AD9874 Analog Devices, AD9874 Datasheet - Page 31

• Manufacturer Part Number: AD9874
• Description: Low Power IF Digitizing Subsystem
• Manufacturer: Analog Devices

Specifications of AD9874

• Resolution (bits): 24bit
• # Chan: 1
• Sample Rate: 26MSPS
• Interface: Ser
• Analog Input Type: Diff-Uni,SE-Uni
• Adc Architecture: Sigma-Delta
• Pkg Type: QFP

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.
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
input signal level (0 dBFS = –18 dBm). Note that the increase
in allowable interferer level occurring beyond 0.04
results from the inherent signal attenuation provided by the
ADC’s signal transfer function.
REV. A
CLK
) tolerated by the AD9874 relative to its maximum target
60
Figure 21c. Maximum Interferer (or Blocker) Input
Level vs. Normalized IF Frequency Offset
120
300
540
900
Table XII. AGCA Limits if the DVGA is Enabled
–12
–15
–3
–6
–9
0
0
M
0
1
4
8
E
NORMALIZED FREQUENCY OFFSET = (
4 5 6
0.01
7 8 9 10 11 12 13 14 15
0.02
AGCA
0.03
f
IN
0.04
–
f
IF
)/
f
CLK
f
CLK
0.05
–31–
Address Bit
(Hex)
0x03
0x04
0x05
0x06
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
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.
CLK
Figure 22a. Nominal System Noise Figure and
Peak SNR vs. AGCG Setting (f
18 MSPS, and 24-bit I/Q data)
Table XIII. SPI Registers Associated with AGC
15
14
13
12
11
10
= 18 MHz. Also shown on the plot is the SNR that
9
8
0
Breakdown
(7)
(6:0)
(7:0)
(7:4)
(3:0)
(7)
(6:4)
(3)
(2:0)
SNR = 82.9dBFS
SNR = 95.1dBFS
3
BW = 150kHz
VGA ATTENUATION – dB
Width
1
7
8
4
4
1
3
1
3
6
BW = 50kHz
Default
Value
0
0x00
0x00
0
0x00
0
0
0
0
BW = 10kHz
IF
= 73.35 MHz, f
SNR = 90.1dBFS
9
SNR = 103.2dB
AD9874
Name
ATTEN
AGCG(14:8)
AGCG(7:0)
AGCA
AGCD
AGCV
AGCO
AGCF
AGCR
12
CLK
=