AD604
Rev. E | Page 21 of 3
At 1 MHz, the attenuation is about −0.2 dB, increasing to −6 dB
at 10 MHz and −28 dB at 100 MHz. Signals less than approximately
1 MHz are not significantly affected.
2
Figure 49 shows the control voltage vs. the input power at 1 MHz to
the circuit shown in Figure 48; note that the AGC threshold is at
−95 dBm. The output signal level is set to 800 mV p-p by applying
−80 mV to the V
SET
connector.
12
11
10
9
8
1
2
3
4
7
6
5
13
16
15
14
24
23
22
21
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AD604
FB
–DSX1
+DSX1
PAI1
FBK1
PAO1
COM1
COM2
PAI2
FBK2
PAO2
+DSX2
–DSX2
VGN1
VREF
VPOS
GND1
OUT1
VNEG
VNEG
VPOS
GND2
OUT2
VOCM
VGN2
C5
0.1µF
R2
499
Ω
C6
560pF
C3
0.1µF
FAIR-RITE
#2643000301
00
540
-04
8
Figure 48. Modifications of the AGC Amplifier to Create 96 dB of Gain Range
4.5
4.0
0
3.5
3.0
1.0
2.5
2.0
1.5
CO
NT
R
O
L
V
O
L
T
AG
E
(
V
)
1MHz
0.5
P
IN
(dBm)
–100 –90 –80 –70 –60 –50 –40 –30 –20 –10
0
10
00
54
0-
0
49
13
16
15
14
19
18
17
24
23
22
21
20
Figure 49. Control Voltage vs. Input Power of the Circuit in Figure 48
ULTRALOW NOISE, DIFFERENTIAL INPUT-
DIFFERENTIAL OUTPUT VGA
Figure 50 shows how to use both preamplifiers and DSXs to
create a high impedance, differential input-differential output
VGA. This application takes advantage of the differential inputs
to the DSXs. Note that the input is not truly differential in the
sense that the common-mode voltage needs to be at ground to
achieve maximum input signal swing. This has largely to do
with the limited output swing capability of the output drivers of
the preamplifiers; they clip around ±2.2 V due to having to drive
an effective load of about 30 Ω. If a different input common-mode
voltage needs to be accommodated, ac coupling (as in Figure 48)
is recommended. The differential gain range of this circuit runs
from 6 dB to 54 dB, which is 6 dB higher than each individual
channel of the AD604 because the DSX inputs now see twice
the signal amplitude compared with when they are driven
single-ended.
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10
9
8
1
2
3
4
7
6
5
AD604
FB
FB
+5V
–5V
ALL SUPPLY PINS ARE DECOUPLED AS SHOWN.
+5V
–5V
–5V
+5V
VG
VREF
VOUT+
VOUT
VIN+
VIN–
–
–DSX1
+DSX1
PAI1
FBK1
PAO1
COM1
COM2
PAI2
FBK2
PAO2
+DSX2
–DSX2
VGN1
VREF
VPOS
GND1
OUT1
VNEG
VNEG
VPOS
GND2
OUT2
VOCM
VGN2
C13
0.1µF
C12
0.1µF
C1
0.1µF
C2
0.1µF
C4
0.1µF
C3
0.1µF
C5
0.1µF
C7
0.1µF
R1
453
Ω
R2
453
Ω
C6
0.1µF
00
540
-0
50
Figure 50. Ultralow Noise, Differential Input-Differential Output VGA
Figure 51 displays the output signals VOUT+ and VOUT− after
a −20 dB attenuator formed between the 453 Ω resistors shown
in Figure 50 and the 50 Ω loads presented by the oscilloscope
plug-in. R1 and R2 are inserted to ensure a nominal load of 500 Ω
at each output. The differential gain of the circuit is set to 20 dB
by applying a control voltage, VGN, of 1 V; the gain scaling is
20 dB/V for a VREF of 2.500 V; the input frequency is 10 MHz,
and the differential input amplitude is 100 mV p-p. The resulting
differential output amplitude is 1 V p-p as can be seen on the
scope photo when reading the vertical scale as 200 mV/div.
100
90
10
0%
20ns
20mV
20mV
NOTES
1. THE OUTPUT AFTER 10× ATTENUATER FORMED
BY 453
Ω
TOGETHER WITH 50
Ω
OF 7A24 PLUG-IN.
–500mV
ACTUAL
V
OUT
+500mV
00
54
0-
05
4
Figure 51. Output of VGA in Figure 50 for VGN = 1 V
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