DAC8222

–8–

REV. C

cal in order to keep DAC errors to a minimum. This is normally

done by connecting AGND to the noninverting input of an op

amp also provides the current-to-voltage conversion for the

DAC’s output current. The output voltage is dependent on the

× D/4096

where D is the digital input code integer number that is between

0 and 4095.

reference voltage or current, ac or dc (positive or negative). It is

resistor be used if a current source is employed.

varies from 90 pF (all digital inputs low) to 120 pF (all digital

inputs high).

Figure 19 shows a transistor switch in series with the R-2R lad-

into the DAC to binarily match the ladder leg switches and im-

prove power supply rejection and gain error temperature coeffi-

cient. The gates of these transistor switches are connected to

This means that an op amp’s output voltage will go to either

Figure 21. Digital Input Structure For One Bit

DIGITAL SECTION

The DAC8222’s digital inputs are CMOS inserters. They were

designed to convert TTL and CMOS input logic levels into

voltage levels to drive the internal circuitry. The digital inputs

mately 5 mA–6 mA.

Figure 21 shows the DAC’s digital input register structure for

one bit. This circuit drives the DAC register. Digital controls

φ

and

φ shown are generated from DAC A/DAC B and WR con-

trol signals.

As shown in Figure 21, these inputs are electrostatic-discharge

protected with two internal distributed diodes; they are con-

cal input current of less than 1 nA.

When the digital inputs are in the region of +1.2 V to +2.8 V

(peaking at +1.8 V) using a +5 V power supply or in the region

of +1.7 V to +12 V (peaking at +3.9 V) with a +15 V power

supply, the input register transistors are operating in their linear

region and draw current from the power supply. It is therefore,

recommended that the digital input voltages be as close to the

supply currents at a minimum. The DAC8222 may be operated

with any supply voltage between the range of +5 V to +15 V.

INTERFACE CONTROL LOGIC

The DAC8222’s input control logic circuitry is shown in Figure

22. Note how the

WR signal is used in conjunction with DAC

A/ DAC B to load data into either input register. LDAC loads

data from the input registers to the DAC register; the DAC’s

analog output voltage is determined by the data contained in

each DAC register.

The truth table for the DAC registers is shown in the Mode Se-

lection Table. Note how the input register is transparent when

WR is low and LDAC is high, and that the DAC register is

transparent when

WR is high and LDAC is low (LDAC updates

the DAC’s analog output voltage). The DAC is transparent

from input to output when

WR and LDAC are both low, and

the DAC is latched (input and output is not being updated)

when

WR and LDAC are both high.

Figure 22. Input Control Logic