2180-DEMO THAT Corporation, 2180-DEMO Datasheet - Page 9

• Manufacturer Part Number: 2180-DEMO
• Description: Audio Modules & Development Tools Pre-Trimmed VCA Demo Board
• Manufacturer: THAT Corporation

Specifications of 2180-DEMO

• Description/function: Audio Amplifiers
• Operating Supply Voltage: 15 V
• Product: Audio Modules
• For Use With/related Products: THAT2180
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

THAT 2180 Series
Blackmer® Pre-trimmed IC VCAs
Positive and Negative
to drive both control ports, either with differential
drive (in which case, the control sensitivities of each
port are summed), or through two different control
signals. There is no reason why both control ports
cannot be used simultaneously.
Control Port Drive Impedance
connected directly to the bases of the logging and/or
antilogging transistors. The accuracy of the logging
and antilogging is dependent on the E
voltages being exactly as desired to control gain. The
base current in the core transistors will follow the
collector currents, of course. Since the collector
currents are 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.
operated with zero source impedance at pins 2 and
3, and an infinite source impedance at pin 4. (pin 4
should be left open.) To realize all the performance
designed into a 2180, keep the source impedance of
the control voltage driver well under 50 Ω.
directly with an opamp. However, the closed-loop
output impedance of an opamp typically rises at high
frequencies because open loop gain falls off as
frequency increases. A typical opamp's output imped-
ance is therefore inductive at high frequencies.
Excessive inductance in the control port source
impedance can cause the VCA to oscillate internally.
In such cases, a 100 Ω resistor in series with a
1.5 nF capacitor from the control port to ground will
usually suffice to prevent the instability.
Noise Considerations
to consider 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 impedance levels as well. High value
resistors have higher inherent thermal noise, and the
noise performance of an otherwise quiet circuit can
be easily spoiled by the wrong choice of impedance
levels.
circuitry and high impedance levels in the control
path of voltage-control circuitry. The 2180 Series
It is also possible (and sometimes advantageous)
The control ports (pins 2 through 4) are
The 2180 Series VCAs are designed to be
This often suggests driving the control port
It is second nature among good audio designers
Less well known, however, is the effect of noisy
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C+
and E
Page 9 of 12
C-
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.
inherent 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 -94 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 92 dBV — 6 dB of
noise modulation.
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 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 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 μV of signal can increase distortion to 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 useful
technique is to temporarily bypass the control port to
ground via a modest-sized capacitor (e.g., 10 μF). If
the distortion diminishes, signal pickup in the
control path is the likely cause.
Temperature Sensitivity
of a 2180 VCA is sensitive to temperature in
The 2180 Series VCAs have a small amount of
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 the equation for A
Document 600029 Rev 02
V
(Page 5), the gain