AD8332 AD [Analog Devices], AD8332 Datasheet - Page 17

• Manufacturer Part Number: AD8332
• Description: Ultralow Noise VGAs with Preamplifier and Programmable RIN
• Manufacturer: AD [Analog Devices]

THEORY OF OPERATION
OVERVIEW
The following discussion applies to all part numbers. Figure 56
and Figure 1 are functional block diagrams of the AD8331 and
AD8332, respectively.
Each channel contains an LNA that provides user-adjustable
input impedance termination, a differential X-AMP VGA, and a
programmable gain postamplifier with adjustable output
voltage limiting. Figure 57 shows a simplified block diagram.
LMD
The linear-in-dB gain control interface is trimmed for slope and
absolute accuracy. The overall gain range is 48 dB, extending
from –4.5 dB to +43.5 dB or from +7.5 dB to +55.5 dB,
depending on the setting of the HILO pin. The slope of the gain
control interface is 50 dB/V, and the gain control range is 40 mV
to 1 V, leading to the following expressions for gain:
The gain characteristics are shown in Figure 58.
INH
GAIN
GAIN
GAIN
COML
LMD
VPSL
INH
(V
BIAS
MID
10
3
6
2
1
PREAMPLIFIER
+
)
–
(
(
LNA
dB
dB
Figure 56. Functional Block Diagram — AD8331
)
)
(V
COMM
LNA
BIAS
LON
LOP
LNA
19dB
=
=
MID
20
LON LOP VIP VIN
GAIN
INTERFACE*
50
50
4
)
Figure 57. Simplified Block Diagram
GAIN
(
(
dB
dB
5
GAIN
COMM
INT
17
V
7
V
*SHARED BETWEEN CHANNELS
VIN
)
)
∆
INTERPOLATOR
ENBL
VIP
×
8
×
G = –48dB to 0dB
19
BIAS AND
V
V
AD8331
[(–48 to 0) + 21] dB
GAIN
+21dB
GAIN
X-AMP VGA
VGA
VPOS
or
14
ENBV
–
18
+
6
5
.
.
5
VCM
5
V
11
dB
dB
MID
VCM
,
,
(
CLAMP
RCLMP
(
POST
AMP1
V
HILO
HILO
HILO
MID
12
19
3.5dB/
15.5dB
POSTAMP
3.5dB/15.5dB
=
=
CLAMP*
LO
HI
15
16
9
VOH
VOL
MODE
)
)
( )
( )
RCLMP
2
1
HILO
VOH
VOL
Rev. C | Page 17 of 32
When MODE is set high, (where available):
The LNA converts a single-ended input to a differential output
with a voltage gain of 19 dB. When only one output is used, the
gain is 13 dB. The inverting output is used for active input
impedance termination. Each of the LNA outputs is capacitively
coupled to a VGA input. The VGA consists of an attenuator
with a range of 48 dB followed by an amplifier with 21 dB of
gain, for a net gain range of –27 dB to +21 dB. The X-AMP
gain-interpolation technique results in low gain error and
uniform bandwidth, and differential signal paths minimize
distortion.
The final stage is a logic programmable amplifier with gains of
3.5 dB or 15.5 dB. The LO and HI gain modes are optimized
for 12-bit and 10-bit A/D converter applications, in terms of
output-referred noise and absolute gain range. Output voltage
limiting may be programmed by the user.
LOW NOISE AMPLIFIER (LNA)
Good noise performance relies on a proprietary ultralow noise
preamplifier 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 matching.
GAIN
GAIN
–10
60
50
40
30
20
10
0
0
(
(
dB
dB
)
)
MODE = LO
=
=
–
–
Figure 58. Gain Control Characteristics
50
50
0.2
(
(
dB
dB
(WHERE AVAILABLE)
V
V
)
MODE = HI
)
0.4
×
×
HILO = HI
HILO = LO
V
V
GAIN
GAIN
V
or
GAIN
+
+
0.6
(V)
45
57
.
.
5
5
AD8331/AD8332
dB
dB
,
,
0.8
(
(
HILO
HILO
=
=
1.0
LO
HI
)
)
1.1
( )
( )
4
3