THAT2181 THAT Corporation, THAT2181 Datasheet - Page 9

• Manufacturer Part Number: THAT2181
• Description: Voltage Controlled Amplifiers
• Manufacturer: THAT Corporation

600030 Rev 01
quiet electronics throughout the control-voltage circuitry.
One useful technique is to process control voltages at a
multiple of the eventual control constant (e.g., 61 mV/dB
— ten times higher than the VCA requires), and then at-
tenuate the control signal just before the final drive am-
plifier. With careful attention to impedance levels,
relatively noisy opamps may be used for all but the final
stage.
Stray Signal Pickup
design circuit boards to minimize the pickup of stray
signals within the signal path. As with noise in the con-
trol path, signal pickup in the control path can ad-
versely effect the performance of an otherwise good VCA.
Because it is a multiplier, the 2181 produces second
harmonic distortion if the audio signal itself is present at
the control port. Only a small voltage at the control port
is required: as little as 10 mV of signal can increase dis-
tortion to over 0.01%. This can frequently be seen at
high frequencies, where capacitive coupling between the
signal and control paths can cause stray signal pickup.
problem can be difficult to diagnose. One clue to the
presence of this problem is that the symmetry null for
minimum THD varies with frequency. It is often possible
to counteract a small amount of pure fundamental
picked up in the control path by "misadjusting" the sym-
metry setting. Since the amount of pickup usually varies
with frequency, the optimum trim setting will vary with
frequency and level. A useful technique to confirm this
problem is to temporarily bypass the control port to
Figure 16. -L (SIP) Version Package Outline Drawing
To avoid excessive noise, one must take care to use
It is also common practice among audio designers to
Because the signal levels involved are very small, this
1
ITEM
A
B
C
D
E
F
G
H
J
K
L
M
N
I
MILLIMETERS
19.5 +0.2/-0
1.25
0.65
0.85
2.54 ±0.2
0.9
1.2
5.8 +0.2/-0
2.8 +0.1/-0
10.5 ±0.5
1.3
0.3
3.5 ±0.5
17.78 ±0.3
C
D
B
A
N
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
Tel: [phone]; Fax: [phone]; Web: www.thatcorp.com
E TYP.
F
INCHES
0.77 +0.008/-0
0.049
0.026
0.033
0.100 ±0.008
0.04
0.05
0.23 +0.008/-0
0.11 +0.004/-0
0.413 ±0.02
0.05
0.012
0.14 ±0.02
0.700 ±0.012
H
G
M
J
L
I
K
ground via a modest-sized capacitor (e.g., 10 mF). If the
distortion diminishes, signal pickup in the control path
is the likely cause.
Temperature Sensitivity
VCA is sensitive to temperature in proportion to the
amount of gain or loss commanded. The constant of pro-
portionality is 0.33% of the decibel gain commanded,
per degree Celsius, referenced to 27°>C (300°K). This
means that at 0 dB gain, there is no change in gain with
temperature. However, at -122 mV, the gain will be
+20 dB at room temperature, but will be 20.66 dB at a
temperature 10°C lower.
perature is of little consequence. However, if necessary, it
may be compensated by a resistor embedded in the con-
trol voltage path whose value varies with temperature at
the same rate of 0.33%/°C. Such parts are available from
RCD Components, Inc, 3301 Bedford St., Manchester,
NH, USA [[phone]], and KOA/Speer Electron-
ics,
[[phone]].
Closing Thoughts
and suggestions regarding these devices, their design
and application. Our engineering staff includes designers
who have decades of experience in applying our parts.
Please feel free to contact us to discuss your applications
in detail.
Figure 17. -S (SO) Version Package Outline Drawing
As shown by Equation 1 (Page 5), the gain of a 2181
For most audio applications, this change with tem-
THAT Corporation welcomes comments, questions
D
PO
ITEM
A
B
C
D
E
F
G
H
Box
E
A
MILLIMETERS
3.81/3.99
5.80/6.20
0.36/0.46
1.27
1.35/1.73
0.19/0.25
0.41/1.27
4.80/4.98
547,
Bradford,
B
C
INCHES
0.189/0.196
0.150/0.157
0.228/0.244
0.014/0.018
0.050
0.053/0.068
0.0075/0.0098
0.016/0.05
PA,
G
16701
F
H
Page 9
USA