參數(shù)資料
型號(hào)： AD9279-65EBZ
廠商： Analog Devices Inc
文件頁(yè)數(shù)： 16/44頁(yè)
文件大?。?/td> 0K
描述： BOARD EVALUATION FOR AD9279
標(biāo)準(zhǔn)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
AD9279
Rev. 0 | Page 23 of 44
0.6
0.7
0.8
0.5
0.4
0.3
0.2
0.1
0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
GAIN+ (V)
IN
P
U
T
FU
L
S
C
A
L
E
(
V
p-p)
09
42
3-
0
49
PGA GAIN = 21dB
PGA GAIN = 24dB
PGA GAIN = 27dB
PGA GAIN = 30dB
Figure 44. LNA with 17.9 dB Gain Setting/VGA Full-Scale Limitations
0
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
0.55
0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
IN
P
U
T
FU
LL
S
C
A
L
E
(
V
p-p)
GAIN+ (V)
PGA GAIN = 21dB
PGA GAIN = 30dB
09
42
3-
0
50
PGA GAIN = 27dB
PGA GAIN = 24dB
Figure 45. LNA with 21.3 dB Gain Setting/VGA Full-Scale Limitations
Low Noise Amplifier (LNA)
Good system sensitivity relies on a proprietary ultralow noise
LNA at the beginning of the signal chain, which minimizes the
noise contribution in the following VGA. Active impedance
control optimizes noise performance for applications that
benefit from input impedance matching.
The LNA input, LI-x, is capacitively coupled to the source.
An on-chip bias generator establishes dc input bias voltages of
approximately 2.2 V and centers the output common-mode
levels at 1.5 V (AVDD2 divided by 2). A capacitor, CLG, of the
same value as the input coupling capacitor, CS, is connected
from the LG-x pin to ground.
It is highly recommended that the LG-x pins form a Kelvin type
connection to the input or probe connection ground. Simply
connecting the LG-x pin to ground near the device can allow
differences in potential to be amplified through the LNA. This
generally shows up as a dc offset voltage that can vary from
channel to channel and part to part depending on the appli-
cation and the layout of the PCB.
The LNA supports a nominal differential output voltage of
4.4 V p-p with positive and negative excursions of ±1.1 V from a
common-mode voltage of 1.5 V. The LNA differential gain sets the
maximum input signal before saturation. One of three gains is
set through the SPI. Overload protection ensures quick recovery
time from large input voltages. Because the inputs are
capacitively coupled to a bias voltage near midsupply, very large
inputs can be handled without interacting with the ESD
protection.
Low value feedback resistors and the current-driving capability
of the output stage allow the LNA to achieve a low input-
referred noise voltage of 0.75 nV/√Hz (at a gain of 21.3 dB).
On-chip resistor matching results in precise single-ended gains,
which are critical for accurate impedance control. The use of a
fully differential topology and negative feedback minimizes
distortion. Low second-order harmonic distortion is particularly
important in second harmonic ultrasound imaging applications.
Differential signaling enables smaller swings at each output,
further reducing third-order harmonic distortion.
Active Impedance Matching
The LNA consists of a single-ended voltage gain amplifier with
differential outputs and the negative output externally available.
For example, with a fixed gain of 8× (17.9 dB), an active input
termination is synthesized by connecting a feedback resistor
between the negative output pin, LO-x, and the positive input
pin, LI-x. This well-known technique is used for interfacing
multiple probe impedances to a single system. The input
resistance is shown in Equation 1.
)
2
1
(
A
R
FB
IN
+
=
(1)
where A/2 is the single-ended gain or the gain from the LI-x
inputs to the LO-x outputs.
Because the amplifier has a gain of 8× from its input to its
differential output, it is important to note that the gain, A/2,
is the gain from Pin LI-x to Pin LO-x and that it is 6 dB less
than the gain of the amplifier, or 12.1 dB (4×). The input
resistance is reduced by an internal bias resistor of 6 kΩ in
parallel with the source resistance connected to Pin LI-x, with
Pin LG-x ac grounded. Equation 2 can be used to calculate the
required RFB for a desired RIN, even for higher values of RIN.
Ω
+
=
k
6
||
)
4
1
(
FB
IN
R
(2)
For example, to set RIN to 200 Ω with a single-ended LNA gain of
12.1 dB (4×), the value of RFB from Equation 1 must be 1000 Ω. If
the simplified equation (Equation 2) is used to calculate RIN, the
value is 194 Ω, resulting in a gain error of less than 0.27 dB.
Some factors, such as the presence of a dynamic source resistance,
may influence the absolute gain accuracy more significantly. At
higher frequencies, the input capacitance of the LNA must be
considered. The user must determine the level of matching
accuracy and adjust RFB accordingly.
RFB is the resulting impedance of the RFB1 and RFB2 combination
(see Figure 41).Using Register 0x2C in the SPI memory, the
AD9279 can be programmed for four impedance matching
options: three active terminations and unterminated. Table 9
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