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AD600/AD602
Rev. F | Page 25 of 3
5
3
–5
4
2
0
1
–1
–3
–2
–4
LOGA
R
IT
H
M
IC
OU
T
P
U
T
(
V
)
1
10
1
100m
10m
1m
100
INPUT SIGNAL (V rms)
2
0
53
8-
05
0
–2.0
Figure 52. VLOG Is Linear over the Full 120 dB Range
Figure 52 shows VLOG 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 VAGC 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
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 VOUT 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, because 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
0.5
1.0
1.5
–1.5
–1.0
–0.5
GA
IN
E
R
OR
(
d
B
)
–0.2
0.2
0
1
10
1
100m
10m
1m
100
INPUT SIGNAL (V rms)
00
53
8-
0
5
400
300
200
350
250
G
A
IN
ER
R
O
R
(m
V)
1
10
1
100m
10m
1m
100
INPUT SIGNAL (V rms)
1
00
53
8
-05
2
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
VC (mV)
S
NR
(
d
B)
Figure 53. Error Ripple Caused by the Individual Gain Functions
Figure 54. VAGC Remains Close to Its Setpoint of
316 mV rms over the Full 120 dB Range
VC SCALE = 10.417mV/dB
90
30
40
50
60
70
80
S
NR
(
d
B)
0
05
38-
053
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.
VC (V)
0
3.817
20
10
–0.558
–1.183
3.192
2.567
1.942
1.317
0.692
0.067
00
53
8-
0
5
4
VC SCALE = 31.25mV/dB
Figure 56. SNR vs. Control Voltage for Sequential Gain Control (See
Figure 51)