AD600/AD602
(This system can, of course, be used as an AGC amplifier, in
which the rms value of the input is leveled.) Figure 21 shows the
“decibel” output voltage. More revealing is Figure 22, which
shows that the
deviation
from the ideal output predicted by
Equation 1 over the input range 80
µV
to 500 mV rms is within
450
425
400
375
350
±
0.5 dB, and within
±
1 dB for the 80 dB range from 80
µV
to
800 mV. By suitable choice of the input attenuator R1 + R2,
this could be centered to cover any range from 25 mV to 250 mV
to, say, 1 mV to 10 V, with appropriate correction to the value
of V
REF
. (Note that V
SCALE
is not affected by the changes in the
range.) The gain ripple of
±
0.2 dB seen in this curve is the re-
sult of the finite interpolation error of the X-AMP. Note that it
occurs with a periodicity of 12 dB—twice the separation be-
tween the tap points (because of the two cascaded stages).
This ripple can be canceled whenever the X-AMP stages are
cascaded by introducing a 3 dB offset between the two pairs of
control voltages. A simple means to achieve this is shown in
Figure 23: the voltages at C1HI and C2HI are “split” by
±
46.875 mV, or
±
1.5 dB. Alternatively, either one of these pins
can be individually offset by 3 dB and a 1.5 dB gain adjustment
made at the input attenuator (R1 + R2).
C1HI
16
15
A1CM
NC
–6V
DEC
+6V DEC
–6V DEC
C2
2µF
NC
NC
V
OUT
– mV
325
300
275
250
225
200
175
150
10µV
100µV
1mV
10mV
100mV
1V
10V
1
2
VINP
INPUT SIGNAL –V RMS
14 A1OP
3 VNEG
4 CAVG
5
6
7
VLOG
BFOP
BFIN
Figure 20. The RMS Output of A2 Is Held Close to the
“Setpoint” 316 mV for an Input Range of Over 80 dB
5
4
3
2
V
OUT
– Volts
1
U1
AD600
13
12
11
10
9
VPOS
VNEG
A2OP
A2CM
C2HI
U2
AD636
–46.875mV
–6V
DEC
+46.875mV
+6V
DEC
10kΩ
78.7Ω
78.7Ω
10kΩ
NC = NO CONNECT
3dB OFFSET
MODIFICATION
0
–1
–2
–3
–4
–5
10µV
Figure 23. Reducing the Gain Error Ripple
100µV
1mV
10mV
100mV
1V
10V
INPUT SIGNAL – V RMS
Figure 21. The dB Output of Figure 19’s Circuit Is Linear
Over an 80 dB Range
2.5
2.0
1.5
The error curve shown in Figure 24 demonstrates that over the
central portion of the range the output voltage can be main-
tained very close to the ideal value. The penalty for this modifi-
cation is the higher errors at the extremities of the range. The
next two applications show how three amplifier sections can be
cascaded to extend the nominal conversion range to 120 dB,
with the inclusion of simple LP filters of the type shown in Fig-
ure 15. Very low errors can then be maintained over a 100 dB
range.
2.5
2.0
1.5
OUTPUT ERROR – dB
0.5
0
–0.5
–1.0
–1.5
–2.0
–2.5
10µV
OUTPUT ERROR – dB
100µV
1mV
10mV
100mV
1V
10V
1.0
1.0
0.5
0
–0.5
–1.0
–1.5
–2.0
–2.5
10µV
100µV
1mV
10mV
100mV
1V
10V
INPUT SIGNAL – V RMS
INPUT SIGNAL – V RMS
Figure 22. Data from Figure 20 Presented as the Deviation
from the Ideal Output Given in Equation 4
Figure 24. Using the 3 dB Offset Network, the Ripple
Is Reduced
REV. A
–13–
AD [ ANALOG DEVICES ]
