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AD7720BRU データシート(PDF) 11 Page - Analog Devices |
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AD7720BRU データシート(HTML) 11 Page - Analog Devices |
11 / 16 page AD7720 –11– REV. 0 R 100 C 2.7nF VIN(+) VIN(–) C 2.7nF C 2.7nF R 100 Figure 24. Differential Input with Antialiasing A capacitor between the two input pins sources or sinks charge to allow most of the charge that is needed by one input to be effectively supplied by the other input. This minimizes undesir- able charge transfer from the analog inputs to and from ground. The series resistor isolates the operational amplifier from the current spikes created during the sampling process and provides a pole for antialiasing. The 3 dB cutoff frequency (f3 dB) of the antialias filter is given by Equation 1, and the attenuation of the filter is given by Equation 2. f3dB = 1/(2 π R EXT CEXT) (1) Attenuation = 20 log 1/ 1+ f / f 3dB ()2 (2) The choice of the filter cutoff frequency will depend on the amount of roll off that is acceptable in the passband of the digital filter and the required attenuation at the first image frequency. The capacitors used for the input antialiasing circuit must have low dielectric absorption to avoid distortion. Film capacitors such as Polypropylene, Polystyrene or Polycarbonate are suitable. If ceramic capacitors are used, they must have NPO dielectric. Applying the Reference The reference circuitry used in the AD7720 includes an on-chip +2.5 V bandgap reference and a reference buffer circuit. The block diagram of the reference circuit is shown in Figure 25. The internal reference voltage is connected to REF1 via a 3 k Ω resistor and is internally buffered to drive the analog modulator’s switched capacitor DAC (REF2). When using the internal reference, connect 100 nF between REF1 and AGND. If the internal reference is required to bias external circuits, use an external precision op amp to buffer REF1. 3k REFERENCE BUFFER 1V 2.5V REFERENCE SWITCHED-CAP DAC REF REF1 REF2 COMPARATOR 100nF Figure 25. Reference Circuit Block Diagram The AD7720 can operate with its internal reference or an external reference can be applied in two ways. An external reference can be connected to REF1, overdriving the internal reference. How- ever, there will be an error introduced due to the offset of the internal buffer amplifier. For lowest system gain errors when using an external reference, REF1 is grounded (disabling the internal buffer) and the external reference is connected to REF2. In all cases, since the REF2 voltage connects to the analog modulator, a 220 nF capacitor must connect directly from REF2 to AGND. The external capacitor provides the charge required for the dynamic load presented at the REF2 pin (Figure 26). A B B 4pF A B A B MCLK REF2 A 4pF SWITCHED-CAP DAC REF 220nF Figure 26. REF2 Equivalent Circuit The AD780 is ideal to use as an external reference with the AD7720. Figure 27 shows a suggested connection diagram. AD780 1 2 3 4 8 7 6 5 NC +VIN TEMP GND O/P SELECT NC VOUT TRIM 22nF 1 F REF2 22 F 220nF REF1 +5V Figure 27. External Reference Circuit Connection Input Circuits Figures 28 and 29 show two simple circuits for bipolar mode operation. Both circuits accept a single-ended bipolar signal source and create the necessary differential signals at the input to the ADC. The circuit in Figure 28 creates a 0 V to 2.5 V signal at the VIN(+) pin to form a differential signal around an initial bias voltage of 1.25 V. For single-ended applications, best THD performance is obtained with VIN(–) set to 1.25 V rather than 2.5 V. The input to the AD7720 can also be driven differen- tially with a complementary input as shown in Figure 29. In this case, the input common-mode voltage is set to 2.5 V. The 2.5 V p-p full-scale differential input is obtained with a 1.25 V p-p signal at each input in antiphase. This configuration minimizes the required output swing from the amplifier circuit and is useful for single supply applications. |
同様の部品番号 - AD7720BRU |
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同様の説明 - AD7720BRU |
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