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AD7724AST Datasheet(PDF) 11 Page - Analog Devices |
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AD7724AST Datasheet(HTML) 11 Page - Analog Devices |
11 / 14 page REV. A AD7724 –11– An alternative circuit configuration for driving the differential inputs to the AD7724 is shown in Figure 24. 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 needed by one input to be effectively supplied by the other input. This minimizes undesirable 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 of the antialias filter is given by Equation 1, and the attenuation of the filter is given by Equation 2. f3 dB = 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 digi- tal 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 or polycarbonate are suitable. If ceramic capacitors are used, they must have NPO dielectric. Applying the Reference The reference circuitry used in the AD7724 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 refer- ence, connect 110 nF between REF1 and AGND. If the internal reference is required to bias external circuits, use an external precision op amp to buffer REF1. 4k REFERENCE BUFFER 1V 2.5V REFERENCE SWITCHED-CAP DAC REF REF1 REF2 COMPARATOR 110nF Figure 25. Reference Circuit Block Diagram The AD7724 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. However, an error will be 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 con- nected to REF2. In all cases, since the REF2 voltage connects to the analog modu- lator, a 110 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 110nF Figure 26. REF2 Equivalent Circuit The AD780 is ideal to use as an external reference with the AD7724. 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 110nF REF1 5V NC = NO CONNECT Figure 27. External Reference Circuit Connection |
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