AD1941 datasheet(18/32 Pages) AD

18 / 32 page

AD1941

Preliminary Technical Data

Rev. PrE | Page 18 of 32

The four ramping curve types are

Linear—Value slews to target using a fixed step size.

Constant dB—Value slews to target using the current value

to calculate the step size. The resulting curve has a constant

rise and decay when measured in dB.

RC-type—Value slews to target using the difference

between target and current values to calculate the step size,

producing a simple RC type curve for rising and falling.

Constant Time—Value slews to the target in a fixed

number of steps in a linear fashion. The control port mute

has no affect on this type.

Table 16 Target/Slew RAM Ramp Type Settings

Setting

Ramp Type

Linear

Constant dB

RC-Type

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-type (34-Bit Write)

The target word for the first three ramp types is broken up 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 described below.

•

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 again written with six

leading zeros to extend the data write to five bytes. The parts of

the constant time target RAM write are described below.

•

Ramp Type (2 bits).

•

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

trigger step value update. Value is automatically reset after

the step value is updated.

• Number of Steps (3 bits). The number of steps that it takes 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, these bits

should all be set to 0.

Target and 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 give a full-scale (1.0 to 0.0) ramp

time of 21.3 ms.

Linear Update Math

Linear math is the addition or subtraction of a constant value

(step). The equation to describe this step size is

()

−

tconst

step

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

This gives 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 13. All slew RAM figure examples,

except the second constant time plot, show a ramp from –80 dB

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

(Figure 16 and Figure 17) use a time constant of 0x7 (0x0 being

the fastest and 0xF being the slowest).

TIME (ms)

0.4

0.6

0.8

0.2

–0.4

–0.2

–1

–0.8

–0.6

010

Figure 13. Slew RAM—Linear Update Plot

Constant dB and RC-type (Exponential) Update Math

Exponential math is accomplished by shifts and adds with a

range from 6.1 ms to 1.27 s (–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 type), the step sizes are equal to the

difference between the values in the target RAM and slew RAM.

Figure 14 and Figure 15 show examples of this type of

target/slew RAM ramping. A decaying plot of both the

constant dB and RC-type ramps would be a mirror image of

what is shown in Figure 14.

AD1941 Datasheet(PDF) 18 Page - Analog Devices

AD1941 Datasheet(HTML) 18 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

Datasheet Download

Link URL