ADAV4601 datasheet(27/60 Pages) AD

27 / 60 page

ADAV4601

Rev. B | Page 27 of 60

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 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 following equation describes this step

size as

()

−

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 39 and Figure 40. All slew RAM figure

examples, except the half-scale constant time ramp plot (Figure 45),

show an increasing or decreasing ramp between −80 dB and 0 dB

(full scale). All figures except the constant time plots (Figure 44,

Figure 45, and Figure 46) 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 39. 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 40. Slew RAM—Linear Update Decreasing Ramp

Constant Decibels and RC Updates (Exponential)

An exponential update is accomplished by shifts and additions

with a range from 6.1 ms to 1.27 sec (−60 dB relative to 0 dB full

scale). When the ramp type is set to 01 (constant decibels), each

step size is set to the current value in the slew data. When the

ramp type bits are set to 10 (RC), the step size is equal to the

difference between the values in the target RAM and the slew

RAM (see Figure 41, Figure 42, and Figure 43).

TIME (ms)

1.0

0.8

0.6

0.4

0.2

–0.6

–0.4

–0.2

–0.8

–1.0

010

520

Figure 41. Slew RAM—Constant Decibels Update Increasing Ramp

TIME (ms)

1.0

0.8

0.6

0.4

0.2

–0.6

–0.4

–0.2

–0.8

–1.0

030

Figure 42. Slew RAM—RC 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 43. Slew RAM—Constant Decibels and RC

Updates Decreasing Ramp, Full Scale

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