Analog Devices AD604 - Manual (Page 21)

AD604
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.
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.
IT -DSX1
IT +DSX1
IT PAO1
IT FBK1
VGN1 ^
VREF 23|
OUT1
GND1 271
R2
499Q
C5.
0.1MF'
C3
0.1 m F
FB
>—0IZh-
FAIR-RITE
#2643000301
^ PAI1 AD604 vpos
-IT
COM1 VNEG 391
VNEG ja]
VPOS 371
GND2 3U
OUT2 35]
VOCM 3U
VGN2 3U
C6 COM2
56 °P^^-|T PAI2
T FBK2
PAO2
+DSX2
n?|-DSX2
Figure 48. Modifications of the AGC Amplifier to Create 96 dB of Gain Range
P in (dBm)
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 O. 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.
AD604
C1
°.1MF
VIN+ (04
C4
°.1MF ■
j-E
°.1 m F-
-TT -DSX1
-IT
+DSX1
IT PAO1
fT FBK1
-|T PAI1
n r COM1
COM2
-IT
PAI2
I FBK2
PAO2
fn +DSX2
VGN1 24}
VREF 23}
OUT1 22}
GND1
VPO^2Q
VNEG^Tâ
VNEG
VPOS
GND2
OUT2 Is}
VOCM
rT2 -DSX2 VGN2
+5V
-5V
-5V
+5V
C7
°.1 mf
C6
°.1 mf
HH
—wv
R1
453Q “
R2
453Q .
-VW-tO) VOUT-
C5
°.1 mf
C1^ ^ -L C13
0.1 u ^ °.1 m F
SEB)— + 5V
W fb )— -5V
ALL SUPPLY PINS ARE DECOUPLED AS SHOWN.
:"T j T0.1U
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 O resistors shown
in Figure 50 and the 50 O loads presented by the oscilloscope
plug-in. R1 and R2 are inserted to ensure a nominal load of 500 O
at each output. The differencial gain .afthe circuit is set to 20 dB
20 dB/VforaVREFof" 2J/00 Vrthe ilpuffrequency is 10 MHz,
and .thediflèrential inpu/amplitudeis T^ImV-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.
NOTES
1. THE output after 10 x attenuater formed
BY 4530 TOGETHER WITH 500 OF 7A24 PLUG-IN.
Figure 51. Output of VGA in Figure 50 for VGN = 1 V
Rev. E | Page 21 of 32
VREF
VOUT+
VIN
VG
ACTUAL
OUT
+500mV
-SQQmV