THAT4305 Datasheet, PDF (15/20 Pages) List of Unclassifed Manufacturers – Pre-trimmed Analog Engine® IC

Document 600067 Rev 00
Page 15
INPUT
C12 _R10
22u
5k1
+15V
C13
C14 10u
U3B
THAT4305
2
RMS In
RMS Out
5
CT
4
U3A
THAT4305
22u
C11 _R8
22u ^20k
R13
5k1
11
EC+
15
13
VCA In VCA Out
EC-
12
C21
220p
C15
47p
R7
20k
U4A
2
1
3
Op-Amp
OUTPUT
Figure 18. 4305 Typical Application Circuit - 1:2 Expander
of -28.5 dBV was chosen as the desired zero dB ref-
erence. The RMS detector's input resistance can be
calculated as:
−28. 5
R_{RMS in}
=
10 ²⁰
7.5µA
≈ 5.1 kΩ .
This value also applies to the decoder.
The Decoder
Figure 18 shows the THAT4305 configured as a
1:2 expander intended to complement the encoder
in Figure 17.
This circuit also uses a static gain of zero dB.
Since the VCA is not stable unless it sees a high fre-
quency source impedance of 5 kΩ or less, the com-
pensation network of R13 and C21 ensures stability.
In this instance, the RMS detector output is con-
nected to E_C+; this reverses the polarity of the con-
trol signal relative to the encoder, and makes this
circuit a 2:1 expander.
System Performance
Table 3 shows the transfer characteristics of this
companding system. The columns labeled Encoder
VCA Gain, Encoder Out, Decoder VCA Gain, and De-
coder Out use the equations derived previously in
the Theory sub-section entitled "The Mathematics of
Log Based Companding Systems". The values in the
column labeled RMS In are derived using the equa-
tion:
Encoder Out
(
)
I_{RMS In}
=
10 ²⁰
R_{RMS In}
The required encoder VCA gain range is -24 dB
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 com-
promised at the lowest input levels.
Encoder
In
(dBV)
+20
+10
0
-10
-20
-30
-40
-50
-60
-70
-80
-90
-100
Encode
VCA
Gain
(In dB)
-24
-19
-14
-9
-4
1
6
11
16
21
26
31
36
Encoder
Out/
Decoder
In
(dBV)
-4
-9
-14
-19
-24
-29
-34
-39
-44
-49
-54
-59
-64
IRMS _In
(mA)
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
Decoder
VCA
Gain
(In dB)
24
19
14
9
4
-1
-6
-11
-16
-21
-26
-31
84
Decoder
Out
(dBV)
20
10
0
-10
-20
-30
-40
-50
-60
-70
-80
-90
139
Table 3. 2:1 compander transfer characteristics
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
Tel: [phone]; Fax: [phone]; Web: www.thatcorp.com