AD9501 Analog Devices, AD9501 Datasheet - Page 6

• Manufacturer Part Number: AD9501
• Description: Digitally Programmable Delay Generator
• Manufacturer: Analog Devices

AD9501
Offset between the two levels is necessary for three reasons.
First, offset allows the ramp to reset and settle without re-
entering the voltage range of the DAC. Second, the DAC may
overshoot as it switches to its most positive value (00
could lead to false output pulses if there were no offset between
the ramp reset voltage and the upper reference. Overshoot on
the ramp could also lead to false outputs without the offset.
Finally, the ramp is slightly nonlinear for a short interval when it
is first started; the offset shifts the most positive DAC level
below this nonlinear region and maintains ramp linearity for
short programmed delay settings.
Pin 8 of the AD9501 is called OFFSET ADJUST (see block
diagram) and allows the user to control the amount of offset
separating the initial ramp voltage and the most positive DAC
reference. This, in turn, causes the Ramp Generator delay to
vary.
Figure 2 shows differences in timing which occur if OFFSET
ADJUST Pin 8 is grounded or open. The variable Ramp
Generator delay is the major component of the three
components which comprise Minimum Delay (t
therefore, is affected by the connection to Pin 8.
It is preferable to ground Pin 8 because the smaller offset that
results from leaving it open increases the possibility of false out-
put pulses. When grounding the pin, it should be grounded
Figure 2. AD9501 Minimum Delay (t
PD
) and,
H
); this
–6–
PD
directly or connected to ground through a resistor or potentiom-
eter with a value of 10 k or less.
Caution is urged when using resistance in series with Pin 8. The
possibility of false output pulses, as discussed above, is in-
creased under these circumstances. Using resistance in series
with Pin 8 is recommended only when matching minimum de-
lays between two or more AD9501 devices; it is not recom-
mended if using a single AD9501. Changing the resistance
between Pin 8 and ground from zero to 10 k varies the Ramp
Generator Delay by approximately 35%.
The Full-Scale Delay Range (t
equation:
Whenever Full-Scale Delay Range is 326 ns or less, C
be left open. Additional capacitance and/or larger values of R
increase the Linear Ramp Settling Time, which reduces the
maximum trigger rate. When delays longer than 326 ns are
required, up to 500 pF can be connected from C
R
1 shows typical Full-Scale Delay Ranges for various values of
R
) vs. Full-Scale Delay Range (t
SET
SET
should be selected in the range from 50
and C
EXT
(t
DFS
.
) R
SET
(C
DFS
EXT
DFS
) can be calculated from the
)
8.5 pF ) 3.84
to 10 k . Graph
EXT
to +V
EXT
REV. A
should
S
.
SET