Data Sheet

AD7228

Rev. D | Page 11 of 15

Timing Deskew

Signal edges slowing or rounding off by the time they reach the

pin driver circuitry is a common problem in automated test

equipment (ATE) applications. Square up the edge at the pin

driver to overcome this problem. However, because each edge is

not rounded off by the same extent, this squaring up may lead

to incorrect timing relationship between signals. This effect is

shown in Figure 13.

BUFFER TRIGGER POINT

HIGH-SPEED

BUFFER

Figure 13. Time Skewing Due to Slowing of Edges

The circuit of Figure 14 shows how two DACs of the AD7228

can help overcome the problem of time skewing. The same two

signals are applied to this circuit as are applied in Figure 14. The

output of each DAC is applied to one input of a high speed

comparator, and the signals are applied to the other inputs.

Varying the output voltage of the DAC effectively varies the

trigger point at which the comparator flips. Therefore, the timing

relationship between the two signals can be programmably

corrected (or deskewed) by varying the code to the DAC of

the AD7228. In a typical application, the code is loaded to the

DACs for correct timing relationships during the calibration

cycle of the instrument.

POSITION OF THIS EDGE

PROGRAMMED BY CODE

TO DAC2

*ADDITIONAL PINS OMITTED FOR CLARITY.

POSITION OF THIS EDGE

PROGRAMMED BY CODE

TO DAC1

HIGH-SPEED

COMPARATORS

AD7228*

GND

11

1

10

12

9

8

Figure 14. AD7228 Timing Deskew Circuit

Coarse/Fine Adjust

Pair the DACs on the AD7228 together to form a coarse/fine

adjust function as shown in Figure 15. The function is achieved

using one external op amp and a few resistors per pair of DACs.

DAC 1 is the most significant or coarse DAC. Data is first loaded to

this DAC to coarsely set the output voltage. DAC 2 is then used

to fine tune this output voltage. Varying the ratio of R1 to R2

varies the relative effect of the coarse and fine DACs on the

output voltage. For the resistor values shown, DAC 2 has a

resolution of 150 μV in a 10 V output range. Because each DAC

on the AD7228 is guaranteed monotonic, the coarse adjustment

and fine adjustment are each monotonic. One application for

this is as a setpoint controller (see the AN-317 Application

Note, “Circuit Applications of the AD7226 Quad CMOS DAC,”

available from Analog Devices, Inc.).

12

8

11

1

GND

*ADDITIONAL PINS OMITTED FOR CLARITY.

–5V

AD7228*

10

DAC 2

9

51.2kΩ

200Ω

51.2kΩ

DAC 1

A1

Figure 15. Coarse/Fine Adjust Circuit

Self Programmable Reference

The circuit of Figure 16 shows how one DAC of the AD7228, in

this case DAC 1, can be used in a feedback configuration to

provide a programmable reference for itself and the other seven

IN

REF

V

D

G

G

V











)

1

(

)

1

(

1

where G = R2/R1.

*ADDITIONAL PINS OMITTED FOR CLARITY.

AD7228*

9

A1

R2

R1

12

GND

–5V

+15V

10

11

1

Figure 16. Self Programmable Reference