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AD7538AQ Datasheet(数据表) 5 Page - Analog Devices

部件型号  AD7538AQ
说明  LC2MOS uP-Compatible 14-Bit DAC
下载  8 Pages
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制造商  AD [Analog Devices]
网页  http://www.analog.com
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AD7538
REV. A
–5–
EQUIVALENT CIRCUIT ANALYSIS
Figure 3 shows an equivalent circuit for the analog section of
the AD7538 D/A converter. The current source ILEAKAGE is
composed of surface and junction leakages. The resistor RO
denotes the equivalent output resistance of the DAC which
varies with input code. COUT is the capacitance due to the cur-
rent 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.
Figure 3. 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 volt-
ages be driven as close as possible to 0 V and 5 V logic levels.
UNIPOLAR BINARY OPERATION (2-QUADRANT
MULTIPLICATION)
Figure 4 shows the circuit diagram for unipolar binary opera-
tion. With an ac input, the circuit performs 2 quadrant multipli-
cation. The code table for Figure 4 is given in Table I.
Capacitor C1 provides phase compensation and helps prevent
overshoot and ringing when high-speed op amps are used.
The R-2R ladder current is 1/8 of the total reference input cur-
rent. 7/8 I flows in the parallel ladder structure. Switches A-G
steer equally weighted currents between IOUT and AGND.
Since the input resistance at VREF is constant, it may be driven
by a voltage source or a current source of positive or negative
polarity.
CIRCUIT INFORMATION
Figure 2. Simplified Circuit Diagram for the AD7538 D/A Section
Figure 4. Unipolar Binary Operation
Table I. Unipolar Binary Code Table for AD7538
Binary Number In
DAC Register
Analog Output, VOUT
MSB
LSB
11 1111 1111 1111
–VIN
16383
16384


10 0000 0000 0000
–VIN
8192
16384


= –1/ 2V
IN
00 0000 0000 0001
–VIN
1
16384


00 0000 0000 0000
0 V
D/A SECTION
Figure 2 shows a simplified circuit diagram for the AD7538
D/A section. The three MSBs of the 14-bit Data Word are de-
coded to drive the seven switches A-G. The 11 LSBs of the
Data Word consist of an R-2R ladder operated in a current
steering configuration.




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