AD603
90
80
70
GAIN ERROR – dB
2.0
1.5
1.0
0.5
0.0
–0.5
–1.0
–1.5
–2.0
–0.2 0.0 0.2
S/N RATIO – dB
60
50
40
30
20
10
–0.2
0.2
0.6
1.0
V
C
1.4
1.8
2.2
0.4 0.6
0.8
1.0
V
C
1.2 1.4
1.6 1.8
2.0 2.2
Figure 9. SNR for Cascaded Stages—Sequential Control
Figure 11. Gain Error for Cascaded Stages–Parallel
Control
90
85
80
2.0
1.5
1.0
GAIN ERROR – dB
0.5
0.0
–0.5
–1.0
–1.5
–2.0
–0.2 0.0 0.2
IS/N RATIO – dB
75
70
65
60
55
50
–0.2
0.4 0.6
0.8
1.0
V
C
1.2 1.4
1.6 1.8
2.0 2.2
0
0.2
0.4
V
C
0.6
0.8
1.0
1.2
Figure 10. Gain Error for Cascaded Stages—Sequential
Control
Figure 12. ISNR for Cascaded Stages–Parallel Control
Parallel Mode (Simplest Gain-Control Interface)
In this mode, the gain-control of voltage is applied to both inputs
in parallel—the GPOS pins of both A1 and A2 are connected to
the control voltage and the GNEW inputs are grounded. The
gain scaling is then doubled to 80 dB/V, requiring only a 1.00 V
change for an 80 dB change of gain:
Gain (dB) =
80
V
G
+
G
O
(4)
where, as before G
O
depends on the range selected; for example,
in the maximum-bandwidth mode, G
O
is +20 dB. Alternatively,
the GNEG pins may be connected to an offset voltage of
+0.500 V, in which case, G
O
is –20 dB.
The amplitude of the gain ripple in this case is also doubled, as
shown in Figure 11, while the instantaneous signal-to-noise ratio
at the output of A2 now decreases linearly as the gain increased
(Figure 12).
Low Gain Ripple Mode (Minimum Gain Error)
As can be seen from Figures 9 and 10, the error in the gain is
periodic, that is, it shows a small ripple. (Note that there is also
a variation in the
output offset voltage,
which is due to the gain
interpolation, but this is not exact in amplitude.) By offsetting
the gains of A1 and A2 by half the period of the ripple, that is,
by 3 dB, the residual gain errors of the two amplifiers can be
made to cancel. Figure 13 shows that much lower gain ripple
when configured in this manner. Figure 14 plots the ISNR as a
function of gain; it is very similar to that in the “Parallel Mode.”
–8–
REV. C
AD [ ANALOG DEVICES ]
