
AD600/AD602
Rev. E | Page 25 of 28 
5
3
–5
4
2
0
1
–1
–3
–2
–4
L
1μ
10μ
10
1
100m
10m
1m
100μ
INPUT SIGNAL (V rms)
0
Figure 52. V
LOG
 Is Linear over the Full 120 dB Range 
Figure 52 shows V
LOG
 to be linear over a full 120 dB range. 
Figure 53 shows the error ripple due to the individual gain 
functions bounded by ±0.2 dB (dotted lines) from 6 μV to 2 V. 
The small perturbations at about 200 μV and 20 mV, caused by 
the impracticality of matching the gain functions perfectly, are 
the only sign that the gains are now sequential. Figure 54 is a 
plot of V
AGC
 that remains very close to its set value of 316 mV 
rms over the full 120 dB range. 
To compare the SNRs in the simultaneous and sequential 
modes of operation more directly, all interstage attenuation was 
eliminated (R2 and R3 in Figure 47 and R2 in Figure 51), the 
input of U1A was shorted, R5 was selected to provide a 20 kHz 
bandwidth (R5 = 7.87 kΩ), and only the gain control was 
varied, using an external source. The rms value of the noise was 
then measured at V
OUT
 and expressed as an SNR relative to 
0 dBV, which is almost the maximum output capability of the 
AD600. Results for the simultaneous mode can be seen in 
Figure 55. The SNR degrades uniformly as the gain is increased. 
Note that since the inverting gain control was used, the gain in 
this curve and in Figure 56 decreases for more positive values of 
the gain-control voltage. 
2.0
–2.0
0.5
1.0
1.5
–1.5
–1.0
–0.5
G
–0.2
0.2
0
1μ
10μ
10
1
100m
10m
1m
100μ
INPUT SIGNAL (V rms)
0
Figure 53. Error Ripple due to the Individual Gain Functions 
400
300
200
350
250
G
1μ
10μ
10
1
100m
10m
1m
100μ
INPUT SIGNAL (V rms)
0
Figure 54. V
AGC
 Remains Close to Its Setpoint of  
316 mV rms over the Full 120 dB Range 
90
0
833.2
20
10
–625.0
–833.2
30
40
50
60
70
80
625.0
416.6
208.3
0
–208.3
–416.6
V
C
 (mV)
S
0
V
C
 SCALE = 10.417mV/dB
Figure 55. SNR vs. Control Voltage for Parallel Gain Control (See Figure 47) 
In contrast, the SNR for the sequential mode is shown in Figure 56. 
U1A always acts as a fixed noise source; varying its gain has no 
influence on the output noise. This is a feature of the X-AMP 
technique. Therefore, for the first 40 dB of control range 
(actually slightly more, as is explained later), when only this 
VCA section has its gain varied, the SNR remains constant. 
During this time, the gains of U1B and U2A are at their 
minimum value of 1.07 dB. 
V
C
 (V)
90
0
3.817
20
10
–0.558
–1.183
30
40
50
60
70
80
3.192
2.567
1.942
1.317
0.692
0.067
S
0
V
C
 SCALE = 31.25mV/dB
Figure 56. SNR vs. Control Voltage for Sequential Gain Control (See Figure 51)