AD9276 Datasheet, PDF (32/48 Pages) Analog Devices – Octal LNA/VGA/AAF/12-Bit ADC and CW I/Q Demodulator

AD9276
GAIN+
GAIN–
100Ω
0.01µF
100Ω
0.01µF
±0.4V DC
AT 0.8V CM
499Ω
AD8138
±0.4V DC
AT 0.8V CM
499Ω
AVDD2
499Ω
0.8V CM
523Ω
31.3kΩ
±0.8V DC
50Ω
10kΩ
Figure 61. Differential GAIN+, GAIN− Pin Configuration
VGA Noise
In a typical application, a VGA compresses a wide dynamic
range input signal to within the input span of an ADC. The
input-referred noise of the LNA limits the minimum resolvable
input signal, whereas the output-referred noise, which depends
primarily on the VGA, limits the maximum instantaneous
dynamic range that can be processed at any one particular gain
control voltage. This latter limit is set in accordance with the
total noise floor of the ADC.
Output-referred noise as a function of GAIN+ is shown in
Figure 11, Figure 12, and Figure 14 for the short-circuit input
conditions. The input noise voltage is simply equal to the output
noise divided by the measured gain at each point in the control
range.
The output-referred noise is a flat 60 nV/√Hz (postamp gain =
24 dB) over most of the gain range because it is dominated by
the fixed output-referred noise of the VGA. At the high end of
the gain control range, the noise of the LNA and of the source
prevails. The input-referred noise reaches its minimum value
near the maximum gain control voltage, where the input-
referred contribution of the VGA is miniscule.
At lower gains, the input-referred noise and, therefore, the
noise figure, increase as the gain decreases. The instantaneous
dynamic range of the system is not lost, however, because the
input capacity increases as the input-referred noise increases.
The contribution of the ADC noise floor has the same depen-
dence. The important relationship is the magnitude of the VGA
output noise floor relative to that of the ADC.
Gain control noise is a concern in very low noise applications.
Thermal noise in the gain control interface can modulate the
channel gain. The resultant noise is proportional to the output
signal level and is usually evident only when a large signal is
present. The gain interface includes an on-chip noise filter,
which significantly reduces this effect at frequencies above
5 MHz. Care should be taken to minimize noise impinging at
the GAIN± inputs. An external RC filter can be used to remove
VGAIN source noise. The filter bandwidth should be sufficient to
accommodate the desired control bandwidth.
Antialiasing Filter (AAF)
The filter that the signal reaches prior to the ADC is used to
reject dc signals and to band limit the signal for antialiasing.
Figure 62 shows the architecture of the filter.
The antialiasing filter is a combination of a single-pole high-
pass filter and a second-order low-pass filter. The high-pass
filter can be configured at a ratio of the low-pass filter cutoff.
This is selectable through the SPI.
The filter uses on-chip tuning to trim the capacitors and, in
turn, to set the desired cutoff frequency and reduce variations.
The default −3 dB low-pass filter cutoff is 1/3 or 1/4.5 the ADC
sample clock rate. The cutoff can be scaled to 0.7, 0.8, 0.9, 1, 1.1,
1.2, or 1.3 times this frequency through the SPI. The cutoff
tolerance is maintained from 8 MHz to 18 MHz.
4kΩ
C
30C
4kΩ
10kΩ/n
4kΩ
30C
2kΩ
4C
2kΩ
C
C = 0.8pF TO 5.1pF
4kΩ
n = 0 TO 7
Figure 62. Simplified Antialiasing Filter Schematic
Tuning is normally off to avoid changing the capacitor settings
during critical times. The tuning circuit is enabled and disabled
through the SPI. Initializing the tuning of the filter must be
performed after initial power-up and after reprogramming the
filter cutoff scaling or ADC sample rate. Occasional retuning
during an idle time is recommended to compensate for
temperature drift.
A total of eight SPI-programmable settings allows the user to
vary the high-pass filter cutoff frequency as a function of the
low-pass cutoff frequency. Two examples are shown in Table 11:
one is for an 8 MHz low-pass cutoff frequency, and the other is
for an 18 MHz low-pass cutoff frequency. In both cases, as the
ratio decreases, the amount of rejection on the low-end fre-
quencies increases. Therefore, making the entire AAF frequency
pass band narrow can reduce low frequency noise or maximize
dynamic range for harmonic processing.
Table 11. SPI-Selectable High-Pass Filter Cutoff Options
High-Pass Cutoff Frequency
Low-Pass Cutoff
SPI Setting Ratio¹ = 8 MHz
Low-Pass Cutoff
= 18 MHz
0
20.65 387 kHz
872 kHz
1
11.45 698 kHz
1.571 MHz
2
7.92 1.010 MHz
2.273 MHz
3
6.04 1.323 MHz
2.978 MHz
4
4.88 1.638 MHz
3.685 MHz
5
4.10 1.953 MHz
4.394 MHz
6
3.52 2.270 MHz
5.107 MHz
7
3.09 2.587 MHz
5.822 MHz
1 _{Ratio = low-pass filter cutoff frequency/high-pass filter cutoff frequency.}
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