AD7538(RevA) Просмотр технического описания (PDF) - Analog Devices

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AD7538
(Rev.:RevA)

LC2MOS μP-Compatible 14-Bit DAC

Analog Devices 

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.

CIRCUIT INFORMATION

AD7538

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.

Figure 2. Simplified Circuit Diagram for the AD7538 D/A Section

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.

Figure 4. Unipolar Binary Operation

Table I. Unipolar Binary Code Table for AD7538

Binary Number In

DAC Register

MSB

LSB

11 1111 1111 1111

10 0000 0000 0000

00 0000 0000 0001

Analog Output, VOUT

 16383 

–VIN  16384 

–VIN

 8192

 16384





=

–1/

2VIN

 1

–VIN  16384 

00 0000 0000 0000

0V

REV. A

–5–

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