4305-DEMO THAT Corporation, 4305-DEMO Datasheet - Page 15

• 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
The Decoder
1:2 expander intended to complement the encoder in
Figure 17.
Since the VCA is not stable unless it sees a high
frequency source impedance of 5 kΩ or less, the
compensation network of R13 and C21 ensures
stability.
connected to E
control 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
Decoder Out use the equtions derived previously in
the Theory sub-section entitled "The Mathematics of
Log Based Companding Systems". The values in the
column labeled I
INPUT
Figure 18 shows the THAT4305 configured as a
This circuit also uses a static gain of zero dB.
In this instance, the RMS detector output is
Table 3 shows the transfer characteristics of this
I
RMS In
=
10
C+
( EncoderOut
R
RMS In
RMS In
; this reverses the polarity of the
20
C12
+15V
C11
22u
22u
are derived using the equation:
)
Tel: [phone]; Fax: [phone]; Web: www.thatcorp.com
C14
22u
THAT Corporation; 45 Sumner Street; Milford, MA 01757-1656; USA
20k
R8
220p
C21
R10
5k1
C13
10u
Figure 18. 4305 typical application circuit - 1:2 expander
R13
5k1
2
RMS In
RMS Out
CT
4
U3B
THAT4305
15
5
VCA In
to +36 dB, and the required decoder VCA gain range
is -36 dB to +14 dB. These gains are easily within
the capabilities of the 4305’s VCA. The range of RMS
input currents is easily accommodated at the high
end, though accuracy may be slightly compromised
at the lowest input levels.
Encoder
EC+
EC-
(dBV)
-100
-10
-20
-30
-40
-50
-60
-70
-80
-90
20
10
In
0
11
12
U3A
THAT4305
The required encoder VCA gain range is -24 dB
VCA Out
Table 3. 2:1 compander transfer characteristics
Encode
(In dB)
Gain
VCA
-24
-19
-14
11
16
21
26
31
36
-9
-4
1
6
13
Encoder
Decoder
(dBV)
Out/
-19
-24
-29
-34
-39
-44
-54
-59
-64
14
49
-4
-9
In
2
3
U4A
Op-Amp
0.1223
0.0688
0.0387
0.0218
0.0122
0.0069
0.0039
0.0022
0.0012
0.0007
0.0004
0.0002
0.0001
C15
47p
20k
I
(mA)
R7
RMS In
1
Decoder
(In dB)
VCA
Gain
-21
-26
-31
-36
24
19
14
11
16
-1
-6
9
4
Page 15 of 20
OUTPUT
Decoder
(dBV)
-100
Out
-10
-20
-30
-40
-50
-60
-70
-80
-90
20
10
0