AD9276-65EBZ Analog Devices Inc, AD9276-65EBZ Datasheet - Page 23

• Manufacturer Part Number: AD9276-65EBZ
• Description: 65MSPS ADC Converter Evaluation Board
• Manufacturer: Analog Devices Inc

Specifications of AD9276-65EBZ

• Silicon Manufacturer: Analog Devices
• Application Sub Type: ADC
• Kit Application Type: Data Converter
• Silicon Core Number: AD9276
• Number Of Adc's: 1
• Number Of Bits: 12
• Sampling Rate (per Second): 65M
• Data Interface: Serial, SPI™
• Inputs Per Adc: 1 Differential
• Input Range: *
• Power (typ) @ Conditions: 195mW @ 40MSPS
• Voltage Supply Source: Analog and Digital
• Operating Temperature: -40°C ~ 85°C
• Utilized Ic / Part: AD9276
• Development Tool Type: Hardware - Eval/Demo Board
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

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).
This is achieved with a current consumption of only 27 mA per
channel (80 mW). 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 ultra-
sound 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.
where:
A/2 is the single-ended gain or the gain from the LI-x inputs to
the LO-x outputs.
R
(see Figure 47).
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 11.9 dB (4×). The input
resistance is reduced by an internal bias resistor of 15 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 R
For example, to set R
If the simplified equation (Equation 2) is used to calculate R
the value is 188 Ω, resulting in a gain error of less than 0.6 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 R
FB
is the resulting impedance of the R
R
R
IN
IN
=
=
FB
1 ( A
1 (
R
for a desired R
+
R
+
FB
) 3
FB
) 2
||
15
IN
FB
k
to 200 Ω, the value of R
Ω
accordingly.
IN
, even for higher values of R
FB1
and R
FB
FB2
must be 1000 Ω.
combination
IN
.
IN
Rev. 0 | Page 23 of 48
(1)
(2)
,
The bandwidth (BW) of the LNA is greater than 100 MHz.
Ultimately, the BW of the LNA limits the accuracy of the
synthesized R
is between 100 kHz and 10 MHz, where the lower frequency
limit is determined by the size of the ac coupling capacitors, and
the upper limit is determined by the LNA BW. Furthermore, the
input capacitance and R
Figure 48 shows R
Note that at the lowest value of R
greater than 10 MHz. This is due to the BW roll-off of the LNA,
as mentioned previously.
However, as can be seen for larger R
tance starts rolling off the signal BW before the LNA can produce
peaking. C
not be used for values of R
lists the recommended values for R
C
and Pin LI-x are unequal.
Table 7. Active Termination External Component Values
LNA Gain
(dB)
15.6
17.9
21.3
15.6
17.9
21.3
15.6
17.9
21.3
FB
is needed in series with R
100
1k
10
100k
R
R
R
R
Figure 48. R
SH
S
S
S
S
= 500Ω, R
= 200Ω, R
= 100Ω, R
= 50Ω, R
further degrades the match; therefore, C
R
50
50
50
100
100
100
200
200
200
IN
IN
. For R
(Effects of R
(Ω)
IN
FB
vs. frequency for various values of R
FB
FB
FB
IN
= 200Ω, C
vs. Frequency for Various Values of R
IN
= 2kΩ
= 800Ω
= 400Ω, C
S
= R
1M
limit the BW at higher frequencies.
S
FREQUENCY (Hz)
IN
R
200
250
350
400
500
700
800
1000
1400
and C
S
FB
FB
that are greater than 100 Ω. Table 7
SH
up to about 200 Ω, the best match
SH
because the dc levels at Pin LO-x
(Ω)
= 70pF
IN
= 20pF
SH
(50 Ω), R
Are Also Shown)
FB
IN
and C
values, parasitic capaci-
10M
Minimum
C
90
70
50
30
20
10
N/A
N/A
N/A
SH
IN
SH
peaks at frequencies
(pF)
in terms of R
AD9276
FB
SH
BW (MHz)
57
69
88
57
69
88
72
72
72
100M
should
FB
.
IN
.