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AD7538 датащи(PDF) 10 Page - Analog Devices |
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AD7538 датащи(HTML) 10 Page - Analog Devices |
10 / 16 page AD7538 Rev. B | Page 10 of 16 CIRCUIT INFORMATION EQUIVALENT CIRCUIT ANALYSIS Figure 5 shows an equivalent circuit for the analog section of the AD7538 DAC. The current source ILEAKAGE is composed of surface and junction leakages. The RO resistor denotes the equivalent output resistance of the DAC, which varies with input code. COUT is the capacitance due to the current steering switches and varies from about 90 pF to 180 pF (typical values) depending upon the digital input. g(VREF, N) is the Thevenin equivalent voltage generator due to the reference input voltage, VREF, and the transfer function of the DAC ladder, N. ILEAKAGE COUT IOUT AGND RFB RO R/4 g (VREF, N) Figure 5. AD7538 Equivalent Analog Output Circuit DIGITAL SECTION The digital inputs are designed to be both TTL and 5 V CMOS compatible. All logic inputs are static protected MOS gates with typical input currents of less than 1 nA. To minimize power supply currents, it is recommended that the digital input voltages be driven as close as possible to 0 V and 5 V logic levels. UNIPOLAR BINARY OPERATION (2-QUADRANT MULTIPLICATION) Figure 6 shows the circuit diagram for unipolar binary operation. With an ac input, the circuit performs 2-quadrant multiplication. The code table for Figure 6 is given in Table 6. Capacitor C1 provides phase compensation and helps prevent overshoot and ringing when high-speed op amps are used. 619 5 24 23 1 20 21 22 2 3 4 DB13 TO DB0 DGND VDD VREF VIN RFB IOUT VDD VO –15V AGND VSS LDAC CS WR LDAC CS WR AD7538 A1 R3 1kΩ R2 10Ω R1 20Ω R4 47kΩ INPUT DATA DIGITAL GND C2 4.7µF C1 33pF ANALOG GND AD711 + Figure 6. Unipolar Binary Operation Table 6. Unipolar Binary Code Table Binary Number In DAC Register Analog Output, VOUT MSB LSB 11 1111 1111 1111 −VIN(16,383/16,384) 10 0000 0000 0000 −VIN(8192/16,384) = −½VIN 00 0000 0000 0001 −VIN(1/16,384) 00 0000 0000 0000 0 V For zero offset adjustment, the DAC register is loaded with all 0s and amplifier offset (VOS) adjusted so that VOUT is 0 V. Adjusting VOUT to 0 V is not necessary in many applications, but it is recommended that VOS be no greater than (25 × 10−6) (VREF) to maintain specified DAC accuracy (see the Application Hints section). Full-scale trimming is accomplished by loading the DAC register with all 1s and adjusting R1 so that VOUTA = −VIN (16,383/16,384). For high temperature operation, resistors and potentiometers should have a low temperature coefficient. In many applications, because of the excellent gain TC and gain error specifications of the AD7538, gain error trimming is not necessary. In fixed reference applications, full scale can also be adjusted by omitting R1 and R2 and trimming the reference voltage magnitude. |
Аналогичный номер детали - AD7538_15 |
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Аналогичное описание - AD7538_15 |
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