Sequential PROPHET-600 - Page 82

APPLICATION HINTS CFM3360
SuppIles
Both positive and negative supplies may range from 3 to 18
volts with little effect on performance. The total supply
across the chip (V . - V F „), however, should be maintained
less than 26 volts. This allows such standard supplies as
*12 volts, *15, -5 volts, *5, -12 volts to be used with
guaranteed reliability. Greater supplies up to 32 volts,
such as +15 volt supplies, may be used but with an occasional
device exhibiting junction breakdown. No guarantee is made
for devices operated over a total supply of 26 volts.
Basic Operation
Each of the two voltage controlled transconductors consists
of a non-inverting current attenuator with a near exponential
control scale (see Control Inputs) and an isolated log converter.
When the outpt of the log converter is connected to the
exponential control input of the current cell, as shown in
the Block Diagram, the composite control response becomes
linear. The signal input, like the output, is a current and
accepts input currents of either polarity; the cell current
gain reaches a maximum of nominally unity and may be decreased
to less than -lOOdB with the control input.
Since the signal input is at a voltage within lOmV from ground,
it may be considered for all practical purposes as a virtual
ground suirsning node. Thus, an input voltage may be converted to
the required input current with a single input resistor, and
multiple input signals may be mixed with multiple input resistors
connected to the input pin, as shown in Figures 1 and 2. In
actuality, the signal input does not exhibit the ideal zero ohm
input impedance, but instead looks like a 900 resistor in
series with forward conducting diodes. Thus, a +100 uA
input current produces approximately +200mV at the input pin.
The output of each cell is a high impedance current output,
able to drive into voltages within 1.2 volts of the negative
supply and .8 volts from the positive supply. Thus, the current
output nay be converted to a voltage simply with a resistor to
ground; if necessary the output may then be buffered with an
op amp follower to provide a low output impedance drive. Such
a non-inverting configuration is shown in Figure 1.
Alternatively, the current outputs may drive the summing input
of an op amp inverter to provide an inverting configuration,
as shown in Figure 2.
Selection of Component Values
The values for the input and output resistors (or feedback
resistor in the case of the inverting configuration) depends
on the abailable input voltage, desired maximum output voltage.
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