TLC7628C

DUAL 8BIT MULTIPLYING

SLAS063B − APRIL 1989 − REVISED MARCH 2007

8

POST OFFICE BOX 655303

• DALLAS, TEXAS 75265

PRINCIPLES OF OPERATION

This device contains two, identical, 8-bit, multiplying DACs: DACA and DACB. Each DAC consists of an inverted

R-2R ladder, analog switches, and input data latches. Binary-weighted currents are switched between the DAC

output and AGND, thus maintaining a constant current in each ladder leg independent of the switch state. Most

applications require only the addition of an external operational amplifier and voltage reference. A simplified D/A

circuit for DACA or DACB with all digital inputs low is shown in Figure 7.

Figure 8 shows the DACA or DACB equivalent circuit. Both DACs share the analog ground terminal 1 (AGND).

With all digital inputs high, the reference current flows to OUTA. A small leakage current (IIkg) flows across

internal junctions, and as with most semiconductor devices, doubles every 10

°C. The Co is caused by the

parallel combination of the NMOS switches and has a value that depends on the number of switches connected

to the output. The range of Co is 25 pF to 60 pF maximum. The equivalent output resistance (ro) varies with the

input code from 0.8R to 3R where R is the nominal value of the ladder resistor in the R-2R network.

The TLC7628C interfaces to a microprocessor through the data bus, CS, WR, and DACA/DACB control signals.

When CS and WR are both low, the analog output on this device, specified by the DACA/DACB control line,

responds to the 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 digital inputs of the TLC7628C provides TTL compatibility when operated from a supply voltage of 10.8 V

to 15.75 V.

DACA Data Latches and Drivers

REF

AGND

OUT

RFB

R

R

R

R

2R

2R

S8

2R

S3

2R

S2

S1

2R

Figure 7. Simplified Functional Circuit for DACA or DACB

R

REF

OUTA

RFB

R

COUT

1/256

Latch A or Latch B Loaded With 11111111

Ilkg

AGND

Figure 8. TLC7628 Equivalent Circuit for DACA or DACB