AD600/AD602
Rev. F | Page 25 of 3
5
3
–5
4
2
0
1
–1
–3
–2
–4
LOGARITHMIC OUTPUT (V)
1µ 10µ 101100m10m1m100µ
INPUT SIGNAL (V rms)
2
00538-050
–2.0
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 V to 2 V.
The small perturbations at about 200 V 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 k), 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, 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
GAIN ER
OR (dB)
–0.2
0.2
0
1µ 10µ 101100m10m1m100µ
INPUT SIGNAL (V rms)
00538-05
400
300
200
350
250
GAIN ERROR (mV)
1µ 10µ 101100m10m1m100µ
INPUT SIGNAL (V rms)
1
00538-052
90
0
833.2
20
10
–625.0–833.2
30
40
50
60
70
80
625.0416.6208.30–208.3–416.6
V
C
(mV)
SNR (dB)
Figure 53. Error Ripple Caused by the Individual Gain Functions
Figure 54. V
AGC
Remains Close to Its Setpoint of
316 mV rms over the Full 120 dB Range
V
C
SCALE = 10.417mV/dB
90
30
40
50
60
70
80
SNR (dB)
00538-053
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.
V
C
(V)
0
3.817
20
10
–0.558–1.183 3.1922.5671.9421.3170.6920.067
00538-054
V
C
SCALE = 31.25mV/dB
Figure 56. SNR vs. Control Voltage for Sequential Gain Control (See Figure 51)
