THAT2180A Datasheet, PDF (8/8 Pages) List of Unclassifed Manufacturers – Pre-Trimmed IC Voltage Controlled Amplifiers

Page 8
THAT2180 Series IC VCAs
Noise Considerations
It is second nature among good audio designers to con-
sider the effects of noisy devices on the signal path. As is
well known, this includes not only active devices such as
opamps and transistors, but extends to the choice of im-
pedance levels as well. High value resistors have higher in-
herent thermal noise, and the noise performance of an
otherwise quiet circuit can be easily spoiled by the wrong
choice of impedance levels.
Less well known, however, is the effect of noisy circuitry
and high impedance levels in the control path of volt-
age-control circuitry. The 2180 Series VCAs act like multi-
pliers: when no signal is present at the signal input, noise at
the control input is rejected. So, when measuring noise (in
the absence of signal – as most everyone does), even very
noisy control circuitry often goes unnoticed. However, noise
at the control port of these parts will cause noise modula-
tion of the signal. This can become significant if care is not
taken to drive the control ports with quiet signals.
The 2180 Series VCAs have a small amount of inherent
noise modulation because of its class AB biasing scheme,
where the shot noise in the core transistors reaches a mini-
mum with no signal, and increases with the square root of
the instantaneous signal current. However, in an optimum
circuit, the noise floor rises only to -94 dBV with a
50 mA rms signal at unity gain — 4 dB of noise modulation.
By contrast, if a unity-gain connected, non-inverting 5534
opamp is used to directly drive the control port, the noise
floor will rise to 92 dBV — 6 dB of noise modulation.
To avoid excessive noise, one must take care to use
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
It is also common practice among audio designers to
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 2180 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.
Because the signal levels involved are very small, this
problem can be difficult to diagnose. One useful tech-
nique is to temporarily bypass the control port to
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
As shown by the equation for A_V (Page 5), the gain of
a 2180 VCA is sensitive to temperature in proportion to
the amount of gain or loss commanded. The constant of
proportionality 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.
For most audio applications, this change with tem-
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 Electronics,
PO Box 547, Bradford, PA, 16701 USA [[phone]].
Closing Thoughts
THAT Corporation welcomes comments, questions
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.
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E TYP.
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ITEM
A
B
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F
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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
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
Figure 16. -L (SIP) Version Package Outline Drawing
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
Tel: [phone]; Fax: [phone]; Web: www.thatcorp.com