AD603
0dB
–28.93dB
INPUT
0dB
0dB
GPOS
V
G1
GNEG
31.07dB
31.07dB
–2.14dB
GPOS
V
G2
GNEG
31.07dB
V
C
= 2.0V
V
O1
= 0.473V
V
O2
= 1.526V
Figure 38. AD603 Gain Control Input Calculations for Sequential Operation V
C
= 2.0 V
+31.07dB
+31.07dB
+10dB
–8.93dB
+28.96dB
70
60
COMBINED
50
OVERALL GAIN (dB)
A1
1
1
A2
–11.07dB
–11.07dB
0.473
1.526
0.5
0
1.0
20
1.50
40
2.0
60
V
C
(V)
62.14
00539-039
40
A1
30
20
10
0
A2
–10
–20
00539-040
00539-041
GAIN
(dB) –22.14
0
–20
1
GAIN
OFFSET OF 1.07dB, OR 26.75mV.
Figure 39. Explanation of Offset Calibration for Sequential Control
With reference to Figure 36, Figure 37, and Figure 38, note that
V
G1
refers to the differential gain control input to A1, and V
G2
refers to the differential gain control input to A2. When V
G
is 0,
V
G1
= −473 mV and thus the gain of A1 is −8.93 dB (recall that
the gain of each individual amplifier in the maximum
bandwidth mode is –10 dB for V
G
= −500 mV and 10 dB for V
G
= 0 V); meanwhile, V
G2
= −1.908 V so the gain of A2 is pinned at
−11.07 dB. The overall gain is thus –20 dB. See Figure 36.
When V
G
= +1.00 V, V
G1
= 1.00 V − 0.473 V = +0.526 V, which
sets the gain of A1 to at nearly its maximum value of 31.07 dB,
while V
G2
= 1.00 V − 1.526 V = 0.526 V, which sets A2’s gain at
nearly its minimum value of −11.07 dB. Close analysis shows
that the degree to which neither AD603 is completely pushed to
its maximum nor minimum gain exactly cancels in the overall
gain, which is now +20 dB. See Figure 37.
When V
G
= 2.0 V, the gain of A1 is pinned at 31.07 dB and that
of A2 is near its maximum value of 28.93 dB, resulting in an
overall gain of 60 dB (see Figure 38). This mode of operation is
further clarified by Figure 40, which is a plot of the separate
gains of A1 and A2 and the overall gain vs. the control voltage.
control voltage. Figure 42 is a plot of the gain error of the
cascaded stages vs. the control voltages.
S/N RATIO (dB)
–30
–0.2
0.2
0.6
1.0
V
C
00539-038
OUTPUT
60dB
1.4
1.8
2.0
Figure 40. Plot of Separate and Overall Gains in Sequential Control
90
80
70
60
50
40
30
20
10
–0.2
0.2
0.6
1.0
V
C
1.4
1.8
2.0
Figure 41. SNR for Cascaded Stages—Sequential Control
Rev. G | Page 15 of 20
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