
. 2
(
/ 2
f
CLK
(
100
/
18
×
π
MHz
×
2
(
AGCA
CLK
)
×
2
= 18 MHz. Each increment
(
2 /
AGCA
)
)
=
2 /
35
)
Hz
/
BW
A
(8)
(9)