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ADAV400

Rev. 0 | Page 21 of 36

Table 14. Target/Slew RAM Ramp Type Settings

Settings

Ramp Type

Linear

Constant dB

Constant time

The following sections detail how the control port writes to the

target/slew RAM to control the time constant and ramp type

parameters.

Ramp Types [1:3]—Linear, Constant dB, RC (34-Bit Write)

The target word for the first three ramp types is broken into

three parts. The 34-bit command is written with six leading 0s

to extend the data write to five bytes. The parts of the target

RAM write are

•

Ramp type (2 bits).

•

Time constant (4 bits).

0000 = fastest

1111 = slowest

•

Data (28 bits): 5.23 format.

Ramp Type 4—Constant Time (34-Bit Write)

The target word for the constant time ramp type is written in

five parts, with the 34-bit command written with six leading 0s

to extend the data write to five bytes. The parts of the constant

time target RAM write are

•

Ramp type (2 bits).

•

Update step (1 bit). Set to 1 when a new target is loaded to

trigger a step value update. The value is automatically reset

after the step value is updated.

•

Number of steps (3 bits). The number of steps needed to

slew to the target value is set by these three bits, with the

number of steps equal to 23-bit setting + 6.

000 = 64

001 = 128

010 = 256

011 = 512

100 = 1024

101 = 2048

110 = 4096

111 = 8196

•

Data (16 bits): 2.14 format.

•

Reserved (12 bits). When writing to the RAM, set all of

these bits to 0.

Target/Slew RAM Initialization

On reset, the target/slew RAM initializes to preset values. The

target RAM initializes to a linear ramp type with a time constant

of 5 and the data set to 1.0. The slew RAM initializes to a value

of 1.0. These defaults result in a full-scale (1.0 to 0.0) ramp time

of 21.3 ms.

Linear Update

A linear update is the addition or subtraction of a constant value,

referred to as a step. The equation to describe this step size is

()

−

CONST

Step

The result of the equation is normalized to a 5.23 data format.

This produces a time constant range from 6.75 ms to 213.4 ms.

(–60 dB relative to 0 dB full scale). An example of this kind of

update is shown in Figure 23 and Figure 24. All slew RAM figure

examples, except the half-scale constant time ramp plot (Figure

29), show an increasing or decreasing ramp between –80 dB and

0 dB (full scale). All figures except the constant time plots

(Figure 28 and Figure 29) use a time constant of 0x7 (0x0 being

the fastest and 0xF being the slowest).

TIME (ms)

1.0

0.4

0.6

0.8

0.2

–0.4

–0.2

–1.0

–0.8

–0.6

030

Figure 23. Slew RAM—Linear Update Increasing Ramp

–1.0

–0.8

–0.6

–0.4

–0.2

0.2

0.4

0.6

0.8

1.0

TIME (ms)

030

Figure 24. Slew RAM—Linear Update Decreasing Ramp

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