AD600 Analog Devices, Inc., AD600 Datasheet - Page 12

• Manufacturer Part Number: AD600
• Description: Dual, Low Noise, Wideband Variable Gain Amplifiers
• Manufacturer: Analog Devices, Inc.

AD600/AD602
ACHIEVING 80 dB GAIN RANGE
The two amplifier sections of the X-AMP can be connected in
series to achieve higher gain. In this mode, the output of A1
(A1OP and A1CM) drives the input of A2 via a high-pass
network (usually just a capacitor) that rejects the dc offset.
The nominal gain range is now –2 dB to +82 dB for the AD600
or −22 dB to +62 dB for the AD602.
There are several options in connecting the gain-control inputs.
The choice depends on the desired SNR and gain error (output
ripple). The following examples feature the AD600; the
arguments generally apply to the AD602, with appropriate
changes to the gain values.
SEQUENTIAL MODE (MAXIMUM SNR)
In the sequential mode of operation, the SNR is maintained at
its highest level for as much of the gain control range as
possible, as shown in Figure 22. Note here that the gain range is
0 dB to 80 dB. Figure 23, Figure 24, and Figure 25 show the
general connections to accomplish this. Both gain-control
inputs, C1HI and C2HI, are driven in parallel by a positive-only,
ground-referenced source with a range of 0 V to 2.5 V.
V
V
C
V
C
C
INPUT
= 1.25V
= 2.5V
INPUT
= 0V
INPUT
0dB
0dB
0dB
Figure 23. AD600 Gain Control Input Calculations for Sequential Control Operation (A)
Figure 25. AD600 Gain Control Input Calculations for Sequential Control Operation (C)
Figure 24. AD600 Gain Control Input Calculations for Sequential Control Operation (B)
C1HI
C1HI
C1HI
–40.00dB
–0.51dB
V
0dB
V
G1
V
G1
G1
C1LO
V
C1LO
C1LO
V
O1
V
–40.00dB
O1
A1
O1
A1
–0.51dB
= 0.592V
A1
= 0.592V
= 0.592V
0dB
41.07dB
41.07dB
41.07dB
Rev. E | Page 12 of 28
1.07dB
41.07dB
40.56dB
An auxiliary amplifier that senses the voltage difference
between input and output commons is provided to reject this
common voltage.
C2HI
Figure 22. SNR vs. Control Voltage Sequential Control (1 MHz Bandwidth)
C2HI
C2HI
–42.14dB
–41.63dB
–2.14dB
V
85
80
75
70
65
60
55
50
45
40
35
30
V
–0.5
G2
V
G2
G2
C2LO
V
C2LO
V
C2LO
O2
V
–41.07dB
O2
O2
–1.07dB
38.93dB
= 1.908V
A2
= 1.908V
= 1.908V
A2
A2
0
0.5
41.07dB
41.07dB
41.07dB
1.0
V
G
OUTPUT
0dB
OUTPUT
80dB
OUTPUT
40dB
1.5
2.0
2.5
3.0