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AD605BN 데이터시트(PDF) 11 Page - Analog Devices |
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AD605BN 데이터시트(HTML) 11 Page - Analog Devices |
11 / 12 page AD605 –11– REV. C Two other easy combinations are possible to provide a gain range of –14 dB to +82.8 dB: (1) make R1 a short and R2 an open; or (2) make R1 an open and R2 a short. The bandwidth for both of these cases will be dominated by the channel that is set to the higher gain and will be about 8 MHz. From a noise standpoint, the second choice is the best since by increasing the gain of the first amplifier, the second amplifier’s noise will have less of an impact on the total output noise. One further observa- tion regarding noise is that by increasing the gain the output noise will increase proportionally; therefore, there is no increase in signal-to-noise ratio. It will actually stay fixed. It should be noted that by selecting the appropriate values of R1 and R2, any gain range between –28 dB to +68.8 dB and 0 dB to +96.8 dB can be achieved with the circuit in Figure 5. When using any value other than shorts and opens for R1 and R2, the final value of the gain range will depend on external resistors matching on-chip resistors. Since the internal resistors can vary by as much as ±20%, the actual values for a particular gain have to be determined empirically. Note that the two channels within one part will match quite well; therefore, R1 will track R2 in Figure 5. C3 is not required since the common-mode voltage at Pin OUT1 should be identical to the one at Pins +IN2 and –IN2. However, since only 1 mV of offset at the output of the first DSX will introduce an offset of 53 mV when the second DSX is set to the maximum gain of the lowest gain range (34.4 dB), and 263 mV when set to the maximum gain of the highest gain range (48.4 dB), it is important to include ac coupling to get the maximum dy- namic range at the output of the cascaded amplifiers. C5 is necessary if the output signal needs to be referenced to any common-mode level other than half of the supply as is provided by Pin OUT2. Figure 6 shows the gain versus VGN for the circuit in Figure 5 at 1 MHz and the lowest gain range (–14 dB to +34.4 dB). Note that the gain scaling is 40 dB/V, double the 20 dB/V of an indi- vidual DSX; this is the result of the parallel connection of the gain control inputs, VGN1 and VGN2. One could of course also sequentially increase the gain by first increasing the gain of Channel 1 and then Channel 2. In that case VGN1 and VGN2 will have to be driven from separate voltage sources, for instance two separate DACs. Figure 7 shows the gain error of Figure 5. VGN (V) 80 –40 0.1 0.5 2.9 f = 1MHz THEORETICAL ACTUAL 0 0.9 1.3 1.7 2.1 2.5 70 20 –10 –20 –30 50 30 60 40 10 Figure 6. Gain vs. VGN for the Circuit in Figure 5 VGN (V) 4 3 –4 0.2 0.7 2.7 1.2 1.7 2.2 0 –1 –2 –3 2 1 f = 1MHz Figure 7. Gain Error vs. VGN for the Circuit in Figure 5 |
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