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.
85
80
75
70
65
SNR (dB)
60
55
50
45
40
35
30
–0.5
0
0.5
1.0
V
G
1.5
2.0
2.5
3.0
00538-022
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.
Figure 22. SNR vs. Control Voltage Sequential Control (1 MHz Bandwidth)
An auxiliary amplifier that senses the voltage difference
between input and output commons is provided to reject this
common voltage.
A1
–40.00dB
INPUT
0dB
–40.00dB
C1HI
V
G1
V
C
= 0V
A2
–41.07dB
–42.14dB
C2HI
V
G2
V
O2
= 1.908V
C2LO
OUTPUT
0dB
00538-023
41.07dB
1.07dB
41.07dB
C1LO
V
O1
= 0.592V
Figure 23. AD600 Gain Control Input Calculations for Sequential Control Operation (A)
A1
–0.51dB
INPUT
0dB
–0.51dB
C1HI
V
G1
V
C
= 1.25V
A2
–1.07dB
–41.63dB
C2HI
V
G2
V
O2
= 1.908V
C2LO
OUTPUT
40dB
00538-055
41.07dB
C1LO
V
O1
= 0.592V
40.56dB
41.07dB
Figure 24. AD600 Gain Control Input Calculations for Sequential Control Operation (B)
A1
0dB
INPUT
0dB
0dB
C1HI
V
G1
V
C
= 2.5V
A2
38.93dB
–2.14dB
C2HI
V
G2
V
O2
= 1.908V
C2LO
OUTPUT
80dB
00538-056
41.07dB
C1LO
V
O1
= 0.592V
41.07dB
41.07dB
Figure 25. AD600 Gain Control Input Calculations for Sequential Control Operation (C)
Rev. E | Page 12 of 28
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