Pentek 6210 Operating Manual page 143

Figure 6. AD603 Gain Control Input Calculations for Sequential Control Operation
+31.07dB
A1
+10dB *
–11.07dB
–8.93dB
0.473
GAIN 0 0.5
(dB) –22.14 –20 0
*
GAIN OFFSET OF 1.07dB, OR 26.75mV
Figure 7. Explanation of Offset Calibration for Sequential
Control
With reference to Figure 6, note that V
tial gain-control input to A1 and V
gain-control input to A2. When V
thus the gain of A1 is –8.93 dB (recall that the gain of each indi-
vidual amplifier in the maximum-bandwidth mode is –10 dB
for V = –500 mV and 10 dB for V
G
–1.908 V so the gain of A2 is "pinned" at –11.07 dB. The over-
all gain is thus –20 dB. This situation is shown in Figure 6a.
When V = +1.00 V, V
G G1
which sets the gain of A1 to at nearly its maximum value of
31.07 dB, while V = 1.00 V – 1.526 V = 0.526 V, which sets
G2
A2's gain at nearly its minimum value –11.07 dB. Close analysis
shows that the degree to which neither AD603 is completely
pushed to its maximum or minimum gain exactly cancels in the
overall gain, which is now +20 dB. This is depicted in Figure 6b.
REV. D
A1
–40.00dB
–42.14dB
INPUT
0dB
GPOS GNEG
V
G1
V = 0.473V
O1
V = 0V
C
0dB
0dB
INPUT
0dB
GPOS GNEG
V
G1
V = 0.473V
O1
V = 1.0V
C
0dB
0dB
INPUT
0dB
GPOS GNEG
V
G1
V = 0.473V
O1
V = 2.0V
C
+31.07dB
+28.96dB
A2
*
–11.07dB
1.526
1.0 1.50 2.0
20 40 60
refers to the differen-
G1
refers to the differential
G2
is zero, V = –473 mV and
G G1
= 0 V); meanwhile, V
G
= 1.00 V – 0.473 V = +0.526 V,
A2
–51.07dB
–8.93dB –42.14dB
31.07dB
GPOS GNEG
V
G2
V = 1.526V
O2
a.
–11.07dB
–42.14dB
31.07dB
31.07dB
GPOS GNEG
V
G2
V
O2
b.
–28.93dB
–2.14dB
31.07dB
31.07dB
GPOS GNEG
V
G2
V = 1.526V
O2
c.
When V = +2.0 V, the gain of A1 is pinned at 31.07 dB and
G
that of A2 is near its maximum value of 28.93 dB, resulting in
an overall gain of 60 dB (see Figure 6c). This mode of operation
is further clarified by Figure 8, which is a plot of the separate
gains of A1 and A2 and the overall gain versus the control voltage.
Figure 9 is a plot of the gain error of the cascaded amplifiers versus
the control voltage. Figure 10 is a plot of the gain error of the
cascaded stages versus the control voltages.
V (V)
C
62.14
70
60
50
40
30
20
10
=
0
G2
–10
–20
–30
–0.2
Figure 8. Plot of Separate and Overall Gains in Sequential
Control
–7–
31.07dB OUTPUT
–20dB
31.07dB OUTPUT
20dB
= 1.526V
31.07dB OUTPUT
60dB
COMBINED
A1
0.2 0.6 0.1
V
C
AD603
A2
1.4 1.8 2.2