THAT4305 Datasheet, PDF (14/20 Pages) List of Unclassifed Manufacturers – Pre-trimmed Analog Engine® IC

Page 14
THAT4305 Pre-trimmed Analog Engine^
The signals in the sidechain, and at the output of
U4, are generally relatively slow moving, so the
sidechain does not usually require wide bandwidth.
Furthermore, noise on the VCA control port can
modulate the VCA signal, thus adding noise to the
signal path. Accordingly, we added C5 in order re-
duce the noise gain of U4 at high frequencies, which
slightly reduces the high-frequency noise floor of the
entire circuit. C5 may be omitted for non-critical ap-
plications.
As described in the Theory of Operation section
("The RMS Detector - In Brief"), the RMS detector is
capable of driving large spikes of current into the av-
eraging capacitor C_TIME. To prevent these currents
from upsetting circuit grounds, it is necessary to by-
pass V_CC to a point very near the grounded end of
C_TIME with a capacitor equal to or greater than the
value of C_TIME. This is C13 in Figure 16. The
grounded ends of these two capacitors should be
connected together before being tied to the rest of
the ground system. Doing so will ensure that the
current spikes flow within the local loop consisting
of the two capacitors, and stay out of the ground sys-
tem.
Companding Systems
The Encoder
Figure 17 shows the 4305 configured as a simple
2:1 encoder or feedback compressor. The encoder in
a companding system is positioned before the noisy
channel (wireless link, storage system, etc.). The
static gain of this circuit is 1, or zero dB, and a
5.1 kΩ resistor (R3) along with a 220 pF capacitor
(C4) comprise the compensation network is re-
quired to keep the VCA's input amplifier stable for
all gains.
Since the RMS detector output is tied directly to
the VCA's E_C-, the compression ratio will be 2:1.
Note that the use of the negative-sense control port,
E_C-, makes this circuit a compressor. The RMS de-
tector timing capacitor is set for a release rate of
-125 dB per second by using a value of 10 uF.
As described in the Theory of Operation section
("The RMS Detector - In Brief"), the RMS detector is
capable of driving large spikes of current into the av-
eraging capacitor C_TIME. To prevent these currents
from upsetting circuit grounds, it is necessary to by-
pass V_CC to a point very near the grounded end of
the C_TIME with a capacitor (C5) equal to or greater
than the value of C_TIME. The grounded ends of these
two capacitors should be connected together before
being tied to the rest of the ground system. Doing so
will ensure that the current spikes flow within the
local loop consisting of the two capacitors, and stay
out of the ground system.
The output of the RMS detector is zero volts
when the RMS input current is equal to the timing
current (internally set to ~7.5 mA). A voltage level
INPUT
C1
R1
22u
20k
R3
5k1
C4
220p
U1A
11 THAT4305
EC+
15 _{VCA In VCA Out} ¹³
EC-
12
C5
22p
R2
20k
U2A
Op-Amp
OUTPUT
5
RMS In
RMS Out 2
U1B
CT
C2
R4
5k1
22u
THAT4305
4
+15V
C3
10u
C5
22u
Figure 17. 4305 simple compander circuit - 2:1 encoder (compressor)
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
Tel: [phone]; Fax: [phone]; Web: www.thatcorp.com