REV. C

–8–

AD603

90

10

80

50

40

30

20

70

60

–0.2

2.2

0.2

0.6

1.0

1.4

1.8

Figure 9. SNR for Cascaded Stages—Sequential Control

2.0

–0.5

–2.0

–0.2

1.5

0.0

–1.0

–1.5

1.0

0.5

0.0 0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

Figure 10. Gain Error for Cascaded Stages—Sequential

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:

(4)

the GNEG pins may be connected to an offset voltage of

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).

2.0

–0.5

–2.0

–0.2

1.5

0.0

–1.0

–1.5

1.0

0.5

0.0 0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

Figure 11. Gain Error for Cascaded Stages–Parallel

Control

–0.2

90

85

80

75

70

65

60

55

50

0

0.2

0.4

0.6

0.8

1.0

1.2

Figure 12. ISNR for Cascaded Stages–Parallel Control

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.”