AD7538

REV. A

–5–

EQUIVALENT CIRCUIT ANALYSIS

Figure 3 shows an equivalent circuit for the analog section of

denotes the equivalent output resistance of the DAC which

rent steering switches and varies from about 90 pF to 180 pF

the Thevenin equivalent voltage generator due to the reference

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

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

MSB

LSB

11 1111 1111 1111

16383

16384









10 0000 0000 0000

8192

16384









= –1/ 2V

IN

00 0000 0000 0001

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.