4305-DEMO THAT Corporation, 4305-DEMO Datasheet - Page 5

• Manufacturer Part Number: 4305-DEMO
• Description: Audio Modules & Development Tools Low-Cost Dynamics Processor Demo Board
• Manufacturer: THAT Corporation

Specifications of 4305-DEMO

• Description/function: Audio DSPs
• Operating Supply Voltage: 15 V
• Product: Audio Modules
• For Use With/related Products: THAT4305
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Document 600067 Rev 01
capacitor (C
an internal current source (I
current source is internally fixed at 7.5 μA. The
resulting time constant in seconds is approximately
equal to 3467 times the value of the timing capacitor
in Farads. Note that, as a result of the mathematics
of RMS detection, the attack and release time
constants are fixed in their relationship to each
other.
spikes of current into C
audio signal input to the RMS detector increases
suddenly. This current is drawn from V
fed through C
supply through the ground end of C
handled properly through layout and bypassing,
these currents can mix with the audio in the circuit’s
ground structure with unpredictable and undesirable
results. As noted in the Applications section, local
bypassing from the V
is strongly recommended in order to keep these
currents out of the ground structure of the circuit
(see C4 in Figure 2.)
same constant of proportionality as the VCA gain
control: 6.2 mV/dB. The detector's 0 dB reference
(i
output to equal 0V), is trimmed during wafer probe
to equal approximately 7.5 μA. The RMS detector
output stage is capable of sinking or sourcing
125 μA. It is also capable of driving up to 150 pF of
capacitance.
across the audio band for a wide range of input
signal levels. Note, however, that it does fall off at
high frequencies at low signal levels like THAT’s
other RMS detectors.
tor circuitry and that of the THAT 2252 RMS detec-
tor include the following.
internally balanced by design, and cannot be
balanced via an external control. The 4305 will
typically balance positive and negative halves of the
input signal within 10% but in extreme cases the
mismatch may reach +40% or -30% (±3dB).
However, even such extreme-seeming mismatches
will
in0
, the input current which causes the detector's
The RMS detector is capable of driving large
The dc output of the detector is scaled with the
Frequency response of the detector extends
Differences between the 4305's RMS level detec-
1. The rectifier in the 4305 RMS Detector is
not
TIME
significantly
TIME
) attached to the C
at pin 4, and returns to the power
CC
pin to the ground end of C
Tel: [phone]; Fax: [phone]; Web: www.thatcorp.com
THAT Corporation; 45 Sumner Street; Milford, MA 01757-1656; USA
TIME
increase
, particularly when the
T
) connected to C
T
pin (pin 4), and
ripple-induced
TIME
CC
.
(pin 9),
T
If not
. The
TIME
distortion in dynamics processors over that caused
by balanced signal ripple alone.
is determined by the combination of an external
capacitor C
internal current source is set to about 7.5 μA. A
resistor is not normally connected directly to the C
pin on the 4305.
also set to approximately 7.5 μA. However, as in the
2252, the level match will be affected by any
additional currents drawn from the C
Compressor (or Limiter) Configurations
blocks required for a wide variety of dynamics
processing applications. The part may be configured
into
system.
4305 is as a compressor or limiter. These circuits
are intended to reduce gain above some determined
signal level in order to prevent subsequent stages
from being overloaded by too high a signal.
Compressors
compression ratios, while limiters have high ratios.
In such applications, the signal path has static gain
so long as the input signal remains below some
threshold, but gain is reduced when the signal rises
above the threshold. Compression ratio is defined as
the number of dB the input signal increases for a
1 dB increase in output signal.
Feedforward Topologies
typically, the input signal is applied to both the VCA
and the RMS detector. The RMS output signal is fed
forward to the VCA's negative control port (E
dc-coupled op-amp based stage. This stage has gain
above some dc level (the threshold), and no trans-
mission below that level. This path, called the
"sidechain," — from detector output to VCA control
port — determines the compression behavior of the
circuit. As signal level rises, the dc voltage at the
RMS' output rises.
threshold, the rms output signal is transmitted
through the sidechain and presented to the VCA
control port, lowering the gain to signals passing
2. The time constant of the 4305's RMS detector
3. The 0 dB reference point, or level match, is
The 4305 provides the two essential building
Perhaps the most common application for the
To make a compressor or limiter with a 4305,
practically any type of dynamics processor
TIME
generally
and an internal current source. The
Once the dc level exceeds the
have
low
T
pin.
to
Page 5 of 20
moderate
C-
) via a
T