Sequential PROPHET-600 - Page 87

Trirraninc the Control Feedthrough
Although the worst case control feedthrough of the CEM3360
without trimming is sufficiently low for the vast majority
of applications, some applications may require even lower
feedthrough. The control feedthrough may be improved by at
least a factor of 10 by injecting a small bipolar current
(+2uA) into the signal input and adjusting its value until
tKere is zero output current when the gain is set for
maximum. This method of feedthrough trimming is shown in
Figure 7.
A potential source of feedthrough in addition to that specified
in the specifications is the signal input offset voltage.
Even though the voltage applied to the input resistor may be
at ground potential (zero volts), a small current may flow into
the input due to an offset voltage at the input pin; this in
turn will cause an output offset at maximum gain. If, for
instance,the input and output resistor are equal valued and the
input resistor is connected to ground, a 5mV input offset will
produce 5mV of control feedthrough as the gain is varied from
minimum to maximum.
The simplest way to eliminate this source of feedthrough is
to capacitively couple to the signal input. For direct coupled
inputs, the resulting control feedthrough may be trimmed out
by using the method of Figure 7. The injected current, however,
should then range over the worst case input offset divided by
the input resistor value, or I TRIM • ♦ 10mV/R It| .
It should be pointed out that this source of feedthrough is
only a problem for small input resistors or maximum voltage
gains greater than unity. Otherwise, the feedthrough produced
by input offset is on the same order or less than the inherent
feedthrough.
Other Applications
The flexibility and simplicity of the CEM3360 lends itself to
numerous applications other than VCA's: V.C. (Voltage
Controlled) filters, V.C. timers, V.C. pulse delay, V.C.
resistors, V.C. lag processors, V.C. sine wave oscillators,
and V.C. waveform generators, to name only a few. Shown in
Figure 8- is one such application in a waveform generator.
This generator features two waveforms with adjustable shape.
The output of A1 can be adjusted from rising ramp to triangle
to falling ramp, while the duty cycle of the comparator A-output
can be varied from 5% to 95%. The frequency can be swept tiver
greater than a 10,000:1 range with a 0 to +2 volt control
input; the control scale is roughly one decade per half volt.
The capacitor can be selected for either the subaudio (LFO)
or audio range.
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