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AD604AR датащи(PDF) 10 Page - Analog Devices |
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AD604AR датащи(HTML) 10 Page - Analog Devices |
10 / 20 page AD604 REV. 0 –10– Since the internal resistors have an absolute tolerance of ±20%, the gain can be in error by as much as 0.33 dB when REXT is 30 Ω, where it was assumed that R EXT is exact. Figure 36 shows how the preamplifier is set to gains of +14, +17.5 and +20 dB. The gain range of a single channel of the AD604 is 0 dB to +48 dB when the preamplifier is set to +14 dB (Figure 36a), 3.5 dB to +51.5 dB for a preamp gain of +17.5 dB (Figure 36b), and 6 dB to 54 dB for the highest preamp gain of +20 dB (Figure 36c). FBK1 PAO1 COM1 R5 32 Ω R7 40 Ω R6 8 Ω PAI1 a. Preamp Gain = 14 dB R10 40 Ω FBK1 PAO1 COM1 R5 32 Ω R7 40 Ω R6 8 Ω PAI1 b. Preamp Gain = 17.5 dB FBK1 PAO1 COM1 R5 32 Ω R7 40 Ω R6 8 Ω PAI1 c. Preamp Gain = 20 dB Figure 36. Preamplifier Gain Programmability For a preamplifier gain of +14 dB, the preamplifier’s –3 dB small-signal bandwidth is 130 MHz; when the gain is at the high end (+20 dB), the bandwidth will be reduced by a factor of two to 65 MHz. Figure 37 shows the ac responses for the three preamp gains discussed above; note that the gain for an REXT of 40 Ω should be 17.5 dB, but the mismatch between the internal resis- tors and the external resistor has caused the actual gain for this particular preamplifier to be 17.7 dB. The –3 dB small-signal bandwidth of one complete channel of the AD604 (preamplifier and DSX) is 40 MHz and is independent of gain. REXT – Ω 20 19 10 100k 1M 10M 100M 18 17 16 15 14 13 12 11 OPEN 40 Ω SHORT 50 Ω 40 Ω REXT 150 Ω 8 Ω 32 Ω IN VIN Figure 37. AC Responses for Preamplifier Gains Shown in Figure 36. To achieve its optimum specifications, power and ground man- agement are critical to the AD604. Large dynamic currents result because of the low resistances needed for the desired noise performance. Most of the difficulty is with the very low gain setting resistors of the preamplifier that allow for a total input referred noise, including the DSX, as low as 0.8 nV/ √Hz. The consequently large dynamic currents have to be carefully handled to maintain performance even at large signal levels. To accommodate these large dynamic currents as well as a ground referenced input, the preamplifier is operated from a dual ±5 V supply. This causes the preamplifiers output to also be ground referenced, which requires a common-mode level shift into the single-supply DSX. The two external coupling ca- pacitors (C1, C2 in Figure 35) connected to nodes PAO1 and +DSX, and –DSX and ground, respectively, perform this func- tion (see AC Coupling Section). In addition, they eliminate any offset that would otherwise be introduced by the preamplifier. It should be noted that an offset of 1 mV at the input of the DSX will get amplified by +34.4 dB ( × 52.5) when the gain-control voltage is at its maximum, this equates to 52.5 mV at the out- put. AC coupling is consequently required to keep the offset from degrading the output signal range. The internal feedback resistors setting the gain of the preampli- fier are so small (nominally 8 Ω and 32 Ω) that even an addi- tional 1 Ω in the “ground” connection at pin COM1, which serves as the input common-mode reference, will seriously degrade gain accuracy and noise performance. This node is very sensitive and careful attention is necessary to minimize the ground impedance. All connections to node COM1 should be as short as possible. The preamplifier including the gain setting resistors has a noise performance of 0.71 nV/ √Hz and 3 pA/√Hz. Note that a signifi- cant portion of the total input referred voltage noise is due to the feedback resistors. The equivalent noise resistance presented by R5 and R6 in parallel is nominally 6.4 Ω, which contributes 0.33 nV/ √Hz to the total input referred voltage noise. The larger portion of the input referred voltage noise is coming from the amplifier with 0.63 nV/ √Hz. The current noise is independent of gain and depends only on the bias current in the input stage of the preamplifier—it is 3 pA/ √Hz. The preamplifier can drive 40 Ω (the nominal feedback resis- tors) and the following 175 Ω ladder load of the DSX with low distortion. For example, at 10 MHz and ±1 V at the output, the preamplifier has less than –45 dB of second and third harmonic distortion when driven from a low (25 Ω) source resistance. In some cases one may need more than 48 dB of gain range, in which case two AD604 channels could be cascaded. Since the preamplifier has limited input signal range, consumes over half (120 mW) of the total power (220 mW), and its ultralow noise is not necessary after the first AD604 channel, a shutdown mechanism that disables only the preamplifier is built in. All that is required to shut down the preamplifier is to tie the COM1 and/or COM2 pin to the positive supply. The DSX will be unaffected and can be used as before (see APPLICATIONS section for further details). |
Аналогичный номер детали - AD604AR |
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Аналогичное описание - AD604AR |
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