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AD9501
REV. A
–5–
Figure 1, the AD9501 Internal T iming diagram, illustrates in
detail how the delay is determined. Minimum Delay (t
PD
) is the
sum of T rigger Circuit delay, Ramp Generator delay, and
Comparator delay.
T he T rigger Circuit delay and Comparator delay are fixed;
Ramp Generator delay is a variable affected by the rate of
change of the linear ramp and (to a lesser degree) the value of
the offset voltage described below.
Maximum Delay is the sum of Minimum Delay (t
PD
) and Full-
Scale Program Delay (t
DFS
).
Ramp Generator delay is the time required for the ramp to slew
from its reset voltage to the most positive DAC reference
voltage (00
H
). T he difference in these two voltages is nominally
18 mV (with OFFSET ADJUST open) or 34 mV (OFFSET
ADJUST grounded).
T HE ORY OF OPE RAT ION
T he AD9501 is a digitally programmable delay device. Its
function is to provide a precise incremental delay between input
and output, proportional to an 8-bit digital word applied to its
delay control port. Incremental delay resolution is 10 ps at the
minimum full-scale range of 2.5 ns. Digital delay data inputs,
latch, trigger and reset are all T T L/CMOS compatible. Output
is T T L-compatible.
Refer to the block diagram of the AD9501.
Inside the unit, there are three main subcircuits: a linear ramp
generator, an 8-bit digital-to-analog converter (DAC) and a
voltage comparator. T he rising edge of the input (T RIGGER)
pulse initiates the delay cycle by triggering the ramp generator.
T he voltage comparator monitors the ramp voltage and switches
the delayed output (Pin 10) HIGH when the ramp voltage
crosses the threshold set by the DAC output voltage. T he DAC
threshold voltage is programmed by the user with digital inputs.
Figure 1. AD9501 Internal Timing