DAC8426

–7–

REV. C

PARAMETER DEFINITIONS

TOTAL UNADJUSTED ERROR (TUE)

This specification includes the Full-Scale-Error, Relative Accu-

racy Zero-Code-Error and the internal reference voltage. The

ideal Full-Scale output voltage is 10 V minus 1 LSB which

equals 9.961 volts. Each LSB equals 10 V

× (1/256) = 0.039 volts.

DIGITAL CROSSTALK

Digital crosstalk is the signal coupled to the output of a DAC

due to a changing digital input from adjacent DACs being up-

dated. It is specified in nano-Volt-seconds (nVs).

CIRCUIT DESCRIPTION

The DAC8426 is a complete quad 8-bit D/A converter. It con-

tains an internal bandgap reference, four voltage switched R-2R

ladder DACs, four DAC latches, four output buffer amplifiers,

and an address decoder. All four DACs share the internal ten

volt reference and analog ground(AGND). Figure 1 provides an

equivalent DAC plus buffer schematic.

Figure 1. Simplified Circuit Configuration for One DAC.

(Switches Are Shown for All “1s” on the Digital Inputs.)

The eleven digital inputs are compatible with both TTL and 5 V

(or higher) CMOS logic. Table I shows the DAC control logic

low the input latch of the selected DAC is transparent, and the

DAC’s output responds to the data present on the eight digital

data inputs (DBx). The data (DBx) is latched into the ad-

dressed DAC’s latch on the positive edge of the WR control sig-

nal. The important timing requirements are shown in the Write

Cycle Timing Diagram, Figure 2.

INTERNAL 10 VOLT REFERENCE

The internal 10 V bandgap reference of the DAC8426 is trimm-

ed to the output voltage and temperature drift specifications.

This internal reference is connected to the reference inputs of

the four internal 8-bit D/A converters. The output terminal of

the internal 10 V reference is available on pin 4. The 10 V out-

put of the reference is produced with respect to the AGND pin.

This reference output can be used to supply as much as 5 mA of

additional current to external devices. Care has been taken in

Table I. DAC Control Logic Truth Table

Logic Control

DAC8426

WR

Operation

H

X

X

No Operation

Device Not Selected

L

L

L

DAC A Transparent

g

L

L

DAC A Latched

L

L

H

DAC B Transparent

g

L

H

DAC B Latched

L

H

L

DAC C Transparent

g

H

L

DAC C Latched

L

H

H

DAC D Transparent

g

H

H

DAC D Latched

L = Low State, H = High State, X = Don’t Care

Figure 2. Write Cycle Timing Diagram

the design of the internal DAC switching to minimize transients

connected to this reference terminal should have well behaved

input loading characteristics. D/A converters such as the PMI

PM7226A have been designed to minimize reference input tran-

sient currents and can be directly connected to the DAC8426

10 V reference. Devices exhibiting large current transients due

to internal switching should be buffered with an op amp to

maintain good overall system noise performance. A 10

µF refer-

ence output bypass capacitor is required.

BUFFER AMPLIFIER SECTION

The four internal unity-gain voltage buffers provide low output

impedance capable of sourcing 5 mA or sinking 350

µA. Typical

output slew rates of

±4 V/µs are achieved with 10 V full-scale out-

Ω. Figure 3 photographs show large sig-

nal and settling time response. Capacitive loads to 3300 pF

maximum, and resistive loads to 2 k

Ω minimum can be applied.