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ADAV400

Rev. 0 | Page 22 of 36

Constant dB 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 dB), 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 slew RAM (see

Figure 25, Figure 26, and Figure 27).

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 25. Slew RAM—Constant dB 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 26. 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 27. Slew RAM—Constant dB and RC

Updates Decreasing Ramp, Full Scale

Constant Time Update

A constant time update is calculated by adding a step value that

is determined after each target is loaded. The equation for this

step size is

Step = (Target Data − Slew Data)/(Number of Steps)

Figure 28 shows a plot of the target/slew RAM operating in

constant time mode. For this example, 128 steps are used to

reach the target value. This type of ramping takes a fixed

amount of time for a given number of steps, regardless of the

difference in the initial state and the target value. Figure 29

shows a plot of a constant time ramp from –80 dB to –6 dB (half

scale) using 128 steps; thus, the ramp takes the same amount of

time as the previous ramp from –80 dB to 0 dB. A constant time

decreasing ramp plot is shown in Figure 30.

TIME (ms)

1.0

0.8

0.6

0.4

0.2

–0.6

–0.4

–0.2

–0.8

–1.0

030

Figure 28. Slew RAM—Constant Time Update Increasing Ramp, Full Scale

TIME (ms)

1.0

0.8

0.6

0.4

0.2

–0.6

–0.4

–0.2

–0.8

–1.0

030

Figure 29. Slew RAM—Constant Time Update Increasing Ramp, Half Scale

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