AD603 Analog Devices, Inc., AD603 Datasheet - Page 14

• Manufacturer Part Number: AD603
• Description: Low Noise, 90 Mhz Variable-gain Amplifier
• Manufacturer: Analog Devices, Inc.

AD603
USING THE AD603 IN CASCADE
Two or more AD603s can be connected in series to achieve
higher gain. Invariably, ac coupling must be used to prevent the
dc offset voltage at the output of each amplifier from overloading
the following amplifier at maximum gain. The required high-
pass coupling network is usually just a capacitor, chosen to set
the desired corner frequency in conjunction with the well-
defined 100 Ω input resistance of the following amplifier.
For two AD603s, the total gain control range becomes 84 dB
(2 × 42.14 dB); the overall −3 dB bandwidth of cascaded stages
is somewhat reduced. Depending on the pin strapping, the gain
and bandwidth for two cascaded amplifiers can range from
−22 dB to +62 dB (with a bandwidth of about 70 MHz) to
+22 dB to +102 dB (with a bandwidth of about 6 MHz).
There are several ways of connecting the gain control inputs
in cascaded operation. The choice depends on whether it is
important to achieve the highest possible instantaneous signal-
to-noise ratio (ISNR), or, alternatively, to minimize the ripple
in the gain error. The following examples feature the AD603
programmed for maximum bandwidth; the explanations apply
to other gain/bandwidth combinations with appropriate
changes to the arrangements for setting the maximum gain.
SEQUENTIAL MODE (OPTIMAL SNR)
In the sequential mode of operation, the ISNR is maintained at
its highest level for as much of the gain control range as possible.
INPUT
V
V
INPUT
INPUT
V
C
C
0dB
0dB
0dB
Figure 36. AD603 Gain Control Input Calculations for Sequential Control Operation V
C
= 1.0V
= 2.0V
= 0V
Figure 37. AD603 Gain Control Calculations for Sequential Control Operation V
Figure 38. AD603 Gain Control Input Calculations for Sequential Operation V
GPOS
GPOS
GPOS
–42.14dB
V
0dB
0dB
V
V
G1
G1
G1
0dB
0dB
GNEG
GNEG
GNEG
–40.00dB
A1
V
V
V
O1
O1
O1
= 0.473V
= 0.473V
= 0.473V
31.07dB
31.07dB
31.07dB
Rev. H | Page 14 of 24
–8.93dB
31.07dB
31.07dB
GPOS
GPOS
GPOS
Figure 35 shows the SNR over a gain range of −22 dB to +62 dB,
assuming an output of 1 V rms and a 1 MHz bandwidth. Figure 36,
Figure 37, and Figure 38 show the general connections to
accomplish this. Here, both the positive gain control inputs
(GPOS) are driven in parallel by a positive-only, ground-referenced
source with a range of 0 V to 2 V, while the negative gain
control inputs (GNEG) are biased by stable voltages to provide
the needed gain offsets. These voltages may be provided by
resistive dividers operating from a common voltage reference.
–42.14dB
–42.14dB
–2.14dB
Figure 35. SNR vs. Control Voltage, Sequential Control (1 MHz Bandwidth)
V
V
V
G2
G2
G2
GNEG
90
85
80
75
70
65
60
55
50
GNEG
GNEG
–0.2
–51.07dB
–11.07dB
–28.93dB
V
V
V
O2
O2
O2
A2
= 1.526V
= 1.526V
= 1.526V
31.07dB
0.2
31.07dB
31.07dB
0.6
C
C
= 2.0 V
OUTPUT
–20dB
OUTPUT
20dB
= 1.0 V
OUTPUT
60dB
C
= 0 V
V
C
1.0
(V)
1.4
1.8
2.2