Sequential PROPHET-600 - Page 84

and to some extent, what parameters are more important.
The specification for maximum input and output current is the
maximum positive input current which the device input can
handle; positive currents above this value will be hard limited
and thus severely distorted.
Hence, it is recommended that for best signal to noise ratio
and minimum feedthrough relative to the maximum output, the
input resistor, or resistors, should be selected so that the
total peak input current is in the 200 - 300 uA range.
However, for applications where distortion is more important,
the inputs should be operated nominally somewhat lower, around
100 uA.
For resistor value calculation purposes, the input may be
assumed to be a virtual ground summing node if the input voltage
is above several volts. For lower input voltages, the following
equation may be used to calculate the input resistor, or
resistors:
V IN - 120 mV
- 900
V I is the available input voltage while I is the selected
iftput current, typically 100 - 300 uA. Node that signals
400mV or less peak may be applied directly to the input pin.
For best distortion performance, however, the input voltage
should be maintained as large as possible and an input resistor
used.
The value of the output resistor, or feedback resistor, is
then selected to give the maximum desired output voltage with
the particular input voltage, with the control inputs set for
maximum gain, the current gain is unity; a +100uA input, for
instance, will produce a nominal maximum output of +100uA.
Note that while the maximum current gain possible Is unity,
the maximum voltage gain can be either greater or less than
unity.
The Control Inputs
Each transconductor may be connected to provide either a linear
control response or an exponential control response.
For a linear control response, the V pin (output of the
log converter) is hardwired to the v2 pin (exponential control
input to the variable gain cell), and the control voltage is
applied to the V c pin (input to the log converter).
The control voltage range to this input is from nominally zero
volts for minimum gain to nominally 1.93 volts for maximum
gain. Since this input is a relatively high impedance, a
resistor divider may be used to accommodate a larger control
range, 0 to +10 V for instance. The resistor divider impedance
- 7 -