that4315 THAT Corporation, that4315 Datasheet - Page 10

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

Page 10
of signal pre-emphasis at 20 kHz. Note additionally
that this added pre-emphasis also compensates the
input to the VCA, so additional components are not
required to implement this feature.
this network is buffered by U2B, and while this is
not strictly necessary, the network must be driven
from a low source impedance to ensure proper
tracking between the encoder pre-emphasis and the
Encoder
Vcc
(dBV)
-100
-110
-10
-20
-30
-40
-50
-60
-70
-80
-90
At 20 kHz, the gain is;
Or 36 dB, which means there is nominally 20 dB
A
-0
In
Encode In
=
Table 2. Hi-fi compander transfer characteristics
C8
100n
21
k
133
Encoder
W
(In dB)
Gain
| | .
VCA
-25
-20
-15
-10
10
15
20
25
30
-5
-0
2 32
5
C7
100n
k
W
k
W
3
2
Op-Amp
@
Encoder
Decoder
Vcc Filt
Gnd
U3A
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
(dBV)
Out/
637
-14
-19
-24
-29
-34
-39
-44
-49
-54
-59
-64
9
8
In
-9
.
Vref
1
Tel: [phone]; Fax: [phone]; Web: www.thatcorp.com
7
6
U1C
THAT4315
0.1375
0.0773
0.0435
0.0244
0.0137
0.0077
0.0043
0.0024
0.0014
0.0008
0.0004
0.0002
C1
1u
I
(mA)
RMS In
C6
22u
1k1
R7
10k0
R1
Decoder
10
(In dB)
C10
9
10n
Gain
VCA
Op-Amp
-10
-15
-20
-25
-30
25
20
15
10
-5
U3C
5
0
The input to
Copyright © 2009, THAT Corporation
15
8
VCA In
Figure 8. 3:1 Encoder circuit
Decoder
(dBV)
-100
-110
Out
Vref
-10
-20
-30
-40
-50
-60
-70
-80
-90
-0
EC+
EC-
12
11
Vref
VCA Out
14
U3D
2k0
C9
R5
1n
U1A
THAT4315
13
12
13
decoder de-emphasis. If this network is driven from
an unbuffered source, then the network should be
adjusted to take into account the impedance of that
source.
tectors in both the encoder and the decoder. The
center frequency of this pre-emphasis circuit is
aligned with the center frequency of the signal path
pre-emphasis when evaluated on a logarithmic fre-
quency scale.
used to “floor out ” the detectors at low levels. There
are a few reasons why a designer might choose to do
this. There is some variability in the low-level per-
formance of these detectors, and some will occasion-
ally perform substantially better than typical, and
this can result in mis-tracking at low levels. A more
common problem is that there is some additional fil-
tering seen by the decoder RMS detector; This is
usually is caused by band limiting in the RF detec-
tor’s signal path. As a result, 2 detectors in the sys-
tem see a different noise floor, and this can result in
mis-tracking.
the detector to stop responding to input signals that
are 27 dB below level match. The value is deter-
mined by
Op-Amp
R6
2k0
Vref
There is 10 dB of RMS pre-emphasis for the de-
R6 in the encoder and R12 in the decoder are
The values of R6 and R12 were chose to cause
R
floor
Vref
Vcc
=
C4
10u
(
7 5
27
.
5
U1B
THAT4315
m
RMS Out
dB
A
×
×
5
6
CT
10
RMS In
200k
80k6
THAT4315 Pre-trimmed Low-voltage
6
22p
dB
C5
R4
4
mV
R3
-
20
U3B
C3
10u
27
Op-Amp
)
7
2
Low-power Analog Engine® IC
3k01
R8
6k65
R2
C11
8n2
C2
1u
Encode
Out