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PRELIMINARY TECHNICAL DATA

AD9953

ASF<15:14> (binary)

Increment/decrement size

Table 5 Auto-Scale Factor Internal Step Size

A special feature of this mode is that the maximum output amplitude allowed is limited by the

contents of the Amplitude Scale Factor Register. This allows the user to ramp to a value less than

full scale.

OSK Ramp Rate Timer

The OSK ramp rate timer is a loadable down counter, which generates the clock signal to the 14-bit

counter that generates the internal scale factor. The ramp rate timer is loaded with the value of the

ASFR every time the counter reaches 1 (decimal). This load and count down operation continues

for as long as the timer is enabled unless the timer is forced to load before reaching a count of 1.

If the Load OSK Timer bit (CFR1<26>) is set, the ramp rate timer is loaded upon an I/O

UPDATE, change in profile input or upon reaching a value of 1. The ramp timer can be loaded

before reaching a count of 1 by three methods.

Method one is by changing the OSK input pin. When the OSK input pin changes state the ASFR

value is loaded into the ramp rate timer, which then proceeds to count down as normal.

The second method in which the sweep ramp rate timer can be loaded before reaching a count of 1

is if the Load OSK Timer bit (CFR1<26>) bit is set and an I/O UPDATE (or change in profile) is

issued.

The last method in which the sweep ramp rate timer can be loaded before reaching a count of 1 is

when going from the inactive AUTO Shaped On-Off Keying mode to the active AUTO Shaped

On-Off Keying mode. That is, when the sweep enable bit is being set.

External Shaped On-Off Keying mode operation:

The external Shaped On-Off Keying mode is enabled by writing CFR1<25> to a logic 1 AND

writing CFR1<24> to a logic 0. When configured for external Shaped On-Off Keying, the content

of the ASFR becomes the scale factor for the data path. The scale factors are synchronized to

sync_clk via the I/O update functionality.

REV. PrB 1/30/03

Page 33

Analog Devices, Inc.

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