that4315 THAT Corporation, that4315 Datasheet - Page 8

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

Page 8
The decoder
2:1 expander, an arrangement intended to comple-
ment the encoder in Figure 4. This circuit is opti-
mized for low-voltage operation, as might be the
case for a decoder in an in-ear monitoring system
which will run from battery power.
Since the VCA is not stable unless it sees a high fre-
quency source impedance of 5 kW or less, another
compensation network is provide to maintain stabil-
ity.
nected to EC+; This reverses the polarity of the con-
trol signal relative to the encoder, and makes this
circuit a 2:1 expander.
System Performance
companding system. The Columns labeled Encoder
VCA Gain, Encoder Out, Decoder VCA Gain, and De-
coder Out use the equations derived previously in
the section titled “The Mathematics of Log Based
Companding Systems”. The values in the column la-
beled RMS In are derived using the equation:
Figure 5 shows the THAT4315 configured as a
This circuit also uses a static gain of zero dB.
In this instance, the RMS detector output is con-
Table 1 shows the transfer characteristics of this
I
RMS In
INPUT
=
22 uF
10
22 uF
R
Encoder Out
RMS In
20
220 pF
20k
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
5k
5k
Tel: [phone]; Fax: [phone]; Web: www.thatcorp.com
16
1
THAT
4315
Figure 5. 4315 Typical Application Circuit - 1:2 Expander
15
2
Copyright © 2009, THAT Corporation
IN
IN
RMS
VCA
14
CT
3
EC+
10 uF
EC-
OUT
OUT
13
4
12
5
22 uF
11
6
±41 dB, which is easily within the capabilities of the
4315 VCA. The range of RMS input currents is eas-
ily accommodated at the high end, but accuracy may
be slightly compromised at the lowest input levels,
below -100 dBV.
Hi-fi Compander
in some applications, a few minor changes can re-
sult in substantially improved overall performance.
The following compander implementation adds
Encoder
(dBV)
-100
-110
-10
-20
-30
-40
-50
-60
-70
-80
-90
One can see that the required gain range is
While the previous circuits perform adequately
-0
In
10
7
Table 1. 2:1 compander transfer characteristics
Ra
Rb
Encode
(In dB)
8
Gain
9
VCA
-14
16
21
26
31
36
41
11
-9
-4
1
6
Vcc
THAT4315 Pre-trimmed Low-voltage
Encoder
Decoder
(dBV)
Out/
-14
-19
-24
-29
-34
-39
-44
-49
-54
-59
-64
-69
In
Low-power Analog Engine® IC
0.0773
0.0435
0.0244
0.0137
0.0077
0.0043
0.0024
0.0014
0.0008
0.0004
0.0002
0.0001
I
(mA)
RMS In
+
_
47 pF
20k
Decoder
(In dB)
VCA
Gain
-11
-16
-21
-26
-31
-36
-41
14
-1
-6
9
4
OUTPUT
Decoder
(dBV)
-100
-110
Out
-10
-20
-30
-40
-50
-60
-70
-80
-90
-0