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| 型號： | AD9279-80KITZ |
| 廠商： | Analog Devices Inc |
| 文件頁數(shù)： | 19/44頁 |
| 文件大?。?/td> | 0K |
| 描述： | KIT EVALUATION FOR AD9279 |
| 標準包裝： | 1 |
| ADC 的數(shù)量： | 8 |
| 位數(shù)： | 12 |
| 采樣率（每秒）： | 80M |
| 數(shù)據(jù)接口： | 串行 |
| 工作溫度： | -40°C ~ 85°C |
| 已用 IC / 零件： | AD9279 |
| 已供物品： | 板 |
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AD9279
Rev. 0 | Page 26 of 44
Variable Gain Amplifier (VGA)
The differential X-AMP VGA provides precise input attenu-
ation and interpolation. It has a low input-referred noise of
2.5 nV/√Hz and excellent gain linearity. The VGA is driven by
a fully differential input signal from the LNA. The X-AMP archi-
tecture produces a linear-in-dB gain law conformance and low
distortion levels—only deviating ±0.5 dB or less from the ideal.
The gain slope is monotonic with respect to the control voltage
and is stable with variations in process, temperature, and supply.
The resulting total gain range is 45 dB, which allows for range
loss at the endpoints.
The X-AMP inputs are part of a programmable gain feedback
amplifier (PGA) that completes the VGA. The PGA in the VGA
can be programmed to a gain of 21 dB, 24 dB, 27 dB, or 30 dB.
This allows for optimization of channel gain for different imaging
modes in the ultrasound system. The VGA bandwidth is
approximately 100 MHz. The input stage is designed to ensure
excellent frequency response uniformity across the gain setting.
For TGC mode, this minimizes time delay variation across the
gain range.
Gain Control
The gain control interface, GAIN±, is a differential input. VGAIN
varies the gain of all VGAs through the interpolator by selecting
the appropriate input stages connected to the input attenuator.
For GAIN at 0.8 V, the nominal GAIN+ range for 29.4 dB/V is
0 V to 1.6 V, with the best gain linearity from approximately
0.16 V to 1.44 V, where the error is typically less than ±0.5 dB.
For GAIN+ voltages greater than 1.44 V and less than 0.16 V,
the error increases. The value of GAIN+ can exceed the supply
voltage by 1 V without gain foldover.
Gain control response time is less than 750 ns to settle within 10%
of the final value for a change from minimum to maximum gain.
There are two ways in which the GAIN+ and GAIN pins can
be interfaced. Using a single-ended method, a Kelvin type of
connection to ground can be used, as shown in Figure 51. For
driving multiple devices, it is preferable to use a differential
method, as shown in Figure 52. In either method, the GAIN+
and GAIN pins should be dc-coupled and driven to accom-
modate a 1.6 V full-scale input.
GAIN+
GAIN–
100
AD9279
0V TO 1.6V DC
0.01F
KELVIN
CONNECTION
094
23
-05
2
Figure 51. Single-Ended GAIN± Pin Configuration
094
23
-05
3
AD8138
499
AD9279
499
±0.4V DC
AT 0.8V CM
±0.8V DC
±0.4V DC
AT 0.8V CM
499
AVDD2
0.8V CM
523
100
0.01F
GAIN+
GAIN–
0.01F
100
31.3k
10k
Figure 52. Differential 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
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 40 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
prevail. 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, increases 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.
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.
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