TLC7524C, TLC7524E, TLC7524I

8-BIT MULTIPLYING DIGITAL-TO-ANALOG CONVERTERS

SLAS061C – SEPTEMBER 1986 – REVISED NOVEMBER 1998

6

POST OFFICE BOX 655303

PRINCIPLES OF OPERATION

The TLC7524C, TLC7524E, and TLC7524I are 8-bit multiplying DACs 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. 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 2. With all digital inputs low, the entire reference

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

be switched to OUT1.

The DAC on these devices interfaces to a microprocessor through the data bus and the CS and WR control

signals. When CS and WR are both low, analog output on these devices 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.

These devices are capable of performing 2-quadrant or full 4-quadrant multiplication. Circuit configurations for

2-quadrant or 4-quadrant multiplication are shown in Figure 3 and Figure 4. Table 1 and Table 2 summarize input

coding for unipolar and bipolar operation respectively.

Iref

REF

OUT2

OUT1

RFB

R

120 pF

30 pF

IIkg

I/256

IIkg

Figure 2. TLC7524 Equivalent Circuit With All Digital Inputs Low