4305-DEMO THAT Corporation, 4305-DEMO Datasheet - Page 4

• 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

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converted to input current via an appropriately sized
resistor (R3 in Figure 2). Because the dc current
associated with dc offsets present at the input pin
plus any dc offset in the preceding stages will be
modulated by gain changes (thereby becoming
audible as thumps), the input pin is normally
ac-coupled. This blocks such offset currents and
reduces dc offset variation with gain.
current, inverted with respect to the input current. In
normal operation, the output current is converted to
a voltage via an external op-amp, where the current-
to-voltage conversion ratio is determined by the
feedback resistor connected between the op-amp's
output and its inverting input (R2 in Figure 2). The
resulting signal path through the VCA plus op-amp is
noninverting.
between E
unused control port should be connected to ground
(as E
proportional to (E
tionality is 6.2 mV/dB for the voltage at E
to E
more than ±0.6 V away from ground.
a predictable way, but due to the way internal bias
currents vary with gain, noise at the output is not
strictly the product of a static input noise times the
voltage gain commanded. At large attenuation, the
noise floor is usually limited by the input noise of the
output op-amp and its feedback resistor. At 0 dB
gain, the noise floor of ~ -97.5 dBV is the result of
the VCA’s output noise current, converted to a
voltage by the typical 20k I-V converter resistor (R2
in Figure 2). In the vicinity of 0 dB gain, the noise
increases more slowly than the gain: approximately
5 dB noise increase for every 10 dB gain increase.
Finally, as gain approaches 30 dB, output noise
begins to increase directly with gain.
that of the THAT 2180 Series VCAs, there are several
important differences, as follows.
the supply voltage. At ±5 V, approximately 800 μA is
available for the sum of input and output signal
currents. This increases to about 1.8 mA at ±15 V.
C-
The VCA output signal, VCA
The VCA gain is controlled by the voltage applied
The VCA's noise performance varies with gain in
While the 4305's VCA circuitry is very similar to
1. Supply current for the 4305 VCA depends on
). Note that neither E
C+
is in Figure 2). The gain (in decibels) is
C+
(pin 11) and E
C+
-E
Tel: [phone]; Fax: [phone]; Web: www.thatcorp.com
C-
THAT Corporation; 45 Sumner Street; Milford, MA 01757-1656; USA
). The constant of propor-
C+
C-
or E
(pin 12). Note that any
OUT
C-
(pin 13), is also a
should be driven
C+
(relative
(Compare this to ~1.8 mA for a 2180 Series VCA
when biased as recommended.)
2180 VCA) is not brought out to an external pin; it is
driven from an internally trimmed current generator.
6.2 mV/dB, due primarily to the higher internal
operating temperature of the 4305 compared to that
of the 2180 Series.
The RMS Detector - in Brief
rectifying input current signals, converting the recti-
fied current to a logarithmic voltage, and applying
that voltage to a log-domain filter. The output signal
is a dc voltage proportional to the decibel-level of the
RMS value of the input signal current. Some ac
component (at twice the input frequency plus higher-
order even harmonics) remains superimposed on the
dc output. The ac signal is attenuated by a log
domain filter, which constitutes a single-pole rolloff
with cutoff determined by an external capacitor (C4
in Figure 2).
small amount of fundamental (and higher odd-order
harmonics) ripple can be present at the detector
output. By design, this ripple contributes less total
ripple
naturally and inevitably present at the output of a
perfectly balanced detector.
RMS
a resistor (R1 in Figure 2) is normally used to
convert input voltages to the desired current. The
level detector is capable of accurately resolving
signals well below 10 mV (with a 5 kΩ
resistor). However, if the detector is to accurately
track such low-level signals, ac coupling (C1 in
Figure 2) is required to prevent dc offsets from
causing a dc current to flow in the detector’s input,
which would obscure low-level ac signal currents.
placed well below the frequency range of interest. For
an audio-band detector, a typical value would be
5 Hz, or a 32 ms time constant (τ). The filter's time
constant is determined by an external timing
2. The SYM control port (similar to that on the
3. The control-voltage constant is approximately
The 4305's detector computes RMS level by
The rectifier is balanced to within ±3 dB, so a
As in the VCA, input signals are currents to the
The log-domain filter cutoff frequency is usually
IN
pin (pin 2). This input is a virtual ground, so
than the
THAT4305 Pre-trimmed Analog Engine®
even-order products
that are
input