REV.

AD7226

–5–

CIRCUIT INFORMATION

D/A SECTION

The AD7226 contains four identical, 8-bit, voltage mode digital-to-

analog converters. The output voltages from the converters have the

same polarity as the reference voltage allowing single supply opera-

tion. A novel DAC switch pair arrangement on the AD7226 allows a

reference voltage range from 2 V to 12.5 V.

Each DAC consists of a highly stable, thin-film, R-2R ladder

and eight high speed NMOS, single-pole, double-throw

switches. The simplified circuit diagram for one channel is

are common to all four DACs.

RR

R

2R

2R

2R

2R

2R

DB0

DB5

DB6

DB7

AGND

SHOWN FOR ALL 1s ON DAC

Figure 1. D/A Simplified Circuit Diagram

parallel combination of the four individual DAC reference input

impedances. It is code dependent and can vary from 2 k

W to

infinity. The lowest input impedance (i.e., 2

KW) occurs when

all four DACs are loaded with the digital code 01010101.

Therefore, it is important that the reference presents a low

output impedance under changing load conditions. The nodal

capacitance at the reference terminals is also code dependent

and typically varies from 100 pF to 250 pF.

voltage source with an output voltage of:

VD V

OUTX

X

REF

=

(1)

and can vary from 0 to 255/256.

The source impedance is the output resistance of the buffer

amplifier.

OP AMP SECTION

Each voltage-mode D/A converter output is buffered by a unity

gain, noninverting CMOS amplifier. This buffer amplifier is

capable of developing 10 V across a 2 k

W load and can drive

capacitive loads of 3300 pF. The output stage of this amplifier

This output stage is shown in Figure 2.

The NPN transistor supplies the required output current drive

(up to 5 mA). The current load consists of NMOS transistors

which normally act as a constant current sink of 400

giving each output a current sink capability of approximately

400

mA if required.

The AD7226 can be operated single or dual supply resulting

in different performance in some parameters from the output

amplifiers.

put approaching AGND (i.e., digital code approaching all 0s)

I/P

O/P

400 A

Figure 2. Amplifier Output Stage

the current load ceases to act as a current sink and begins to act

as a resistive load of approximately 2 k

W to AGND. This occurs

as the NMOS transistors come out of saturation. This means

that, in single supply operation, the sink capability of the ampli-

fiers is reduced when the output voltage is at or near AGND. A

typical plot of the variation of current sink capability with out-

put voltage is shown in Figure 3.

500

010

2

468

400

300

200

100

0

If the full sink capability is required with output voltages at or

and thereby maintain the 400

mA current sink as indicated in

in the output amplifier which allows for better zero code error

performance on each output. Also improved is the slew rate and

negative-going settling time of the amplifiers (discussed later).

Each amplifier offset is laser trimmed during manufacture to

eliminate any requirement for offset nulling.

DIGITAL SECTION

The digital inputs of the AD7226 are both TTL and CMOS

are static protected MOS gates with typical input currents of

less than 1 nA. Internal input protection is achieved by an

on-chip distributed diode from DGND to each MOS gate. To

minimize power supply currents, it is recommended that the

and DGND) as practically possible.

4

o

f

1

4

D