AD1953YSTRL7 Analog Devices Inc, AD1953YSTRL7 Datasheet - Page 16

• Manufacturer Part Number: AD1953YSTRL7
• Description: IC DSP DAC AUDIO3CH/26BIT 48LQFP
• Manufacturer: Analog Devices Inc
• Series: SigmaDSP®r

Specifications of AD1953YSTRL7

• Rohs Status: RoHS non-compliant
• Number Of Bits: 26
• Data Interface: Serial
• Number Of Converters: 3
• Voltage Supply Source: Analog and Digital
• Power Dissipation (max): 540mW
• Operating Temperature: -40°C ~ 105°C
• Mounting Type: Surface Mount
• Package / Case: 48-LQFP
• For Use With: EVAL-AD1953EBZ - BOARD EVAL FOR AD1953 3CH 24BIT
• Settling Time: -

AD1953
Using this idea of a modified rms algorithm, the true rms value
is still obtained for all but the lowest frequency signals, while the
distortion due to rms ripple is reduced. It also allows the user to
set the hold and release times of the compressor independently.
The detector path of the AD1953 is shown in Figure 10. The rms
detector is controlled by three parameters stored in parameter
RAM: the rms time constant, the hold time, and the release rate.
The log output of the rms detector is applied to a look-up table
with interpolation. The higher bits of the rms output form an
offset into this table, and the lower bits are used to interpolate
between the table entries to form a high precision gain word.
The look-up table resides in the parameter RAM and is loaded
by the user to give the desired curve. The look-up table contains
33 data locations, and the LSB of the address into the look-up
table corresponds to a 3 dB change in the amplitude of the detec-
tor signal. This gives the user the ability to program an input/
output curve over a 99 dB range. For the main compressor, the
table resides in locations 110 to 142 in the SPI parameter RAM.
CONSTANT
One subtlety of the table look-up involves the difference between
the rms value of a sine wave and that of a square wave. If a
full-scale square wave is applied to the AD1953, the rms value of
this signal will be 3 dB higher than the rms value of a 0 dBFS sine
wave. Therefore, the table will range from +9 dB (location 142)
to –87 dB (location 110).
The entries in the table are linear gain words in 2.20 format.
Figure 11 shows an example of the table entries for a simple
above-threshold compressor.
Figure 10. Gain Derived from Interpolated Look-Up Table
TIME
Figure 9. Using the Hold and Release Time Feature
DETECTOR WITH
MODIFIED RMS
LOG OUTPUT
HOLD RELEASE
HIGH BITS (1LSB = 3dB)
LOOK-UP TABLE
LOW BITS
PROGRAMMABLE
HOLD TIME, SPI-
INPUT WAVEFORM
INTERPOLATION
LINEAR
RELEASE TIME, SPI-
PROGRAMMABLE
GAIN STAGE
OUTPUT TO
–16–
Note that the maximum gain that can be entered in the table is 2.0
(minus 1 LSB). If more gain is required, the entire compression
curve may be shifted upward by using the post-compression gain
block following the compressor/limiter.
The AD1953 compressor/limiter also includes a look-ahead
compression feature. The idea behind look-ahead compression
is to prevent compressor overshoots by applying some digital
delay to the signal before the gain-control multiplier, but not to
the detector path. In this way, the detector can acquire the new
amplitude of the input signal before the signal actually reaches
the multiplier. A comparison of a tone burst fed to a conventional
compressor versus a look-ahead compressor is shown in Figure 12.
Figure 12. Conventional Compression vs. Look-Ahead
Compression
LOOK-AHEAD COMPRESSOR GAIN
CONVENTIONAL COMPRESSOR GAIN
Figure 11. Example of Table Entry for a Given
Compression Curve
1.0
INPUT LEVEL – 3dB/TABLE ENTRY
INPUT LEVEL – 3dB/TABLE ENTRY
HOLD TIME
COMPRESSION
DESIRED
CURVE
REV. 0