AD9874BST AD [Analog Devices], AD9874BST Datasheet - Page 29

• Manufacturer Part Number: AD9874BST
• Description: IF Digitizing Subsystem
• Manufacturer: AD [Analog Devices]

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 2
decay time relative to the attack time. The decay time may be
computed from:
Figure 21a shows the AGC response to a 30 Hz pulse-modulated
IF burst for different AGCA and AGCD settings.
The 3-bit value in the AGCO field determines the amount of
attenuation added in response to a reset event in the ADC.
Each increment in AGCO doubles the weighting factor. At the
highest AGCO setting, the attenuation will change from 0 dB to
12 dB in approximately 10 µs, while at the lowest setting the
attenuation will change from 0 dB to 12 dB in approximately
1.2 ms. Both times assume f
the AGC attack time response for different AGCO settings.
REV. 0
Figure 21a. AGC Response for Different AGCA and AGCD
Settings with f
mate by 900, and AGCO = 0
t
t
BW
attack
decay
A
16
16
32
16
=
32
32
80
64
48
96
80
64
48
96
80
64
48
96
0
0
0
=
=
50
0
t
2 2 100
attack
.
×
(


f
AGCD = 8
CLK
CLK
× (
10
= 18 MSPS, f
2
AGCD = 0
AGCD = 8
×
AGCD = 0
18
AGCD
π
MHz
×
CLK
2
1/2
2
20
)
(
AGCA = 0
AGCA = 8
AGCA
, allowing a 180:1 range in the
AGCA = 4
)
TIME – ms
= 18 MHz. Figure 21b shows
AGCD = 0
× (
2
CLKOUT
2
)
AGCA

 =
30
AGCD = 8
0 35
= 20 kSPS, Deci-
2
.
)
Hz
BW
40
A
50
(10)
(8)
(9)
–29–
Figure 21b. AGC Response for Different AGCO Settings
with f
AGCA = AGCD = 0
Lastly, the AGCF bit reduces the DAC source resistance by at
least a factor of 10. This facilitates fast acquisition by lowering
the RC time constant that is formed with the external capacitors
connected from the GCP pin-to-ground (GCN Pin). For an
overshoot-free step response in the AGC loop, the capacitor
connected from the GCP pin to the GCN ground pin should be
chosen so that the RC time constant is less than one quarter
that of the raw loop. Specifically:
where R is the resistance between the GCP Pin and ground (64 kW
± 20% if AGCF = 0, < 7 kW if AGCF = 1) and BW is the raw
loop bandwidth. Note that with C chosen at this upper limit, the
loop bandwidth increases by approximately 30%.
Now consider the same case as above but with the DVGA enabled
to minimize the effects of 16-bit truncation. With the DVGA
enabled, a control loop based on the larger of the two estimated
signal levels (i.e., output of DEC1 and DVGA) is used to control
the DVGA gain. The DVGA multiplies the output of the deci-
mation filter by a factor ranging from 1 to 4 (i.e., 0 dB to 12 dB).
When signals are small, the DVGA gain is 4 and the 16-bit output
is extracted from the 24-bit data produced by the decimation
filter by dropping 2 MSBs and taking the next 16-bits. As signals
get larger, the DVGA gain decreases until the point where the
DVGA gain is 1 and the 16-bit output data is simply the 16 MSBs
of the internal 24-bit data. As signals get even larger, attenuation
is accomplished by the normal method of increasing the ADC’s
full scale.
The extra 12 dB of gain range provided by the DVGA reduces
the input-referred truncation noise by 12 dB and makes the data
more tolerant of LSB corruption within the DSP. The price
paid for this extension to the gain range is that the start of AGC
action is 12 dB lower and that the AGC loop will be unstable if
its bandwidth is set too wide. The latter difficulty results from
the large delay of the decimation filters, DEC2 and DEC3,
when one implements a large decimation factor. As a result, given
an option, the use of 24-bit data is preferable to using the DVGA.
RC
CLK
< 1 8 (
128
112
64
96
80
48
32
16
= 18 MSPS, f
0
0
π
BW
0.1
AGCO = 7
)
0.2
AGCD = 0
CLKOUT
AGCO = 4
0.3
0.4
= 300 kSPS, Decimate by 60, and
TIME – msec
0.5
0.6
0.7
0.8
AD9874
0.9
1.0
(11)