Data Sheet AD7228
Rev. C | 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
13034-013
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
*ADDITION
L PINS OMITTED FOR CLARITY.
POSITION OF THIS EDGE
PROGRAMMED BY CODE
TO DAC1
HIGH-SPEED
COMPARATORS
V
REF
AD7228*
V
SS
V
DD
V
OUT
2
V
OUT
1
GND
11 1
10 12
9
8
13034-014
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.
V
SS
–5V
DD
V
OUT
2
REF
AD7228*
10
DAC 2
9
V
OUT
1
200Ω
51.2kΩ 200Ω
V
OUT
51.2kΩ
DAC 1
A1
13034-015
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
converters. The relationship of V
REF
to V
IN
is expressed by
INREF
V
DG
G
V
)1(
)1(
1
where G = R2/R1.
*ADDITIONAL PINS OMITTED FOR CLARITY.
AD7228*
9
V
OUT
1
A1
R2R1
V
IN
12
GND
V
SS
–5V
+15V
10
11 1
V
DD
V
REF
13034-016
Figure 16. Self Programmable Reference
