AD604
C1
0.1µF
1
2
3
4
–DSX1
+DSX1
PAO1
FBK1
PAI1
COM1
COM2
PAI2
FBK2
VGN1
24
VREF
23
OUT1
22
GND1
21
VPOS
20
VNEG
19
VNEG
18
VPOS
17
GND2
16
OUT2
15
VOCM
14
VGN2
13
+5V
–5V
–5V
+5V
– (V1)
2
C8
0.33µF
R4
2kΩ
+5V
1V
LOW-
PASS
FILTER
R7
1kΩ
C10
1µF
Z
4
C2
0.1µF
AD604
VREF
VSET (<0V)
VIN
(MAX
800mV p-p)
R1
49.9Ω
5
6
7
8
9
R8
2kΩ
C11
1µF
1
2
3
OFFS
NULL
NC
8
V1 = V
IN
× G
C7
0.33µF
8
7
6
5
X1
X2
VP
W
–5V
AD711
+V
S 7
+5V
VG
OUT
6
–V
S
OFFS
5
NULL
C3
0.1µF
10
PAO2
11
+DSX2
12
–DSX2
C7
0.1µF
AD835
C6
0.56µF
R2
453Ω
C9
0.33µF
R3
1kΩ
Y1
1
4
Y2
2
VN
3
– (A)
2
2
IF V1 = A × cos (wt)
C4
0.1µF
FB
C12
0.1µF
+5V
–5V
RF OUT
R5
2kΩ
R6
2kΩ
Figure 44. AGC Amplifier with 82 dB of Gain Range
AN ULTRALOW NOISE AGC AMPLIFIER WITH
82 dB TO 96 dB GAIN RANGE
82 dB of gain range using a single AD604. The signal is applied
to connector VIN, and because the signal source is 50 Ω, a
terminating resistor (R1) of 49.9 Ω is added. The signal is then
amplified by 14 dB (Pin FBK1 shorted to PAO1) through the
Channel 1 preamplifier and is further processed by the Channel 1
DSX. Next, the signal is applied directly to the Channel 2 DSX. The
second preamplifier is powered down by connecting its COM2 pin
to the positive supply as explained in the Preamplifier section. C1
and C2 level shift the signal from the preamplifier into the first
DSX and at the same time eliminate any offset contribution of the
preamplifier. C3 and C4 have the same offset cancellation purpose
for the second DSX. Each set of capacitors, combined with the
175 Ω input resistance of the corresponding DSX, provides a
high-pass filter with a −3 dB corner frequency of about 9.1 kHz.
VOCM is decoupled to ground by a 0.1 μF capacitor, while
VREF can be externally provided; in this application, the gain
scale is set to 20 dB/V by applying 2.500 V. Because each of the
DSX amplifiers operates from a single 5 V supply, the output is
ac-coupled via C6 and C7. The output signal can be monitored
at the connector labeled RF OUT.
the AD604 connected as shown. The gain range is −14 dB to
+82 dB; the useful range is 0 dB to +82 dB if the RF output
amplitude is controlled to ±400 mV (+2 dBm). The main limitation
on the lower end of the signal range is the input capability of
the preamplifier. This limitation can be overcome by adding an
attenuator in front of the preamplifier, but that would defeat the
advantage of the ultralow noise preamplifier. It should be noted
that the second preamplifier is not used because its ultralow
noise and the associated high power consumption are overkill
after the first DSX stage. It is disabled in this application by
connecting the COM2 pin to the positive supply. Nevertheless,
the second preamplifier can be used, if so desired, and the
useful gain range increases by 14 dB to encompass 0 dB to
96 dB of gain. For the same +2 dBm output, this allows signals
as small as −94 dBm to be measured.
To achieve the highest gains, the input signal must be band-
limited to reduce the noise; this is especially true if the second
preamplifier is used. If the maximum signal at OUT2 of the AD604
is limited to ±400 mV (+2 dBm), the input signal level at the
AGC threshold is +25 μV rms (−79 dBm). The circuit as shown in
of input referred voltage noise spectral density of the AD604
results in an rms noise of 5.05 μV in the 40 MHz bandwidth.
Rev. D | Page 19 of 32
00540-044
FB
–5V
C13
0.1µF ALL SUPPLY PINS ARE DECOUPLED AS SHOWN.
AD [ ANALOG DEVICES ]
