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AD7226

LC2MOS Quad 8-Bit D/A Converter

Analog Devices 

AD7226

STAIRCASE WINDOW COMPARATOR

In many test systems, it is important to be able to determine

whether some parameter lies within defined limits. The staircase

window comparator of Figure 14a is a circuit that can be used,

for example, to measure the VOH and VOL thresholds of a TTL

device under test. Upper and lower limits on both VOH and VOL

can be programmably set using the AD7226. Each adjacent pair

of comparators forms a window of programmable size. If VTEST

lies within a window, then the output for that window will be

high. With a reference of 2.56 V applied to the VREF input, the

minimum window size is 10 mV.

VREF VDD

VTEST

FROM D.U.T.

1/4 CA339

10k⍀

5V

WINDOW 1

VOUTA

VOH (HIGH)

10k⍀

5V

WINDOW 2

VOUTB

VOH (LOW)

AD7226

VOUTC

VOL (HIGH)

10k⍀

5V

WINDOW 3

10k⍀

5V

WINDOW 4

VOUTD

VOL (LOW)

AGND

10k⍀

5V

WINDOW 5

Figure 14a. Logic Level Measurement

VREF

VOUTA

VOUTB

WINDOW 1

WINDOW 2

WINDOW 3

VOUTC

VOUTD

WINDOW 4

WINDOW 5

AGND

Figure 14b. Window Structure

The circuit can easily be adapted to allow for overlapping of

windows as shown in Figure 15a. If the three outputs from this

circuit are decoded then five different nonoverlapping program-

mable windows can again be defined.

VREF VDD

VOUTA

VOUTB

AD7226

VOUTC

VOUTD

AGND

VTEST

FROM D.U.T.

10k⍀

5V

WINDOW 1

10k⍀

5V

WINDOW 2

10k⍀

5V

WINDOW 3

Figure 15a. Overlapping Windows

VREF

VOUTB

VOUTA

VOUTD

VOUTC

WINDOW 1

WINDOW 2

WINDOW 3

AGND

Figure 15b. Window Structure

+4V

–4V

+15V

15k⍀

10k⍀

*DIGITAL INPUTS OMITTED

FOR CLARITY

VREF

AD7226*

DAC A

VDD

VOUTA

VSS AGND

DGND

Figure 16. Varying Reference Signal

VARYING REFERENCE SIGNAL

In some applications, it may be desirable to have a varying signal

applied to the reference input of the AD7226. The AD7226 has

multiplying capability within upper and lower limits of reference

voltage when operated with dual supplies. The upper and lower

limits are those required by the AD7226 to achieve its linearity

specification. Figure 16 shows a sine wave signal applied to the

reference input of the AD7226. For input signal frequencies up

to 50 kHz, the output distortion typically remains less than 0.1%.

Typical 3 dB bandwidth figure is 700 kHz.

REV. D

–11–

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