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AD7390AR Datasheet(PDF) 10 Page - Analog Devices |
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AD7390AR Datasheet(HTML) 10 Page - Analog Devices |
10 / 12 page REV. A AD7390/AD7391 –10– UNIPOLAR OUTPUT OPERATION This is the basic mode of operation for the AD7390. As shown in Figure 10, the AD7390 has been designed to drive loads as low as 5 k Ω in parallel with 100 pF. The code table for this operation is shown in Table IV. AD7390 0.1 F CLK VOUT REF VDD GND R 10 F 6 7 5 1 2 3 4 SDI CLR LD 2.7V TO 5.5V RL 5k CL 100pF C RS EXT REF 0.01 F Figure 10. AD7390 Unipolar Output Operation Table IV. AD7390 Unipolar Code Table Hexadecimal Decimal Output Number Number Voltage (V) in DAC Register in DAC Register VREF = 2.5 V FFF 4095 2.4994 801 2049 1.2506 800 2048 1.2500 7FF 2047 1.2494 000 0 0 The circuit can be configured with an external reference plus power supply, or powered from a single dedicated regulator or reference, depending on the application performance requirements. BIPOLAR OUTPUT OPERATION Although the AD7391 has been designed for single-supply opera- tion, the output can be easily configured for bipolar operation. A typical circuit is shown in Figure 11. This circuit uses a clean regulated 5 V supply for power, which also provides the circuit’s reference voltage. Since the AD7391 output span swings from ground to very near 5 V, it is necessary to choose an external amplifier with a common-mode input voltage range that extends to its positive supply rail. The micropower consumption OP196 has been designed just for this purpose and results in only 50 micro- amps of maximum current consumption. Connection of the equally valued 470 k Ω resistors results in a differential amplifier mode of operation with a voltage gain of two, which results in a circuit output span of ten volts, that is, 25 V to 15 V. As the DAC is programmed with zero-code 000H to midscale 200H to full-scale 3FFH, the circuit output voltage VO is set at 25 V, 0 V and 15 V (minus 1 LSB). The output voltage VO is coded in offset binary according to Equation 4. V D O = × 512 15 – (4) where D is the decimal code loaded in the AD7391 DAC register. Note that the LSB step size is 10/1024 = 10 mV. This circuit has been optimized for micropower consumption including the 470 k Ω gain setting resistors, which should have low temperature coeffi- cients to maintain accuracy and matching (preferably the same material, such as metal film). If better stability is required, the power supply could be substituted with a precision reference voltage such as the low dropout REF195, which can easily supply the circuit’s 162 µA of current, and still provide additional power for the load connected to VO. The micropower REF195 is guaranteed to source 10 mA output drive current, but only consumes 50 µA internally. If higher resolution is required, the AD7390 can be used with the addition of two more bits of data inserted into the software coding, which would result in a 2.5 mV LSB step size. Table V shows examples of nominal output voltages VO provided by the Bipolar Operation circuit application. C ISY < 162 A BIPOLAR OUTPUT SWING VO +5V –5V –5V VOUT AD7391 VDD REF GND +5V < 100 A 470k 470k < 50 A OP196 DIGITAL INTERFACE CIRCUITRY OMITTED FOR CLARITY Figure 11. Bipolar Output Operation Table V. Bipolar Code Table Hexadecimal Decimal Analog Number Number Output in DAC Register in DAC Register Voltage (V) 3FF 1023 4.9902 201 513 0.0097 200 512 0.0000 1FF 511 –0.0097 000 0 –5.0000 MICROCOMPUTER INTERFACES The AD7390 serial data input provides an easy interface to a variety of single-chip microcomputers ( µCs). Many µCs have a built-in serial data capability which can be used for communi- cating with the DAC. In cases where no serial port is provided, or it is being used for some other purpose (such as an RS-232 communications interface), the AD7390/AD7391 can easily be addressed in software. Twelve data bits are required to load a value into the AD7390. If more than 12 bits are transmitted before the load LD input goes high, the extra (i.e., the most-significant) bits are ignored. This feature is valuable because most µCs only transmit data in 8-bit increments. Thus, the µC sends 16 bits to the DAC instead of 12 bits. The AD7390 will only respond to the last 12 bits clocked into the SDI input, however, so the serial-data interface is not affected. Ten data bits are required to load a value into the AD7391. If more than 10 bits are transmitted before load LD returns high, the extra bits are ignored. |
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