Datasheet

AD9852
Rev. E | Page 28 of 52
The two fixed elements of the transition time are the period of
the system clock (which drives the ramp rate counter) and the
number of amplitude steps (4096). For example, if the system
clock of the AD9852 is 100 MHz (10 ns period) and the ramp
rate counter is programmed for a minimum count of 3, two system
clock periods are required: one rising edge loads the countdown
value, and the next edge decrements the counter from 3 to 2. If the
countdown value is less than 3, the ramp rate counter stalls and
therefore produces a constant scaling value to the digital multiplier.
This stall condition may have an application for the user.
The relationship of the 8-bit countdown value to the time between
output pulses is given as
(N + 1) × System Clock Period
where N is the 8-bit countdown value.
A total of 4096 output pulses is required to advance the 12-bit
up-counter from zero scale to full scale. Therefore, the minimum
output shaped keying ramp time for a 100 MHz system clock is
4096 × 4 × 10 ns ≈ 164 µs
The maximum ramp time is
4096 × 256 × 10 ns ≈ 10.5 ms
Finally, by changing the logic state of Pin 30, output shaped
keying automatically performs the programmed output envelope
functions when OSK INT is high. A logic high on Pin 30 causes
the outputs to linearly ramp up to full-scale amplitude and hold
until the logic level is changed to low, causing the outputs to
ramp down to zero scale.
12-BIT DIGITAL
MULTIPLIER
12 12
(BYPASS MULTIPLIER)
OSK EN = 0
OSK EN = 1
OSK EN = 0
OSK EN = 1
12
12
DIGITAL
SIGNAL IN
USER-PROGRAMMABLE
12-BIT MULTIPLIER
OUTPUT SHAPED
KEYING MULTIPLIER
REGISTER
12
OSK INT = 1
OSK INT = 0
1
8-BIT RAMP
RATE
COUNTER
SYSTEM
CLOCK
ON/OFF OUTPUT SHAPED
KEYING PIN
12-BIT
UP/DOWN
COUNTER
DDS DIGITAL
OUTPUT
COSINE
DAC
00634-047
Figure 47. Block Diagram of the Digital Multiplier Section Responsible for the Output Shaped Keying Function