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AD605BR датащи(PDF) 9 Page - Analog Devices |
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AD605BR датащи(HTML) 9 Page - Analog Devices |
9 / 12 page AD605 –9– REV. C From these equations one can see that all gain curves intercept at the same –19 dB point; this intercept will be 14 dB higher (–5 dB) if the FBK to OUT connection is left open. Outside of the central linear range, the gain starts to deviate from the ideal control law but still provides another 8.4 dB of range. For a given gain scaling one can calculate VREF as shown in Equation 6. V REF = 2.500V × 20 dB /V Gain Scale (6) 35 30 25 20 15 10 5 0 –5 –10 –15 –20 40dB/V 30dB/V 20dB/V LINEAR-IN-dB RANGE OF AD605 1.0 0.5 1.5 2.0 2.5 3.0 GAIN CONTROL VOLTAGE Figure 3. Ideal Gain Curves vs. VREF Usable gain control voltage ranges are 0.1 V to 2.9 V for 20 dB/V scale and 0.1 V to 1.45 V for the 40 dB/V scale. VGN voltages of less than 0.1 V are not used for gain control since below 50 mV the channel is powered down. This can be used to con- serve power and at the same time gate-off the signal. The supply current for a powered-down channel is 1.9 mA, the response time to power the device on or off is less than 1 µs. Active Feedback Amplifier (Fixed Gain Amp) To achieve single-supply operation and a fully differential input to the DSX, an active feedback amplifier (AFA) was utilized. The AFA is basically an op amp with two gm stages; one of the active stages is used in the feedback path (therefore the name), while the other is used as a differential input. Note that the differential input is an open-loop gm stage which requires that it be highly linear over the expected input signal range. In this design, the gm stage that senses the voltages on the attenuator is a distributed one; for example, there are as many gm stages as there are taps on the ladder network. Only a few of them are on at any one time, depending on the gain control voltage. R –6.908dB R 1.5R 1.5R R R –13.82dB R 1.5R 1.5R R –20.72dB R 1.5R 1.5R R –27.63dB R 1.5R 1.5R R –34.54dB R 1.5R 1.5R R –41.45dB R 1.5R 1.5R R –48.36dB 1.5R 1.5R 175 175 +IN MID –IN NOTE: R = 96 1.5R = 144 Figure 2. R-1.5R Dual Ladder Network One feature of the X-AMP architecture is that the output referred noise is constant versus gain over most of the gain range. This can be easily explained by looking at Figure 2 and observing that the tap resistance is equal for all taps after only a few taps away from the inputs. The resistance seen looking into each tap is 54.4 Ω which makes 0.95 nV/√Hz of Johnson noise spectral density. Since there are two attenuators, the overall noise contribution of the ladder network is √2 times 0.95 nV/√Hz or 1.34 nV/ √Hz, a large fraction of the total DSX noise. The rest of the DSX circuit components contribute another 1.20 nV/ √Hz which together with the attenuator produces 1.8 nV/ √Hz of total DSX input referred noise. AC Coupling The DSX is a single, single-supply circuit and therefore its inputs need to be ac-coupled to accommodate ground-based signals. External capacitors C1 and C2 in Figure 1 level shift the input signal from ground to the dc value established by VOCM (nominal 2.5 V). C1 and C2, together with the 175 Ω looking into each of DSX inputs (+IN and –IN), will act as high-pass filters with corner frequencies depending on the values chosen for C1 and C2. For example, if C1 and C2 are 0.1 µF, then together with the 175 Ω input resistance of each side of the differential ladder of the DSX, a –3 dB high-pass corner at 9.1 kHz is formed. If the DSX output needs to be ground referenced, then another ac-coupling capacitor will be required for level shifting. This capacitor will also eliminate any dc offsets contributed by the DSX. With a nominal load of 500 Ω and a 0.1 µF coupling capacitor, this adds a high-pass filter with –3 dB corner fre- quency at about 3.2 kHz. The choice for all three of these coupling capacitors depends on the application. They should allow the signals of interest to pass unattenuated, while at the same time they can be used to limit the low frequency noise in the system. Gain Control Interface The gain control interface provides an input resistance of approximately 2 M Ω at pin VGN1 and gain scaling factors from 20 dB/V to 40 dB/V for VREF input voltages of 2.5 V to 1.25 V, respectively. The gain varies linearly-in-dB for the cen- ter 40 dB of gain range, that is for VGN equal to 0.4 V to 2.4 V for the 20 dB/V scale, and 0.25 V to 1.25 V for the 40 dB/V scale. Figure 3 shows the ideal gain curves when the FBK to OUT connection is shorted as described by the following equations: G (20 dB/V ) = 20 × VGN – 19, VREF = 2.500 V (3) G (30 dB/V ) = 30 × VGN – 19, VREF = 1.6666 V (4) G (40 dB/V ) = 40 × VGN – 19, VREF = 1.250 V (5) |
Аналогичный номер детали - AD605BR |
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Аналогичное описание - AD605BR |
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