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AD883B Datasheet(PDF) 8 Page - Analog Devices |
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AD883B Datasheet(HTML) 8 Page - Analog Devices |
8 / 14 page REV. C –8– AD603 VC 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 VC 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: Gain (dB) = 80 VG + GO (4) where, as before GO depends on the range selected; for example, in the maximum-bandwidth mode, GO is +20 dB. Alternatively, the GNEG pins may be connected to an offset voltage of +0.500 V, in which case, GO 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). VC 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 VC –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.” |
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