that4315 THAT Corporation, that4315 Datasheet - Page 7

• Manufacturer Part Number: that4315
• Description: Pre-trimmed Low-voltage Low-power Analog Engine
• Manufacturer: THAT Corporation

Document 600065 Rev 04
site polarity of A in the Encoder
both the compression ratio (C.R.) of a feedback
compressor
feedforward expander is
side chain.
The encoder
compressor. The encoder in a wireless companding
system is located in the transmitter and generally
operates from a battery supply. The static gain of
this circuit is 1, or zero dB, and a 5 kW resistor
along with a 220 pF capacitor comprise the compen-
sation network is required to keep the VCA’s input
amplifier stable for all gains. Since the RMS detec-
tor output is tied directly to the VCA’s EC-, the com-
pression ratio will be 2:1. Note that the use of the
negative-sense control port, EC-, makes this circuit
a compressor. The RMS detector timing capacitor is
set for a release rate of -125 dB per second by using
a value of 10 uF.
Out
Note that for the decoder, A will have the oppo-
Using these methods, it can also be shown that
E R
where k is the absolute value of the gain of the
Figure 4 shows a simple 2:1 encoder or feedback
. .
INPUT
=
=
In
C R
. .
(
1
22 uF
+
and
= +
k
1
) (
+ -
k
220 pF
20k
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
the
A
5k
)
Tel: [phone]; Fax: [phone]; Web: www.thatcorp.com
- ×
k L M
expansion
. .
Figure 4. 4315 Typical Application Circuit - 2:1 Compressor
16
1
THAT
4315
22 uF
5k
15
2
ratio
Copyright © 2009, THAT Corporation
IN
IN
VCA
RMS
14
CT
3
EC+
10 uF
for
EC-
OUT
OUT
13
4
a
12
5
22 uF
11
6
“The RMS Detector - In Brief ”, the RMS detector is
capable of driving large spikes of current into the av-
eraging capacitor C
from upsetting circuit grounds, it is necessary to by-
pass VCC to a point very near the grounded end of
the C
the value of C
capacitors should be connected together before be-
ing 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. This requirement applies
to the decoder and other applications of the
THAT4315 as well.
when the RMS input current is equal to the timing
current (internally set to ~7.5 mA). A low-frequency
voltage level of -28.5 dBV was chosen as the desired
zero dB reference. The RMS detector’s input resis-
tance can be calculated as:
As described in the Theory of Operation section
The output of the RMS detector is zero volts
R
This value also applies to the decoder.
10
RMS in
7
TIME
Ra
Rb
with a capacitor equal to or greater than
8
9
=
10
7 5
TIME
. m
-
28 5
20
Vcc
A
.
. The grounded ends of these two
@
TIME
5
k
. To prevent these currents
W
+
_
47 pF
20k
OUTPUT
Page 7