Datasheet

AD9277
Rev. 0 | Page 29 of 48
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
(ADC Noise Floor/LNA Input Noise Floor) + Margin =
20 log(224/3.9) + 11 dB = 46 dB
The minimum gain required is determined by
(ADC Input FS/LNA Input FS) + Margin =
20 log(2/0.55) − 10 dB = 3 dB
Therefore, 42 dB of gain range for a 14-bit, 50 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 Nominal Gain (dB)
LNA 15.6/17.9/21.3
Attenuator −42 to 0
VGA Amplifier 21/24/27/30
Filter 0
ADC 0
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
GAIN
, and Equation 4 is
the expression for the channel gain.
V
GAIN
(V) = (GAIN+) – (GAIN−) (3)
Gain (dB) = 28.5 dB/V × V
GAIN
+ ICPT (4)
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
FB
= 350 Ω). However,
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.
LNA FULL SCALE
(0.55V p-p SINGLE-ENDED)
LNA INPUT-REFERRED
NOISE FLOOR
(3.9µV rms) @ AAF BW = 15MHz
LNA + VGA NOISE = 1.0nV/ Hz
A
DC FULL SCALE (2V p-p)
~10dB MARGIN
>11dB MARGIN
ADC NOISE FLOOR
(224µV rms)
MINIMUM GAIN
MAXIMUM GAIN
LNA
ADC
73dB
VGA GAIN RANGE > 42dB
MAX CHANNEL GAIN > 48dB
94dB
08181-049
Figure 55. Gain Requirements of TGC Operation for a 14-Bit, 50 MSPS ADC