
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. 
Rev. E | Page 12 of 28 
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
G
S
85
30
–0.5
3.0
45
35
0
40
60
50
55
65
70
75
80
2.5
2.0
1.5
1.0
0.5
0
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. 
V
O2
 = 1.908V
41.07dB
INPUT
0dB
–40.00dB
–40.00dB
C1HI        C1LO
1.07dB
V
G2
OUTPUT
0dB
41.07dB
–41.07dB
–42.14dB
C2HI        C2LO
A1
A2
V
O1
 = 0.592V
V
G1
V
C
 = 0V
0
Figure 23. AD600 Gain Control Input Calculations for Sequential Control Operation (A) 
–0.51dB
OUTPUT
40dB
A1
41.07dB
–0.51dB
C1HI        C1LO
41.07dB
–1.07dB
–41.63dB
C2HI        C2LO
40.56dB
A2
V
O2
 = 1.908V
V
G2
V
O1
 = 0.592V
V
G1
INPUT
0dB
V
C
 = 1.25V
0
Figure 24. AD600 Gain Control Input Calculations for Sequential Control Operation (B) 
38.93dB
A1
41.07dB
0dB
0dB
C1HI        C1LO
41.07dB
–2.14dB
C2HI        C2LO
41.07dB
A2
INPUT
0dB
V
C
 = 2.5V
V
O1
 = 0.592V
V
G1
V
O2
 = 1.908V
V
G2
OUTPUT
80dB
0
Figure 25. AD600 Gain Control Input Calculations for Sequential Control Operation (C)