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AD7880CQ Datasheet(PDF) 11 Page - Analog Devices |
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AD7880CQ Datasheet(HTML) 11 Page - Analog Devices |
11 / 16 page AD7880 REV. 0 –11– V+ + – C1 10µF C2 0.1µF IC1 ANALOG INPUT V+ V– AB V– AB LK2 LK3 TO ADC LK1 SKT1 C3 10µF C4 0.1µF V+ V DD Figure 21. Analog Input Buffering When it is required to drive the AD7880 with the 0 V to 10 V input range, an external supply must be connected to V+ (see Figure 21). In bipolar operation, positive and negative supplies must be connected to V+ and V–. The AD711 is a general purpose op amp which could be used to drive the analog input of the AD7880. POWER-DOWN CONTROL (MODE INPUT) The AD7880 is designed for systems which need to have mini- mum power consumption. This includes such applications as hand held, portable battery powered systems and remote moni- toring systems. As well as consuming minimum power under normal operating conditions, typically 20 mW, the AD7880 can be put into a power-down or sleep mode when not required to convert signals. When in this power-down mode, the AD7880 consumes approximately 2 mW of power. The AD7880 is powered down by bringing the MODE input pin to a Logic Low in conjunction with keeping the RD input control High. The AD7880 will remain in the power-down mode until MODE is brought to a Logic High again. The MODE input should be driven with CD4000 or HCMOS logic levels. It is recommended that one “dummy” conversion be imple- mented before reading conversion data from the AD7880 after it has been in the power-down mode. This is required to reset all internal logic and control circuitry. In a remote monitoring system where, say, 10 conversions are required to be taken with a sampling interval of 1 second, an additional 11th conversion must be carried out. Figure 22 gives a plot of power consumption 01 2 TIME – secs POWER CONSUMPTION – mW 20 2 CONVERTING POWER-DOWN CONVERTING POWER-DOWN 1.65 x 10 4 – Figure 22. Power Consumption for Normal Operation and Power-Down Operation vs. Time APPLICATION HINTS Good printed circuit board (PCB) layout is as important as the circuit design itself in achieving high speed A/D performance. The AD7880’s comparator is required to make bit decisions on an LSB size of 1.22 mV. To achieve this, the designer must be conscious of noise both in the ADC itself and in the preceding analog circuitry. Switching mode power supplies are not recom- mended, as the switching spikes will feed through to the com- parator causing noisy code transitions. Other causes of concern are ground loops and digital feedthrough from microprocessors. These are factors which influence any ADC, and a proper PCB layout which minimizes these effects is essential for best performance. LAYOUT HINTS Ensure that the layout for the printed circuit board has the digi- tal and analog signal lines separated as much as possible. Take care not to run digital tracks alongside analog signal tracks. Guard (screen) the analog input with AGND. Establish a single point analog ground (star ground) separate from the logic system ground at the AD7880 AGND pin or as close as possible to the AD7880. Connect all other grounds and the AD7880 DGND to this single analog ground point. Do not connect any other digital grounds to this analog ground point. Low impedance analog and digital power supply common re- turns are essential to low noise operation of the ADC, so make the foil width for these tracks as wide as possible. The use of ground planes minimizes impedance paths and also guards the analog circuitry from digital noise. The circuit layout of Fig- ures 26 and 27 have both analog and digital ground planes which are kept separated and only joined together at the AD7880 AGND pin. NOISE Keep the input signal leads to VIN and signal return leads from AGND as short as possible to minimize input noise coupling. In applications where this is not possible, use a shielded cable be- tween the source and the ADC. Reduce the ground circuit im- pedance as much as possible since any potential difference in grounds between the signal source and the ADC appears as an error voltage in series with the input signal. ANALOG INPUT BUFFERING To achieve specified performance, it is recommended that the analog input (VINA, VINB) be driven from a low impedance source. This necessitates the use of an input buffer amplifier. The choice of op amp will be a function of the particular appli- cation and the desired analog input range. The data acquisition circuit, described in this data sheet allows for various op amp configurations. Figure 21 shows the analog input buffer circuit. The options available to drive the supply of the op amp are: Single +5 V (derived from PCB 5 V supply) Dual Supply (externally supplied to V+ and V–) ±5 V, ±12 V or ±15 V The simplest configuration is the 0 V to 5 V range of Figure 5. A single supply 5 V op amp is recommended for such an imple- mentation. This will allow for operation of the AD7880 in the 0 V to 5 V unipolar range without supplying an external supply to V+ and V–. The 5 V supply is derived from the systems +5 V VDD supply. |
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