AD9874-EB Datasheet, PDF (31/40 Pages) Analog Devices – IF Digitizing Subsystem

AD9874
Table XII indicates which AGCA values are reasonable for
various decimation factors. The white cells indicate that the
(decimation factor/AGCA) combination works well; the light
gray cells indicate ringing and an increase in the AGC settling
time; and the dark gray cells indicate that the combination
results in instability or near instability in the AGC loop. Setting
AGCF = 1 improves the time-domain behavior at the expense
of increased spectral spreading.
Table XII. AGCA Limits if the DVGA is Enabled
Table XIII. SPI Registers Associated with AGC
Address Bit
(Hex) Breakdown
0x03
(7)
(6:0)
0x04
(7:0)
0x05
(7:4)
(3:0)
Width
1
7
8
4
4
Default
Value
0
0x00
0x00
0
0x00
Name
ATTEN
AGCG(14:8)
AGCG(7:0)
AGCA
AGCD
AGCA
M
4 5 6 7 8 9 10 11 12 13 14 15
60 0
120 1
300 4
540 8
900 E
Lastly, consider the case of a strong out-of-band interferer (i.e.,
–18 dBm to –32 dBm for matched IF input) that is larger than
the target signal and large enough to be tracked by the control
loop based on the output of the DEC1. The ability of the con-
trol loop to track this interferer and set the VGA attenuation to
prevent clipping of the ADC is limited by the accuracy of the
digital signal estimation occurring at the output of DEC1. The
accuracy of the digital signal estimation is a function of the
frequency offset of the out-of-band interferer relative to the IF
frequency as shown in Figure 20. Interferers at increasingly
higher frequency offsets incur larger measurement errors, poten-
tially causing the control loop to inadvertently reduce the
amount of VGA attenuation that may result in clipping of the
ADC. Figure 21c shows the maximum measured interferer
signal level versus the normalized IF offset frequency (relative to
f_CLK) tolerated by the AD9874 relative to its maximum target
input signal level (0 dBFS = –18 dBm). Note that the increase
in allowable interferer level occurring beyond 0.04 Ï« f_CLK
results from the inherent signal attenuation provided by the
ADC’s signal transfer function.
0
0x06
(7)
(6:4)
(3)
(2:0)
1
0
3
0
1
0
3
0
AGCV
AGCO
AGCF
AGCR
System Noise Figure (NF) vs. VGA (or AGC) Control
The AD9874’s system noise figure is a function of the ACG
attenuation and output signal bandwidth. Figure 22a plots the
nominal system NF as a function of the AGC attenuation for
both narrow-band (20 kHz) and wideband (150 kHz) modes
with f_CLK = 18 MHz. Also shown on the plot is the SNR that
would be observed at the output for a –2 dBFS input. The
high dynamic range of the ADC within the AD9874 ensures
that the system NF increases gradually as the AGC attenuation
is increased. In narrow-band (BW = 20 kHz) mode, the system
noise figure increases by less than 3 dB over a 12 dB AGC
range, while in wideband (BW = 150 kHz) mode, the degra-
dation is about 5 dB. As a result, the highest instantaneous
dynamic range for the AD9874 occurs with 12 dB of AGC
attenuation, since the AD9874 can accommodate an addi-
tional 12 dB peak signal level with only a moderate increase
in its noise floor.
As Figure 22a shows, the AD9874 can achieve an SNR in
excess of 100 dB in narrow-band applications. To realize the
full performance of the AD9874 in such applications, it is recom-
mended that the I/Q data be represented with 24 bits. If 16-bit
data is used, the effective system NF will increase because of the
quantization noise present in the 16-bit data after truncation.
15
SNR = 90.1dBFS
14
13
BW = 50kHz
12
–3
–6
–9
–12
–15
0
0.01
0.02
0.03
0.04
0.05
NORMALIZED FREQUENCY OFFSET = (f_IN – f_IF)/f_CLK
BW = 150kHz
11
SNR = 82.9dBFS
10
SNR = 103.2dB
BW = 10kHz
9
SNR = 95.1dBFS
8
0
3
6
9
12
VGA ATTENUATION – dB
Figure 22a. Nominal System Noise Figure and
Peak SNR vs. AGCG Setting (f_IF = 73.35 MHz, f_CLK =
18 MSPS, and 24-bit I/Q data)
Figure 21c. Maximum Interferer (or Blocker) Input
Level vs. Normalized IF Frequency Offset
REV. A
–31–