that2162 THAT Corporation, that2162 Datasheet - Page 8

• Manufacturer Part Number: that2162
• Description: Dual Pre-trimmed Blackmer Voltage Controlled Amplifier
• Manufacturer: THAT Corporation

Page 8 of 12
Control Port Drive Impedance
bases of the logging and/or antilogging transistors.
The accuracy of the logging and antilogging is
dependent on the E
as desired to control gain. The base current in the
core transistors will follow the collector currents, of
course.
signal-related, the base currents are therefore also
signal-related. Should the source impedance of the
control voltage(s) be large, the signal-related base
currents will cause signal-related voltages to appear
at the control ports, which will interfere with precise
logging and antilogging, in turn causing distortion.
zero source impedance at pins 4 [13] and 6 [11], and
a high (> 100 kΩ) source impedance at pin 3 [14].
To realize all the performance designed into a 2162,
keep the source impedance of the control voltage
driver well under 50 Ω.
Noise Considerations
a predictable way (shown in Figure 3), but due to the
way internal bias currents vary with gain, noise at the
output is not strictly the product of a static input
noise times the voltage gain commanded. At large
attenuation, the noise floor is usually limited by the
input noise of the output op-amp and its feedback
resistor. At 0 dB gain, the noise floor of ~ -97.5 dBV
is the result of the VCA’s output noise current,
converted to a voltage by the typical 20 kΩ I-V
converter resistor (R
of 0 dB gain, the noise increases more slowly than
the gain: approximately 7.5 dB noise increase for
every
approaches 30 dB, output noise begins to increase
directly with gain.
2162 VCAs act like multipliers: 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 modulation of the signal. This can become
significant if care is not taken to drive the control
ports with quiet signals.
noise modulation because of its class AB biasing
scheme, where the shot noise in the core transistors
reaches a minimum 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 -93.5 dBV with a 50 μA 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 91.5 dBV — 6 dB of
noise modulation.
The control ports are connected directly to the
The 2162 VCAs are designed to be operated with
Another factor that influences noise is that the
The 2162 VCAs have a small amount of inherent
The VCA's noise performance varies with gain in
10
Since
dB
gain
the
C+
2
[R
and E
increase.
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THAT Corporation; 45 Sumner Street; Milford, MA 01757-1656; USA
collector
C-
voltages being exactly
Finally,
currents
Copyright © 2007, THAT Corporation
as
gain
are
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., 64 mV/dB — ten times higher than the VCA
requires), and then attenuate the control signal just
before the final drive amplifier. With careful attention
to impedance levels, relatively noisy opamps may be
used for all but the final stage.
Stray Signal Pickup
ers to design circuit boards to minimize the pickup
of stray signals within the signal path. As with noise
in the control path, signal pickup in the control path
can adversely effect the performance of an otherwise
good VCA. Because it is a multiplier, the 2162
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 μV of signal can increase distortion by over
0.01%. This can frequently be seen at high frequen-
cies, where capacitive coupling between the signal
and control paths can cause stray signal pickup.
this 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 symmetry 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 ground via
a modest-sized capacitor (e.g., 10 μF). If the distor-
tion diminishes, signal pickup in the control path is
the likely cause.
Temperature Sensitivity
2162 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 tempera-
ture, but will be 20.66 dB at a temperature 10 °C
lower.
temperature is of little consequence. However, if
necessary, it may be compensated by a resistor
embedded in the control voltage path whose value
varies with temperature at the same rate of 0.33%/°C.
Such parts are available from RCD Components, Inc,
Manchester,
[www.rcd-comp.com] and KOA/Speer Electronics,
Bradford,
[www.koaspeer.com].
To avoid excessive noise, one must take care to
It is also common practice among audio design-
Because the signal levels involved are very small,
As shown by Equation 1 (Page 5), the gain of a
For most audio applications, this change with
PA,
2162 Dual Pre-trimmed Blackmer® VCA
NH,
16701
USA
USA
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