AD9276 Analog Devices, AD9276 Datasheet - Page 29

• Manufacturer Part Number: AD9276
• Description: Octal LNA/VGA/AAF/12-Bit ADC and CW I/Q Demodulator
• Manufacturer: Analog Devices

Specifications of AD9276

• Resolution (bits): 12bit
• # Chan: 8
• Sample Rate: 80MSPS
• Interface: LVDS,Ser
• Analog Input Type: SE-Uni
• Ain Range: 2 V p-p
• Adc Architecture: n/a
• Pkg Type: QFP

TGC OPERATION
The TGC signal path is fully differential throughout to maxi-
mize signal swing and reduce even-order distortion; however,
the LNAs are designed to be driven from a single-ended signal
source. Gain values are referenced from the single-ended LNA
input to the differential ADC input. A simple exercise in under-
standing the maximum and minimum gain requirements is
shown in Figure 55.
The maximum gain required is determined by
The minimum gain required is determined by
Therefore, 42 dB of gain range for a 12-bit, 40 MSPS ADC with
15 MHz of bandwidth should suffice in achieving the dynamic
range required for most of today’s ultrasound systems.
The system gain is distributed as listed in Table 9.
Table 9. Channel Gain Distribution
Section
LNA
Attenuator
VGA Amplifier
Filter
ADC
(ADC Noise Floor/LNA Input Noise Floor) + Margin =
20 log(224/3.9) + 11 dB = 46 dB
(ADC Input FS/LNA Input FS) + Margin =
20 log(2/0.55) − 10 dB = 3 dB
Nominal Gain (dB)
15.6/17.9/21.3
−42 to 0
21/24/27/30
0
0
(3.9µV rms) @ AAF BW = 15MHz
LNA + VGA NOISE = 1.0nV/ Hz
(0.55V p-p SINGLE-ENDED)
Figure 55. Gain Requirements of TGC Operation for a 12-Bit, 40 MSPS ADC
LNA INPUT-REFERRED
LNA FULL SCALE
NOISE FLOOR
LNA
MINIMUM GAIN
94dB
Rev. 0 | Page 29 of 48
The linear-in-dB gain (law conformance) range of the TGC path
is 42 dB. The slope of the gain control interface is 28.5 dB/V,
and the gain control range is −0.8 V to +0.8 V. Equation 3 is the
expression for the differential voltage V
the expression for the channel gain.
where ICPT is the intercept point of the TGC gain.
In its default condition, the LNA has a gain of 21.3 dB (12×), and
the VGA postamp gain is 24 dB if the voltage on the GAIN+ pin
is 0 V and the voltage on the GAIN− pin is 0.8 V (42 dB attenua-
tion). This results in a total gain (or ICPT) of 3.6 dB through
the TGC path if the LNA input is unmatched, or a total gain of
−2.4 dB if the LNA is matched to 50 Ω (R
if the voltage on the GAIN+ pin is 1.6 V and the voltage on the
GAIN− pin is 0.8 V (0 dB attenuation), the VGA gain is 24 dB.
This results in a total gain of 45 dB through the TGC path if the
LNA input is unmatched or in a total gain of 39 dB if the LNA
input is matched.
Each LNA output is dc-coupled to a VGA input. The VGA
consists of an attenuator with a range of −42 dB to 0 dB followed
by an amplifier with 21 dB/24 dB/27 dB/30 dB of gain. The
X-AMP gain interpolation technique results in low gain error
and uniform bandwidth, and differential signal paths minimize
distortion.
VGA GAIN RANGE > 42dB
MAX CHANNEL GAIN > 48dB
MAXIMUM GAIN
V
Gain (dB) = 28.5 dB/V × V
GAIN
70dB
(V) = (GAIN+) – (GAIN−)
>11dB MARGIN
ADC NOISE FLOOR
(224µV rms)
ADC FULL SCALE (2V p-p)
~10dB MARGIN
ADC
GAIN
+ ICPT
GAIN
FB
, and Equation 4 is
= 350 Ω). However,
AD9276
(3)
(4)