AD7524MFKB datasheet(5/12 Pages) TI

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AD7524M

Advanced LinCMOS

™ 8-BIT MULTIPLYING

DIGITAL-TO-ANALOG CONVERTER

SGLS028A – SEPTEMBER 1989 – REVISED MARCH 1995

POST OFFICE BOX 655303

• DALLAS, TEXAS 75265

PRINCIPLES OF OPERATION

The AD7524M is an 8-bit multiplying D/A converter consisting of an inverted R-2R ladder, analog switches, and

data input latches. Binary weighted currents are switched between the OUT1 and OUT2 bus lines, thus

maintaining a constant current in each ladder leg independent of the switch state. The high-order bits are

decoded and these decoded bits, through a modification in the R-2R ladder, control three equally weighted

current sources. Most applications only require the addition of an external operational amplifier and a voltage

reference.

The equivalent circuit for all digital inputs low is seen in Figure 1. With all digital inputs low, the entire reference

current, Iref, is switched to OUT2. The current source 1/256 represents the constant current flowing through the

termination resistor of the R-2R ladder, while the current source IIkg represents leakage currents to the

substrate. The capacitances appearing at OUT1 and OUT2 are dependent upon the digital input code. With all

digital inputs high, the off-state switch capacitance (30 pF maximum) appears at OUT2 and the on-state switch

capacitance (120 pF maximum) appears at OUT1. With all digital inputs low, the situation is reversed as shown

in Figure 1. Analysis of the circuit for all digital inputs high is similar to Figure 1; however, in this case, Iref would

be switched to OUT1.

Interfacing the AD7524M D/A converter to a microprocessor is accomplished via the data bus and the CS and

WR control signals. When CS and WR are both low, the AD7524M analog output responds to the data activity

on the DB0–DB7 data bus inputs. In this mode, the input latches are transparent and input data directly affects

the analog output. When either the CS signal or WR signal goes high, the data on the DB0–DB7 inputs are

latched until the CS and WR signals go low again. When CS is high, the data inputs are disabled regardless

of the state of the WR signal.

The AD7524M is capable of performing 2-quadrant or full 4-quadrant multiplication. Circuit configurations for

2-quadrant or 4-quadrant multiplication are shown in Figures 2 and 3. Input coding for unipolar and bipolar

operation are summarized in Tables 1 and 2, respectively.

AD7524MFKB Datasheet(PDF) 5 Page - Texas Instruments